Work energy and power

Multiple Choice Questions (MCQs)

1. An electron and a proton are moving under the influence of mutual forces. In calculating the change in the kinetic energy of the system during motion, one ignores the magnetic force of one on another. This is, because

(a) the two magnetic forces are equal and opposite, so they produce no net effect

(b) the magnetic forces do not work on each particle

(c) the magnetic forces do equal and opposite (but non-zero) work on each particle

(d) the magnetic forces are necessarily negligible

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Thinking Process

In this problem as the electron and proton are moving under the influence of mutual forces, they will perform circular motion about their centre (i.e., about middle point of the line joining them).

Answer (b) When electron and proton are moving under influence of their mutual forces, the magnetic forces will be perpendicular to their motion hence no work is done by these forces.

  • (a) The two magnetic forces are equal and opposite, but they do not cancel out in terms of work done because work depends on the direction of force and displacement, not just the force itself.
  • (c) The magnetic forces do not do work on each particle because the force is perpendicular to the direction of motion, hence no work is done.
  • (d) The magnetic forces are not necessarily negligible; they are simply perpendicular to the motion, resulting in no work being done.

2. A proton is kept at rest. A positively charged particle is released from rest at a distance d in its field. Consider two experiments; one in which the charged particle is also a proton and in another, a positron. In the same time t, the work done on the two moving charged particles is

(a) same as the same force law is involved in the two experiments

(b) less for the case of a positron, as the positron moves away more rapidly and the force on it weakens

(c) more for the case of a positron, as the positron moves away a larger distance

(d) same as the work done by charged particle on the stationary proton

Show Answer

Answer (c) Force between two protons is same as that of between proton and a positron.

As positron is much lighter than proton, it moves away through much larger distance compared to proton.

We know that work done = force × distance. As forces are same in case of proton and positron but distance moved by positron is larger, hence, work done will be more.

  • (a) The work done is not the same in both experiments because, although the force law is the same, the distance moved by the positron is larger due to its smaller mass, resulting in more work done on the positron.
  • (b) The work done is not less for the positron. While it is true that the positron moves away more rapidly and the force weakens, the overall work done depends on both the force and the distance moved. Since the positron moves a larger distance, the work done is actually more.
  • (d) The work done by the charged particle on the stationary proton is not the same as the work done on the moving particles. The stationary proton does not move, so no work is done on it. The work done on the moving particles depends on the distance they travel, which is greater for the positron.

3. A man squatting on the ground gets straight up and stand. The force of reaction of ground on the man during the process is

(a) constant and equal to mg in magnitude

(b) constant and greater than mg in magnitude

(c) variable but always greater than mg

(d) at first greater than mg and later becomes equal to mg

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Answer (d) When the man is squatting on the ground he is tilted somewhat, hence he also has to balance frictional force besides his weight in this case.

R= reactional force = friction +mg

R>mg

When the man gets straight up in that case friction 0

Reactional force mg

  • (a) The force of reaction of the ground on the man is not constant and equal to ( mg ) in magnitude because when the man is squatting and then gets up, there is a change in the forces acting on him. Initially, the reaction force must counteract not only his weight but also the additional forces due to his movement, making it variable and not constant.

  • (b) The force of reaction of the ground on the man is not constant and greater than ( mg ) in magnitude because the reaction force changes during the process. It is greater than ( mg ) when the man is accelerating upwards but eventually becomes equal to ( mg ) when he stands still.

  • (c) The force of reaction of the ground on the man is not always greater than ( mg ) because once the man stands up and is no longer accelerating, the reaction force equals ( mg ). Therefore, it is not always greater than ( mg ); it is only greater during the upward acceleration phase.

4. A bicyclist comes to a skidding stop in 10m. During this process, the force on the bicycle due to the road is 200N and is directly opposed to the motion. The work done by the cycle on the road is

(a) +2000 J

(b) -200 J

(c) zero

(d) 20,000 J

Show Answer

Thinking Process

In this problem energy will be lost due to dissipation by friction.

Answer (c) Here, work is done by the frictional force on the cycle and is equal to 200×10=2000J.

As the road is not moving, hence, work done by the cycle on the road = zero.

Note We should be aware that here the energy of bicyclist is lost during the motion, but it is lost due to friction in the form of heat.

  • Option (a) +2000 J: This option is incorrect because the work done by the frictional force is negative, as it opposes the motion. Therefore, the work done cannot be positive.

  • Option (b) -200 J: This option is incorrect because the work done by the frictional force is calculated as the force multiplied by the distance, which is 200N×10m=2000J. The correct value should be 2000J, not 200J.

  • Option (d) 20,000 J: This option is incorrect because it overestimates the work done by the frictional force. The correct calculation is 200N×10m=2000J, so the work done should be 2000J, not 20,000J.

5. A body is falling freely under the action of gravity alone in vaccum. Which of the following quantities remain constant during the fall?

(a) Kinetic energy

(b) Potential energy

(c) Total mechanical energy

(d) Total linear momentum

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Answer (c) As the body is falling freely under gravity, the potential energy decreases and kinetic energy increases but total mechanical energy ( PE+KE ) of the body and earth system will be constant as external force on the system is zero.

  • Kinetic energy: The kinetic energy of the body increases as it falls because its velocity increases due to the acceleration caused by gravity.

  • Potential energy: The potential energy of the body decreases as it falls because its height above the ground decreases.

  • Total linear momentum: The total linear momentum of the body increases as it falls because its velocity increases due to the acceleration caused by gravity.

6. During inelastic collision between two bodies, which of the following quantities always remain conserved?

(a) Total kinetic energy

(b) Total mechanical energy

(c) Total linear momentum

(d) Speed of each body

Show Answer

Thinking Process

In an inelastic collision between two bodies due to some deformation, energy may be lost in the form of heat and sound etc.

Answer (c) When we are considering the two bodies as system the total external force on the system will be zero.

Hence, total linear momentum of the system remain conserved.

  • (a) Total kinetic energy: In an inelastic collision, some of the kinetic energy is converted into other forms of energy such as heat, sound, or deformation energy. Therefore, the total kinetic energy is not conserved.

  • (b) Total mechanical energy: Total mechanical energy, which includes both kinetic and potential energy, is not conserved in an inelastic collision because some of the mechanical energy is transformed into other forms of energy like heat or sound.

  • (d) Speed of each body: The speed of each body changes during an inelastic collision due to the transfer of momentum and energy between the colliding bodies. Therefore, the speed of each body is not conserved.

7. Two inclined frictionless tracks, one gradual and the other steep meet at A from where two stones are allowed to slide down from rest, one on each track as shown in figure.

Which of the following statement is correct?

(a) Both the stones reach the bottom at the same time but not with the same speed

(b) Both the stones reach the bottom with the same speed and stone I reaches the bottom earlier than stone II

(c) Both the stones reach the bottom with the same speed and stone II reaches the bottom earlier than stone I

(d) Both the stones reach the bottom at different times and with different speeds

Show Answer

Answer (c) As the given tracks are frictionless, hence, mechanical energy will be conserved. As both the tracks having common height, h.

From conservation of mechanical energy,

12mv2=mgh (for both tracks I and II) v=2gh

Hence, speed is same for both stones. For stone I,a1= acceleration along inclined plane =gsinθ1

Similarly, for stone |a2=gsinθ2 as θ2>θ1 hence, a2>a1.

And both length for track II is also less hence, stone II reaches earlier than stone I.

  • Option (a): This option is incorrect because although both stones will reach the bottom with the same speed due to conservation of mechanical energy, they will not reach the bottom at the same time. The stone on the steeper track (stone II) will have a greater acceleration and a shorter path, causing it to reach the bottom earlier than the stone on the gradual track (stone I).

  • Option (b): This option is incorrect because while it correctly states that both stones will reach the bottom with the same speed, it incorrectly states that stone I will reach the bottom earlier than stone II. In reality, stone II, which is on the steeper track, will reach the bottom earlier due to its greater acceleration and shorter path.

  • Option (d): This option is incorrect because it states that both stones will reach the bottom at different times and with different speeds. However, due to the conservation of mechanical energy, both stones will reach the bottom with the same speed. The difference lies only in the time taken to reach the bottom, with stone II reaching earlier than stone I.

8. The potential energy function for a particle executing linear SHM is given by V(x)=12kx2 where k is the force constant of the oscillator (Fig). For k=0.5N/m, the graph of V(x) versus x is shown in the figure. A particle of total energy E turns back when it reaches x=±xm. If V and K indicate the PE and KE, respectively of the particle at x=+xm, then which of the following is correct?

(a) V=O,K=E

(b) V=E,K=O

(c) V<E,K=O

(d) V=O,K<E

Show Answer

Answer (b) Total energy is E=PE+KE

When particle is at x=xm i.e., at extreme position, returns back. Hence, at x=xm; x=0;KE=0

From Eq. (i) E=PE+0=PE=V(xm)=12kxm2

  • Option (a) V=0,K=E: This option is incorrect because at the extreme position x=±xm, the potential energy V is at its maximum value, not zero. The kinetic energy K is zero at this point because the particle momentarily stops before reversing direction.

  • Option (c) V<E,K=0: This option is incorrect because at the extreme position x=±xm, the potential energy V is equal to the total energy E, not less than E. The kinetic energy K is indeed zero, but the potential energy should be equal to the total energy.

  • Option (d) V=0,K<E: This option is incorrect because at the extreme position x=±xm, the potential energy V is not zero; it is equal to the total energy E. The kinetic energy K is zero at this point, not less than the total energy.

9. Two identical ball bearings in contact with each other and resting on a frictionless table are hit head-on by another ball bearing of the same mass moving initially with a speed v as shown in figure.

If the collision is elastic, which of the following (figure) is a possible result after collision?

Show Answer

Answer (b) When two bodies of equal masses collides elastically, their velocities are interchanged.

When ball 1 collides with ball-2, then velocity of ball-1, v1 becomes zero and velocity of ball-2, v2 becomes v, i.e., similarly.

when ball 2 collides will ball 3v2=0,v3=v

  • Option (a): This option shows all three balls moving with the same velocity after the collision. This is incorrect because, in an elastic collision involving identical masses, the velocities are exchanged, not shared equally among all the balls.

  • Option (c): This option shows the incoming ball stopping and the two initially stationary balls moving apart with equal and opposite velocities. This is incorrect because, in an elastic collision, the incoming ball’s velocity would be transferred to one of the stationary balls, not split between them.

  • Option (d): This option shows the incoming ball continuing to move with the same velocity while the two initially stationary balls remain stationary. This is incorrect because, in an elastic collision, the incoming ball’s velocity would be transferred to one of the stationary balls, causing it to move.

10. A body of mass 0.5kg travels in a straight line with velocity v=ax3/2 where a=5m1/2s1. The work done by the net force during its displacement from x=0 to x=2m is

(a) 1.5J

(b) 50J

(c) 10J

(d) 100J

Show Answer

Answer Given, v=ax3/2

m=0.5kg,a=5m1/2s1, work done (W)=?

We know that

Acceleration

a0=dvdt=vdvdx=ax3/2ddx(ax3/2)=ax3/2×a×32×x1/2=32a2x2

Now,

 Force =ma0=m32a2x2

Work done =x=0x=2Fdx=0232ma2x2dx

=32ma2×(x3/3)02=12ma2×8=12×(0.5)×(25)×8=50J

  • Option (a) 1.5J is incorrect because the calculated work done, based on the given parameters and the integration of the force over the displacement, results in a value of 50J, not 1.5J. The mathematical steps and integration clearly show that the work done is significantly higher than 1.5J.

  • Option (c) 10J is incorrect because, similar to option (a), the integration of the force over the displacement from x=0 to x=2 results in 50J. The value of 10J does not match the derived result from the given equations and constants.

  • Option (d) 100J is incorrect because the work done, as calculated through the integration process, is 50J. The value of 100J is double the correct answer, indicating a possible misunderstanding or miscalculation in the problem-solving process.

11. A body is moving unidirectionally under the influence of a source of constant power supplying energy. Which of the diagrams shown in figure correctly shown the displacement-time curve for its motion?

Show Answer

Answer (b) Given, power = constant

We know that power (P)

P=dWdt=Fdsdt=Fdsdt( body is moving unidirectionally )

Hence,

Fds=Fdscos0

P=Fdsdt= constant (P= constant by question )

Now, writing dimensions

[F][v]= constant  [MLT2][LT1]= constant  L2T3= constant ( mass is constant ) LT3/2 Displacement (d)t3/2

  • Option (a): This option shows a linear relationship between displacement and time, which implies a constant velocity. However, since the power is constant and not the velocity, the displacement cannot be linear with time.

  • Option (c): This option shows a parabolic relationship between displacement and time, which implies a constant acceleration. However, with constant power, the acceleration is not constant; instead, the velocity increases as the square root of time, leading to a displacement-time relationship of ( t^{3/2} ).

  • Option (d): This option shows an exponential relationship between displacement and time, which implies an exponentially increasing velocity. However, with constant power, the velocity increases as the square root of time, leading to a displacement-time relationship of ( t^{3/2} ), not an exponential one.

12. Which of the diagrams shown in figure most closely shows the variation in kinetic energy of the earth as it moves once around the sun in its elliptical orbit?

Show Answer

Answer (d) When the earth is closest to the sun, speed of the earth is maximum, hence, KE is maximum. When the earth is farthest from the sun speed is minimum hence, KE is minimum but never zero and negative.

This variation is correctly represented by option(d).

  • Option (a): This option shows a constant kinetic energy, which is incorrect because the kinetic energy of the Earth varies as it moves in its elliptical orbit around the Sun. The speed of the Earth changes, leading to changes in kinetic energy.

  • Option (b): This option shows a linear increase and decrease in kinetic energy, which is incorrect because the variation in kinetic energy is not linear. The kinetic energy changes more rapidly when the Earth is closer to the Sun and more slowly when it is farther away.

  • Option (c): This option shows the kinetic energy dropping to zero, which is incorrect because the Earth’s speed never becomes zero in its orbit around the Sun. The kinetic energy is always positive and never reaches zero.

13. Which of the diagrams shown in figure represents variation of total mechanical energy of a pendulum oscillating in air as function of time?

Show Answer

Answer (c) When a pendulum oscillates in air, it will lose energy continuously in overcoming resistance due to air. Therefore, total mechanical energy of the pendulum decreases continuously with time.

The variation is correctly represented by curve (c).

  • Option (a): This diagram shows a constant total mechanical energy over time, which is incorrect because a pendulum oscillating in air loses energy due to air resistance, causing its total mechanical energy to decrease continuously.

  • Option (b): This diagram shows an increasing total mechanical energy over time, which is incorrect because a pendulum oscillating in air cannot gain energy on its own; it loses energy due to air resistance.

  • Option (d): This diagram shows a periodic variation in total mechanical energy, which is incorrect because the total mechanical energy of a pendulum oscillating in air should decrease continuously due to energy loss from air resistance, not oscillate periodically.

14. A mass of 5kg is moving along a circular path of radius 1m. If the mass moves with 300rev/min, its kinetic energy would be

(a) 250π2

(b) 100π2

(c) 5π2

(d) 0

Show Answer

Answer (a) Given, mass =m=5kg

ω=300rev/min=(300×2π)rad/min=(300×2×3.14)rad/60s=300×2×3.1460rad/s=10πrad/s linear speed =v=ωR=(300×2π60)(1)=10πm/sKE=12mv2=12×5×(10π)2=100π2×5×12=250π2J

  • Option (b) 100π2: This option is incorrect because it underestimates the kinetic energy. The correct calculation shows that the kinetic energy is 250π2 J, not 100π2 J. The error likely comes from an incorrect application of the kinetic energy formula or a miscalculation of the linear speed.

  • Option (c) 5π2: This option is incorrect because it significantly underestimates the kinetic energy. The correct kinetic energy is 250π2 J. This large discrepancy suggests a fundamental error in the calculation process, possibly in the conversion of angular velocity to linear speed or in the application of the kinetic energy formula.

  • Option (d) 0: This option is incorrect because it implies that the mass has no kinetic energy. Since the mass is moving with a non-zero velocity along a circular path, it must have kinetic energy. The correct kinetic energy is 250π2 J, not zero.

15. A raindrop falling from a height h above ground, attains a near terminal velocity when it has fallen through a height (3/4)h. Which of the diagrams shown in figure correctly shows the change in kinetic and potential energy of the drop during its fall up to the ground?

Show Answer

Thinking Process

During fall of a raindrop first velocity of the drop increases and then become constant after sometime.

Answer (b) When drop falls first velocity increases, hence, first KE also increases. After sometime speed (velocity) is constant this is called terminal velocity, hence, KE also become constant. PE decreases continuously as the drop is falling continuously.

The variation in PE and KE is best represented by (b).

  • Option (a): This option shows the kinetic energy (KE) increasing linearly throughout the fall, which is incorrect because the raindrop reaches terminal velocity after falling through a height of ((3/4)h). At terminal velocity, KE should become constant, not continue to increase. Additionally, the potential energy (PE) should decrease continuously, but the graph does not show this correctly.

  • Option (c): This option shows the kinetic energy (KE) increasing and then decreasing, which is incorrect. Once the raindrop reaches terminal velocity, KE should remain constant, not decrease. The potential energy (PE) is shown as decreasing correctly, but the incorrect representation of KE makes this option invalid.

  • Option (d): This option shows the kinetic energy (KE) increasing and then decreasing, similar to option (c), which is incorrect. KE should remain constant after reaching terminal velocity. Additionally, the potential energy (PE) is shown as decreasing correctly, but the incorrect representation of KE makes this option invalid.

16. In a shotput event an athlete throws the shotput of mass 10kg with an initial speed of 1ms1 at 45 from a height 1.5m above ground. Assuming air resistance to be negligible and acceleration due to gravity to be 10ms2, the kinetic energy of the shotput when it just reaches the ground will be

(a) 2.5J

(b) 5.0J

(c) 52.5J

(d) 155.0J

Show Answer

Thinking Process

As air resistance is negligible, total mechanical energy of the system will remain constant.

Answer (d) Given, h=1.5m,v=1m/s,m=10kg,g=10ms2

From conservation of mechanical energy.

(PE)i+(KE)i=(PE)f+(KE)fmgh+12mv2=0+(KE)f(KE)f=mgh+12mv2(KE)f=10×10×1.5+12×10×(1)2=150+5=155J

Note We should be careful about the reference taken for PE, it may or may not be the ground.

  • Option (a) 2.5J: This value is too low because it only considers a small fraction of the total energy. The initial potential energy due to the height and the initial kinetic energy due to the speed are both significant and add up to much more than 2.5J.

  • Option (b) 5.0J: This value is also too low. It only accounts for the initial kinetic energy of the shotput (12mv2=5J) and ignores the potential energy due to the height from which the shotput is thrown.

  • Option (c) 52.5J: This value is incorrect because it underestimates the total energy. It might be considering only part of the potential energy or a miscalculation. The correct total energy should include both the potential energy from the height and the initial kinetic energy, which sums up to 155J.

17. Which of the diagrams in figure correctly shows the change in kinetic energy of an iron sphere falling freely in a lake having sufficient depth to impart it a terminal velocity?

(a)

(b)

(c)

(d)

Show Answer

Answer (b) First velocity of the iron sphere increases and after sometime becomes constant, called terminal velocity. Hence, accordingly first KE increases and then becomes constant which is best represented by (b).

  • Option (a): This diagram shows a linear increase in kinetic energy without any indication of it becoming constant. This does not accurately represent the scenario where the iron sphere reaches terminal velocity, at which point the kinetic energy should level off.

  • Option (c): This diagram shows a decrease in kinetic energy after an initial increase. This is incorrect because once the iron sphere reaches terminal velocity, its kinetic energy should remain constant, not decrease.

  • Option (d): This diagram shows a constant kinetic energy from the beginning, which is incorrect. Initially, the kinetic energy of the iron sphere should increase as it accelerates, and only after reaching terminal velocity should it become constant.

18. A cricket ball of mass 150g moving with a speed of 126km/h hits at the middle of the bat, held firmly at its position by the batsman. The ball moves straight back to the bowler after hitting the bat. Assuming that collision between ball and bat is completely elastic and the two remain in contact for 0.001s, the force that the batsman had to apply to hold the bat firmly at its place would be

(a) 10.5N

(b) 21N

(c) 1.05×104N

(d) 2.1×104N

Show Answer

Answer (c) Given, m=150g=1501000kg=320kg

Δt= time of contact =0.001su=126km/h=126×100060×60m/s=35m/sv=126km/h=35m/s

Change in momentum of the ball

Δp=m(vu)=320(3535)kgm/s=320(70)=212

We know that force F=ΔpΔt

=21/20.001N=1.05×104N

Here, - ve sign shown that force will be opposite to the direction of movement of the ball before hitting.

  • Option (a) 10.5N: This option is incorrect because it significantly underestimates the force required. The calculation of force involves a change in momentum divided by the time of contact. Given the mass and velocity of the ball, the resulting force is much higher than 10.5 N.

  • Option (b) 21N: This option is incorrect because it also underestimates the force required. The change in momentum for the given mass and velocity, when divided by the very short contact time of 0.001 seconds, results in a force that is much greater than 21 N.

  • Option (d) 2.1×104N: This option is incorrect because it overestimates the force required. The correct calculation of the force, based on the given mass, velocity, and contact time, results in a force of 1.05×104N, which is half of this value.

Multiple Choice Questions (More Than One Options)

19. A man of mass m, standing at the bottom of the staircase, of height L climbs it and stands at its top.

(a) Work done by all forces on man is equal to the rise in potential energy mgL

(b) Work done by all forces on man is zero

(c) Work done by the gravitational force on man is mgL

(d) The reaction force from a step does not do work because the point of application of the force does not move while the force exists

Show Answer

Answer (b,d)

When a man of mass m climbs up the staircase of height L, work done by the gravitational force on the man is-mgl work done by internal muscular forces will be mgL as the change in kinetic energy is almost zero.

Hence, total work done =mgL+mgL=0

As the point of application of the contact forces does not move hence work done by reaction forces will be zero.

Note Here work done by friction will also be zero as there is no dissipation or rubbing is involved.

  • (a) Work done by all forces on man is equal to the rise in potential energy mgL: This is incorrect because the work done by all forces on the man is zero. The work done by the gravitational force is (-mgL) and the work done by the internal muscular forces is (mgL), which cancel each other out, resulting in a net work of zero.

  • (c) Work done by the gravitational force on man is (mgL): This is incorrect because the work done by the gravitational force on the man is actually (-mgL). The gravitational force acts downward while the man moves upward, resulting in negative work done by gravity.

20. A bullet of mass m fired at 30 to the horizontal leaves the barrel of the gun with a velocity v. The bullet hits a soft target at a height h above the ground while it is moving downward and emerge out with half the kinetic energy it had before hitting the target.

Which of the following statements are correct in respect of bullet after it emerges out of the target?

(a) The velocity of the bullet will be reduced to half its initial value

(b) The velocity of the bullet will be more than half of its earlier velocity

(c) The bullet will continue to move along the same parabolic path

(d) The bullet will move in a different parabolic path

(e) The bullet will fall vertically downward after hitting the target

(f) The internal energy of the particles of the target will increase

Show Answer

Answer (b,d,f)

Consider the adjacent diagram for the given situation in the question.

(b) Conserving energy between " O " and " A "

Ui+Ki=Uf+Kf

0+12mv2=mgh+12mv

(v)22=v22=gh

(v)2=v22ghv=v22gh

where v is speed of the bullet just before hitting the target. Let speed after emerging from the target is v then,

 By question, =12(mv)2=12[12m(v)2]12m(v)2=14m(v)2=14m[v22gh](v)2=v22gh2=v22ghv=v22gh

From Eqs. (i) and (ii)

vv=v22ghv22gh2=2

v=v2=v2(v2)

vv=2=1.414>1

v2=v2

Hence, after emerging from the target velocity of the bullet (v) is more than half of its earlier velocity v (velocity before emerging into the target).

(d) As the velocity of the bullet changes to v which is less than v1 hence, path, followed will change and the bullet reaches at point B instead of A, as shown in the figure.

(f) As the bullet is passing through the target the loss in energy of the bullet is transferred to particles of the target. Therefore, their internal energy increases.

  • (a) The velocity of the bullet will be reduced to half its initial value: This statement is incorrect because the kinetic energy of the bullet is reduced to half after emerging from the target, not the velocity. Since kinetic energy is proportional to the square of the velocity, reducing the kinetic energy to half results in the velocity being reduced by a factor of (\sqrt{2}), not 2.

  • (c) The bullet will continue to move along the same parabolic path: This statement is incorrect because the velocity of the bullet changes after it emerges from the target. Since the velocity is different, the trajectory of the bullet will also change, resulting in a different parabolic path.

  • (e) The bullet will fall vertically downward after hitting the target: This statement is incorrect because the bullet still retains a horizontal component of velocity after emerging from the target. Therefore, it will continue to follow a parabolic trajectory rather than falling straight down vertically.

21. Two blocks M1 and M2 having equal mass are free to move on a horizontal frictionless surface. M2 is attached to a massless spring as shown in figure. Initially M2 is at rest and M1 is moving toward M2 with speed v and collides head-on with M2.

(a) While spring is fully compressed all the KE of M1 is stored as PE of spring

(b) While spring is fully compressed the system momentum is not conserved, though final momentum is equal to initial momentum

(c) If spring is massless, the final state of the M1 is state of rest

(d) If the surface on which blocks are moving has friction, then collision cannot be elastic

Show Answer

Answer (c) Consider the adjacent diagram when M1 comes in contact with the spring, M1 is retarded by the spring force and M2 is accelerated by the spring force.

(a) The spring will continue to compress until the two blocks acquire common velocity.

(b) As surfaces are frictionalless momentum of the system will be conserved. (c) If spring is massless whole energy of M1 will be imparted to M2 and M1 will be at rest, then

(d) Collision is inelastic, even if friction is not involved.

  • (a) While spring is fully compressed all the KE of M1 is stored as PE of spring: This statement is incorrect because, at the point of maximum compression, both blocks M1 and M2 will have the same velocity. Therefore, not all the kinetic energy of M1 is converted into the potential energy of the spring; some of it is also converted into the kinetic energy of M2.

  • (b) While spring is fully compressed the system momentum is not conserved, though final momentum is equal to initial momentum: This statement is incorrect because the momentum of the system is conserved at all times, including when the spring is fully compressed. The conservation of momentum is a fundamental principle that holds true regardless of the state of compression of the spring.

  • (d) If the surface on which blocks are moving has friction, then collision cannot be elastic: This statement is incorrect because the presence of friction does not necessarily make the collision inelastic. The collision can still be elastic if the kinetic energy is conserved during the collision. Friction would affect the motion of the blocks after the collision, but it does not inherently change the nature of the collision itself.

Very Short Answer Type Questions

22. A rough inclined plane is placed on a cart moving with a constant velocity u on horizontal ground. A block of mass M rests on the incline. Is any work done by force of friction between the block and incline? Is there then a dissipation of energy?

Show Answer

Answer Consider the adjacent diagram. As the block M is at rest. Hence, f= frictional force =Mgsinθ

The force of friction acting between the block and incline opposes the tendency of sliding of the block. Since, block is not in motion, therefore, no work is done by the force of friction. Hence, no dissipation of energy takes place.

23. Why is electrical power required at all when the elevator is descending? Why should there be a limit on the number of passengers in this case?

Show Answer

Answer When the elevator is descending, then electric power is required to prevent it from falling freely under gravity.

Also, as the weight inside the elevator increases, its speed of descending increases, therefore, there should be a limit on the number of passengers in the elevator to prevent the elevator from descending with large velocity.

24. A body is being raised to a height h from the surface of earth. What is the sign of work done by

(a) applied force and

(b) gravitational force?

Show Answer

Answer (a) Force is applied on the body to lift it in upward direction and displacement of the body is also in upward direction, therefore, angle between the applied force and displacement is θ=0

Work done by the applied force

W=Fscosθ=Fscos0=Fs(cos0=1)

 i.e., W= Positive 

(b) The gravitational force acts in downward direction and displacement in upward direction, therefore, angle between them is θ=180.

Work done by the gravitational force

W=Fscos180=Fs

(cos180=1)

25. Calculate the work done by a car against gravity in moving along a straight horizontal road. The mass of the car is 400kg and the distance moved is 2m.

Show Answer

Answer Force of gravity acts on the car vertically downward while car is moving along horizontal road, i.e., angle between them is 90.

Work done by the car against gravity

W=FScos90=0(cos90=0)

26. A body falls towards earth in air. Will its total mechanical energy be conserved during the fall? Justify.

Show Answer Answer No, total mechanical energy of the body falling freely under gravity is not conserved, because a small part of its energy is utilised against resistive force of air, which is non-conservative force. In this condition, gain in KE< loss in PE.

27. A body is moved along a closed loop. Is the work done in moving the body necessarily zero? If not, state the condition under which work done over a closed path is always zero.

Show Answer Answer No, work done in moving along a closed loop is not necessarily zero. It is zero only when all the forces are conservative forces.

28. In an elastic collision of two billiard balls, which of the following quantities remain conserved during the short time of collision of the balls (i.e., when they are in contact)?

(a) Kinetic energy.

(b) Total linear momentum.

Give reason for your answer in each case.

Show Answer Answer Total linear momentum of the system of two balls is always conserved. While balls are in contact, there may be deformation which means elastic PE which came from part of KE Therefore, KE may not be conserved.

29. Calculate the power of a crane in watts, which lifts a mass of 100kg to a height of 10m in 20s.

Show Answer

Answer Given,

 mass =m=100kg height =h=10m time duration t=20s power = Rate of work done = change of PE  time =mght=100×9.8×1020=5×98=490W

30. The average work done by a human heart while it beats once is 0.5J. Calculate the power used by heart if it beats 72 times in a minute.

Show Answer

Answer Given, average work done by a human heart per beat =0.5J

Total work done during 72 beats

=72×0.5J=36J Power = Work done  Time =36J60s=0.6W

31. Give example of a situation in which an applied force does not result in a change in kinetic energy.

Show Answer

Answer When a charged particle moves in a uniform normal magnetic field, the path of the particle is circular, as given field is uniform hence, radius of the circular path is also constant.

As the force is central and movement is tangential work done by the force is zero. As speed is also constant we can say that ΔK=0.

32. Two bodies of unequal mass are moving in the same direction with equal kinetic energy. The two bodies are brought to rest by applying retarding force of same magnitude. How would the distance moved by them before coming to rest compare?

Show Answer

Answer According to work-energy theorem,

Change in KE= Work done by the retarding force

KE of the body = Retarding force × Displacement

As KE of the bodies and retarding forces applied on them are same, therefore, both bodies will travel equal distances before coming to rest.

33. A bob of mass m suspended by a light string of length L is whirled into a vertical circle as shown in figure. What will be the trajectory of the particle, if the string is cut at

(a) point B ?

(b) point C ?

(c) point X ?

Show Answer

Thinking Process

In a uniform circular motion, velocity is always tangential in the direction of motion at any point.

Answer When bob is whirled into a vertical circle, the required centripetal force is obtained from the tension in the string. When string is cut, tension in string becomes zero and centripetal force is not provided, hence, bob start to move in a straight line path along the direction of its velocity

(a) At point B, the velocity of B is vertically downward, therefore, when string is cut at B, bob moves vertically downward.

(b) At point C, the velocity is along the horizontal towards right, therefore, when string is cut at C, bob moves horizontally towards right.

Also, the bob moves under gravity simultaneously with horizontal uniform speed. So, it traversed on a parabolic path with vertex at C.

(c) At point X, the velocity of the bob is along the tangent drawn at point X, therefore when string is cut at point C, bob moves along the tangent at that point X.

Also, the bob move under gravity simultaneously with horizontal uniform speed. So, it traversed on a parabolic path with vertex higher than C.

Short Answer Type Questions

34. A graph of potential energy V(x) versus x is shown in figure. A particle of energy E0 is executing motion in it. Draw graph of velocity and kinetic energy versus x for one complete cycle AFA.

Show Answer

Thinking Process

We will assume total mechanical energy of the system to be constant.

Answer KE versus x graph

We know that Total ME = KE + PE

Eo=KE+V(x)

KE=EoV(x)

at A1x=0,V(x)=E0

KE = EoEo

at B1V(x)<E0

KE > 0

at C and D1V(x)=0

KE is maximum at F1V(x)=E0

Hence, KE=0

The variation is shown in adjacent diagram.

Velocity versus x graph

As

KE=12mv2

At A and F, where KE=0,v=0.

At C and D,KE is maximum. Therefore, v is ± max.

KE>0 (positive)

At B,KE is positive but not maximum.

Therefore,

v is ± some value

(<max.

The variation is shown in the diagram.

35. A ball of mass m, moving with a speed 2v0, collides inelastically (e>0) with an identical ball at rest. Show that

(a) For head-on collision, both the balls move forward.

(b) For a general collision, the angle between the two velocities of scattered balls is less than 90.

Show Answer

Answer (a) Let v1 and v2 are velocities of the two balls after collision.

Now, by the principle of conservation of linear momentum,

or

2mv0=mv1+mv2

2v0=v1+v2

e=v2v12v0

 and e=v2v12v0

v2=v1+2v0e

2v1=2v02ev0

v1=v0(1e)

Since, e<1v1 has the same sign as v0, therefore, the ball moves on after collision.

(b) Consider the diagram below for a general collision.

By principle of conservation of linear momentum,

P=P1+P2

For inelastic collision some KE is lost, hence p22m>p122m+p222m

p2>p12+p22

Thus, p,p1 and p2 are related as shown in the figure.

θ is acute (less than .90)(p2=p12+p22. would given .θ=90)

36. Consider a one-dimensional motion of a particle with total energy E. There are four regions A,B,C and D in which the relation between potential energy V, kinetic energy (K) and total energy E is as given below

Region A: V>E

Region B : V<E

Region C:K<E

Region D : V>E

State with reason in each case whether a particle can be found in the given region or not.

Show Answer

Thinking Process

A particle cannot be found in the given region when KE<0.

Answer We know that

 Total energy E=PE+KE

E=V+K

For region A Given, V>E, From Eq. (i)

K=EV

as V>EEV<0

Hence, K<0, this is not possible.

For region B Given, V<EEV>0

This is possible because total energy can be greater than PE (V).

For region C Given, K>EKE>0

from Eq. (i) PE=V=EK<0

Which is possible, because PE can be negative.

For region D Given, V>K

This is possible because for a system PE (V) may be greater than KE(K).

37. The bob A of a pendulum released from horizontal to the vertical hits another bob B of the same mass at rest on a table as shown in figure.

If the length of the pendulum is 1m, calculate

(a) the height to which bob A will rise after collision.

(b) the speed with which bob B starts moving.

Neglect the size of the bobs and assume the collision to be elastic.

Show Answer

Thinking Process

When two bodies of equal masses collides elastically momentum is interchanged. At the bottom point bob A is having almost horizontal velocity.

Answer When ball A reaches bottom point its velocity is horizontal, hence, we can apply conservation of linear momentum in the horizontal direction. (a) Two balls have same mass and the collision between them is elastic, therefore, ball A transfers its entire linear momentum to ball B. Hence, ball A will come to at rest after collision and does not rise at all.

(b) Speed with which bob B starts moving

= Speed with which bob A hits bob B=2gh=2×9.8×1=19.6=4.42m/s

Note When the bob A is at the bottommost point, its velocity is horizontal and tension is the external force on the bob but still momentum can be considered to be conserved in horizontal direction, because the tension has no effect in horizontal direction at the bottommost point.

38. A raindrop of mass 1.00g falling from a height of 1km hits the ground with a speed of 50ms1. Calculate

(a) the loss of PE of the drop.

(b) the gain in KE of the drop.

(c) Is the gain in KE equal to loss of PE? If not why?

Take, g=10ms2.

Show Answer

Answer Given, mass of the rain drop (m)=1.00g

=1×103kg

Height of falling (h)=1km=103m

g=10m/s2

Speed of the rain drop(v)=50m/s

(a) Loss of PE of the drop =mgh

=1×103×10×103=10J

(b) Gain in KE of the drop =12mv2

=12×1×103×(50)2=12×103×2500=1.250J

(c) No, gain in KE is not equal to the loss in its PE, because a part of PE is utilised in doing work against the viscous drag of air.

39. Two pendulums with identical bobs and lengths are suspended from a common support such that in rest position the two bobs are in contact (figure). One of the bobs is released after being displaced by 10 so that it collides elastically head-on with the other bob.

(a) Describe the motion of two bobs.

(b) Draw a graph showing variation in energy of either pendulum with time, for 0t2T. where T is the period of each pendulum.

Show Answer

Thinking Process

As collision is elastic, mechanical energy of the system is conserved. We have to apply energy conservation principle to describe the motion of the two bobs.

Answer (a) Consider the adjacent diagram in which the bob B is displaced through an angle θ and released.

At t=0, suppose bob B is displaced by θ=10 to the right. It is given potential energy E1=E. Energy of A,E2=0.

When B is released, it strikes A at t=T/4. In the head-on elastic collision between B and A comes to rest and A gets velocity of B. Therefore, E1=0 and E2=E. At t=2T/4,B reaches its extreme right position when KE of A is converted into PE=E2=E. Energy of B,E1=0.

At t=3T/4,A reaches its mean position, when its PE is converted into KE=E2=E. It collides elastically with B and transfers whole of its energy to B. Thus, E2=0 and E1=E. The entire process is repeated.

(b) The values of energies of B and A at different time intervals are tabulated here. The plot of energy with time 0t2T is shown separately for B and A in the figure below.

Time (t) Energy of A
(E1)
Energy of B
(E2)
0 E 0
T/4 0 E
2T/4 0 E
3T/4 E 0
4T/4 E 0
5T/4 0 E
6T/4 0 E
7T/4 E 0
8T/4 E 0

40. Suppose the average mass of raindrops is 3.0×105kg and their average terminal velocity 9ms1. Calculate the energy transferred by rain to each square metre of the surface at a place which receives 100cm of rain in a year.

Show Answer

Answer Given, average mass of rain drop

(m)=3.0×105kg

Average terminal velocity =(V)=9m/s.

 Height (h)=100cm=1m

 Density of water (ρ)=103kg/m3

Area of the surface (A)=1m2

Volume of the water due to rain (V)= Area × height

=A×h=1×1=1m3

Mass of the water due to rain (M)= Volume × density

=V×ρ=1×103=103kg

Energy transferred to the surface =12mv2

=12×103×(9)2=40.5×103J=4.05×104J

41. An engine is attached to a wagon through a shock absorber of length 1.5m. The system with a total mass of 50,000kg is moving with a speed of 36kmh1 when the brakes are applied to bring it to rest. In the process of the system being brought to rest, the spring of the shock absorber gets compressed by 1.0m. If 90 of energy of the wagon is lost due to friction, calculate the spring constant.

Show Answer

Answer Given, mass of the system (m)=50,000kg

Speed of the system (v)=36km/h

=36×100060×60=10m/s

Compression of the spring (x)=1.0m

KE of the system =12mv2=12×50000×(10)2=25000×100J=2.5×106J

Since, 90 of KE of the system is lost due to friction, therefore, energy transferred to shock absorber, is given by

ΔE=12kx2=10=10100×2.5×106J or k=2×2.5×10610×(1)2=5.0×105N/m

42. An adult weighting 600N raises the centre of gravity of his body by 0.25m while taking each step of 1m length in jogging. If he jogs for 6km, calculate the energy utilised by him in jogging assuming that there is no energy loss due to friction of ground and air. Assuming that the body of the adult is capable of converting 10 of energy intake in the form of food, calculate the energy equivalents of food that would be required to compensate energy utilised for jogging.

Show Answer

Thinking Process

Here, shift in centre of gravity of his body is equal to the height of each step.

Answer Given, weight of the adult (w)=mg=600N

Height of each step =h=0.25m

Length of each step =1m

Total distance travelled =6km=6000m

Total number of steps =60001=6000

Total energy utilised in jogging =n×mgh

=6000×600×0.25J=9×105J

Since, 10 of intake energy is utilised in jogging.

Total intake energy =10×9×105J=9×106J.

43. On complete combustion a litre of petrol gives off heat equivalent to 3×107J. In a test drive, a car weighing 1200kg including the mass of driver, runs 15km per litre while moving with a uniform speed on a straight track. Assuming that friction offered by the road surface and air to be uniform, calculate the force of friction acting on the car during the test drive, if the efficiency of the car engine were 0.5 .

Show Answer

Answer Energy is given by the petrol in the form of heat of combustion.

Thus, by question,

Energy given by 1 litre of petrol =3×107J

Efficiency of the car engine =0.5

Energy used by the car =0.5×3×107J

E=1.5×107J

Total distance travelled (s)=15km=15×103m

If f is the force of friction then,

E=f×s( Energy is utilised in working against friction )1.5×107=f×15×103f=1.5×10715×103=103Nf=1000N

Long Answer Type Questions

44. A block of mass 1kg is pushed up a surface inclined to horizontal at an angle of 30 by a force of 10N parallel to the inclined surface (figure). The coefficient of friction between block and the incline is 0.1 . If the block is pushed up by 10m along the incline, calculate

(a) work done against gravity

(b) work done against force of friction

(c) increases in potential energy

(d) increase in kinetic energy

(e) work done by applied force

Show Answer

Answer Consider the adjacent diagram the block is pushed up by applying a force F.

Normal reaction (N) and frictional force (f) is shown.

Given, mass =m=1kg,θ=30

F=10N,μ=0.1 and s= distance moved by the block along the inclined plane =10m

(a) Work done against gravity = Increase in PE of the block

=mg× Vertical distance travelled =mg×s(sinθ)=(mgs)sinθ=1×10×10×sin30=50J

(b) Work done against friction

wf=f×s=μN×s=μmgcosθ×s=0.1×1×10×cos30×10=10×0.866=8.66J

(c) Increase in PE=mgh=mg(ssinθ)

=1×10×10×sin30=100×12=50J

(d) By work-energy theorem, we know that work done by all the forces = change in KE

(W)=ΔKΔk=Wg+Wf+Wf=mghfs+FS=508.66+10×10=508.66=41.34J

(e) Work done by applied force, F=FS

=(10)(10)=100J

45. A curved surface is shown in figure. The portion BCD is free of friction. There are three spherical balls of identical radii and masses. Balls are released from rest one by one from A which is at a slightly greater height than C.

With the surface AB, ball 1 has large enough friction to cause rolling down without slipping; ball 2 has a small friction and ball 3 has a negligible friction.

(a) For which balls is total mechanical energy conserved?

(b) Which ball (s) can reach D ?

(c) For balls which do not reach D, which of the balls can reach back A ?

Show Answer

Answer (a) As ball 1 is rolling down without slipping there is no dissipation of energy hence, total mechanical energy is conserved.

Ball 3 is having negligible friction hence, there is no loss of energy.

(b) Ball 1 acquires rotational energy, ball 2 loses energy by friction. They cannot cross at C. Ball 3 can cross over.

(c) Ball 1,2 turn back before reaching C. Because of loss of energy, ball 2 cannot reach back to A. Ball 1 has a rotational motion in “wrong” sense when it reaches B. It cannot roll back to A, because of kinetic friction.

46. A rocket accelerates straight up by ejecting gas downwards. In a small time interval Δt, it ejects a gas of mass Δm at a relative speed u. Calculate KE of the entire system at t+Δt and t and show that the device that ejects gas does work =(1/2)Δmu2 in this time interval (negative gravity).

Show Answer

Thinking Process

As the gas is ejected, the rocket gets propelled in forward direction due to upward thrust.

Answer Let M be the mass of rocket at any time t and v1 the velocity of rocket at the same time t. Let Δm= mass of gas ejected in time interval Δt.

Relative speed of gas ejected =u.

Consider at time t+Δt

(KE)t+Δt=KE of rocket +KE of gas 

=12(MΔm)(v+Δv)2+12Δm(vu)2

=12Mv2+MvΔvΔmvu+12Δmu2

(KE)t=KE of the rocket at time t=12Mv2

ΔK=(KE)t+Δt(KE)t

=(MΔvΔmu)v+12Δmu2

Since, action-reaction forces are equal.

Hence,

Mdvdt=dmdt|u|MΔv=ΔmuΔK=12Δmu2

Now, by work-energy theorem,

ΔK=ΔWΔW=12Δmu2

47. Two identical steel cubes (masses 50g, side 1cm ) collide head-on face to face with a space of 10cm/s each. Find the maximum compression of each. Young’s modulus for steel =Y=2×1011N/m2.

Show Answer

Answer Let

m=50g=50×103kg Side =L=1cm=0.01m Speed =V=10cm/s=0.1m/s Young’s modulus =Y=2×1011N/m2 Maximum compression ΔL=?

In this case, all KE will be converted to PE By Hooke’s law,

FA=YΔLL

where A is the surface area and L is length of the side of the cube. If k is spring or compression constant, then

 force F=KΔLK=YAL=YL Initial KE=2×12mv2=5×104J Final PE=2×12k(ΔL)2ΔL=KEK=KEYL=5×1042×1011×0.1=1.58×107m[PE=KE]

48. A balloon filled with helium rises against gravity increasing its potential energy. The speed of the baloon also increases as it rises. How do you reconcile this with the law of conservation of mechanical energy? You can neglect viscous drag of air and assume that density of air is constant.

Show Answer

Thinking Process

In this problem, as viscous drag of air is neglected, hence there is no dissipation of energy.

Answer Let m= Mass of balloon

V= Volume of balloon ρHe = Density of helium ρair = Density of air 

Volume V of balloon displaces volume V of air.

So,

V(ρair ρHe)g=ma=mdvdt= up thrust 

Integrating with respect to t,

V(ρair ρHe)gt=mv12mv2=12mV2m2(ρair ρHe)2g2t=12mV2(ρair ρHe)2g2t2

12mv2=12mV2m2(ρair ρHe)2g2t2

If the balloon rises to a height h, from s=ut+12at2,

 We get h=12at2=12V(ρair ρHe)mgt2

From Eqs. (iii) and (ii),

Rearranging the terms,

12mv2=[V(ρaρHe)g][12mV(ρair ρHe)gt2]=V(ρaρHe)gh

12mv2+VρHegh=Vρair hg

KEballoon +PEballoon = Change in PE of air.

So, as the balloon goes up, an equal volume of air comes down, increase in PE and KE of the balloon is at the cost of PE of air [which comes down].