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
Show Answer
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
(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
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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
- (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
(b) constant and greater than
(c) variable but always greater than
(d) at first greater than
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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.
When the man gets straight up in that case friction
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(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.
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(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.
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(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
(a) +2000 J
(b) -200 J
(c) zero
(d)
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
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.
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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.
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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
. The correct value should be , not . -
Option (d)
J: This option is incorrect because it overestimates the work done by the frictional force. The correct calculation is , so the work done should be , not .
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 (
-
Kinetic energy: The kinetic energy of the body increases as it falls because its velocity increases due to the acceleration caused by gravity.
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Potential energy: The potential energy of the body decreases as it falls because its height above the ground decreases.
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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.
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(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.
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(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.
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(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
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
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Answer
(c) As the given tracks are frictionless, hence, mechanical energy will be conserved. As both the tracks having common height,
From conservation of mechanical energy,
Hence, speed is same for both stones. For stone
Similarly, for stone
And both length for track II is also less hence, stone II reaches earlier than stone I.
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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).
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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.
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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

(a)
(b)
(c)
(d)
Show Answer
Answer
(b) Total energy is
When particle is at
From Eq. (i)
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Option (a)
: This option is incorrect because at the extreme position , the potential energy is at its maximum value, not zero. The kinetic energy is zero at this point because the particle momentarily stops before reversing direction. -
Option (c)
: This option is incorrect because at the extreme position , the potential energy is equal to the total energy , not less than . The kinetic energy is indeed zero, but the potential energy should be equal to the total energy. -
Option (d)
: This option is incorrect because at the extreme position , the potential energy is not zero; it is equal to the total energy . The kinetic energy 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

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

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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,
when ball 2 collides will ball
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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.
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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.
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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
(a)
(b)
(c)
(d)
Show Answer
Answer
Given,
We know that
Acceleration
Now,
Work done
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Option (a)
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 , not . The mathematical steps and integration clearly show that the work done is significantly higher than . -
Option (c)
is incorrect because, similar to option (a), the integration of the force over the displacement from to results in . The value of does not match the derived result from the given equations and constants. -
Option (d)
is incorrect because the work done, as calculated through the integration process, is . The value of 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?

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Answer (b) Given, power = constant
We know that power
Hence,
Now, writing dimensions
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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.
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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} ).
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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?

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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).
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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.
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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.
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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?

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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).
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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.
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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.
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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
(a)
(b)
(c)
(d) 0
Show Answer
Answer
(a) Given, mass
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Option (b)
: This option is incorrect because it underestimates the kinetic energy. The correct calculation shows that the kinetic energy is J, not J. The error likely comes from an incorrect application of the kinetic energy formula or a miscalculation of the linear speed. -
Option (c)
: This option is incorrect because it significantly underestimates the kinetic energy. The correct kinetic energy is 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
J, not zero.
15. A raindrop falling from a height

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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
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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.
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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.
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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
(a)
(b)
(c)
(d)
Show Answer
Thinking Process
As air resistance is negligible, total mechanical energy of the system will remain constant.
Answer
(d) Given,
From conservation of mechanical energy.
Note We should be careful about the reference taken for
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Option (a)
: 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 . -
Option (b)
: This value is also too low. It only accounts for the initial kinetic energy of the shotput ( ) and ignores the potential energy due to the height from which the shotput is thrown. -
Option (c)
: 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 .
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).
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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.
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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.
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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
(a)
(b)
(c)
(d)
Show Answer
Answer
(c) Given,
Change in momentum of the ball
We know that force
Here, - ve sign shown that force will be opposite to the direction of movement of the ball before hitting.
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Option (a)
: 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)
: 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)
: 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 , which is half of this value.
Multiple Choice Questions (More Than One Options)
19. A man of mass
(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
(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
When a man of mass
Hence, total work done
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.
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(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.
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(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
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
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Answer
Consider the adjacent diagram for the given situation in the question.

(b) Conserving energy between "
where
From Eqs. (i) and (ii)
Hence, after emerging from the target velocity of the bullet
(d) As the velocity of the bullet changes to
(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.
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(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.
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(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.
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(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
(a) While spring is fully compressed all the KE of
(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
(d) If the surface on which blocks are moving has friction, then collision cannot be elastic

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Answer
(c) Consider the adjacent diagram when
(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
(d) Collision is inelastic, even if friction is not involved.

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

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
(a) applied force and
(b) gravitational force?
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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
(b) The gravitational force acts in downward direction and displacement in upward direction, therefore, angle between them is
25. Calculate the work done by a car against gravity in moving along a straight horizontal road. The mass of the car is
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Answer
Force of gravity acts on the car vertically downward while car is moving along horizontal road, i.e., angle between them is
Work done by the car against gravity
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 in27. 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
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Answer Given,
30. The average work done by a human heart while it beats once is
Show Answer
Answer
Given, average work done by a human heart per beat
Total work done during 72 beats
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

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
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
(a) point
(b) point
(c) point

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) At point
Also, the bob moves under gravity simultaneously with horizontal uniform speed. So, it traversed on a parabolic path with vertex at
(c) At point
Also, the bob move under gravity simultaneously with horizontal uniform speed. So, it traversed on a parabolic path with vertex higher than

Short Answer Type Questions
34. A graph of potential energy

Show Answer
Thinking Process
We will assume total mechanical energy of the system to be constant.
Answer
KE versus
We know that Total ME = KE + PE
at
at
at
Hence,
The variation is shown in adjacent diagram.
Velocity versus
As
At

At
Therefore,
The variation is shown in the diagram.

35. A ball of mass
(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
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Answer
(a) Let
Now, by the principle of conservation of linear momentum,
or
Since,
(b) Consider the diagram below for a general collision.

By principle of conservation of linear momentum,
For inelastic collision some KE is lost, hence
Thus,
36. Consider a one-dimensional motion of a particle with total energy
Region A:
Region B :
Region
Region D :
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
Answer We know that
For region
as
Hence,
For region
This is possible because total energy can be greater than PE (V).
For region
from Eq. (i)
Which is possible, because
For region
This is possible because for a system PE
37. The bob

If the length of the pendulum is
(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.
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Thinking Process
When two bodies of equal masses collides elastically momentum is interchanged. At the bottom point bob
Answer
When ball

(b) Speed with which bob B starts moving
Note When the bob
38. A raindrop of mass
(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,
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Answer
Given, mass of the rain drop
Height of falling
Speed of the rain
(a) Loss of PE of the drop
(b) Gain in KE of the drop
(c) No, gain in
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

(a) Describe the motion of two bobs.
(b) Draw a graph showing variation in energy of either pendulum with time, for
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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
At
When
At

(b) The values of energies of
Time |
Energy of |
Energy of |
---|---|---|
0 | 0 | |
0 | ||
0 | ||
0 | ||
0 | ||
0 | ||
0 | ||
0 | ||
0 |

40. Suppose the average mass of raindrops is
Show Answer
Answer Given, average mass of rain drop
Average terminal velocity
Area of the surface
Volume of the water due to rain
Mass of the water due to rain
41. An engine is attached to a wagon through a shock absorber of length
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Answer
Given, mass of the system
Speed of the system
Compression of the spring
Since,
42. An adult weighting
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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
Height of each step
Length of each step
Total distance travelled
Total energy utilised in jogging
Since,
43. On complete combustion a litre of petrol gives off heat equivalent to
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Answer Energy is given by the petrol in the form of heat of combustion.
Thus, by question,
Energy given by 1 litre of petrol
Efficiency of the car engine
Total distance travelled
If
Long Answer Type Questions
44. A block of mass

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

Normal reaction
Given, mass
(a) Work done against gravity = Increase in PE of the block
(b) Work done against friction
(c) Increase in
(d) By work-energy theorem, we know that work done by all the forces = change in KE
(e) Work done by applied force,
45. A curved surface is shown in figure. The portion

With the surface
(a) For which balls is total mechanical energy conserved?
(b) Which ball (s) can reach
(c) For balls which do not reach
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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 1,2 turn back before reaching
46. A rocket accelerates straight up by ejecting gas downwards. In a small time interval
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Thinking Process
As the gas is ejected, the rocket gets propelled in forward direction due to upward thrust.
Answer
Let
Relative speed of gas ejected
Consider at time
Since, action-reaction forces are equal.
Hence,
Now, by work-energy theorem,
47. Two identical steel cubes (masses
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Answer Let
In this case, all
where
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.
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Thinking Process
In this problem, as viscous drag of air is neglected, hence there is no dissipation of energy.
Answer
Let
Volume
So,
Integrating with respect to
If the balloon rises to a height
From Eqs. (iii) and (ii),
Rearranging the terms,
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].