Work and Energy
Multiple Choice Questions
1. When a body falls freely towards the earth, then its total energy
(a) increases
(b) decreases
(c) remains constant
(d) first increases and then decreases
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Answer
Answer is (c) remains constant
Explanation :
Body falling freely towards the earth possess same kinetic and potential energy and it follows Law of conservation of energy hence its total energy remains constant.
(a) does not change
(b) becomes twice to that of initial
(c) becomes 4 times that of initial
(d) becomes 16 times that of initial
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Answer
Answer is (a) does not change
Explanation:
Potential energy is the product of height, mass and gravity. Hence height is a factor in determining potential energy. Here the height does not change hence potential energy of the car does not change.
(a)
(b)
(c)
(d)
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Answer
Answer is (d )
Explanation:
workdone
Hence the answer is
(a) acceleration
(b) momenta
(c) potential energy
(d) kinetic energy
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Answer
Momentum, potential energy and kinetic energy varies with weight. But in this case acceleration is because of acceleration due to gravity which is independent of mass hence acceleration is the right answer.
(a)
(b)
(c)
(d) zero
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Answer
Here Direction of workdone is perpendicular to gravitational force direction.Hence the workdone against the gravity is zero.
(a) joule
(b) newton metre
(c) kilowatt
(d) kilowatt hour
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Answer
Answer is (c) kilowatt
Explanation:
Kilowatt is SI Unit of electrical power
(a) displacement
(b) force applied
(c) angle between force and displacement
(d) initial velocity of the object
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Answer
Answer is (d) initial velocity of the object
Explanation:
Workdone is the product of force and displacement hence force and displacement are the required attributes for the work to be done. But workdone is not dependent of initial velocity.
(a) no energy
(b) electrical energy
(c) kinetic energy
(d) potential energy
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Answer
Answer is (d) potential energy
Explanation:
Potential energy is the energy stored in an object because of the position. Water in the dam possess energy because of position hence it is potential energy.
(a) only potential energy
(b) only kinetic energy
(c) half potential and half kinetic energy
(d) more kinetic and less potential energy
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Answer
Answer is (c) half potential and half kinetic energy
Explanation:
When body is at height h, its potential will be hundred percent and kinetic energy will be zero. In the same way when body reached ground its potential energy will be zero and kinetic energy will be maximum. At height h/2 both potential energy and kinetic energy of the body will be half.
Short Answer Questions
10. A rocket is moving up with a velocity
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Answer
Initial velocity
Initial kinetic energy
Kinetic energy
Ratio between Initial kinetic energy and final kinetic energy is 1:9
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Hence Avinash is more powerful than Kapil
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Total distance travelled by the boy
Total distance travelled by the boy
Frictional force
Workdone
Workdone
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Momentum is the product of mass and velocity. If the body is at rest its velocity will be zero. If it is reast at height it possess gravitational potential energy hence mechanical energy.
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No, Because its potential and kinetic energy are zero.
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Power of pump
Time
Height
Power =work done per unit time.
Work done
Therefore,
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Person on Planet A can jump a height of
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Answer
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Force always acts in perpendicular to the displacement direction. If the object is moving in circular path then no work will be done despite of action of force.
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when
Hence
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Power of electric iron
Usage per day
Number of days in the month of April=30days
Electrical energy consumed, E=Pxt 1200x0.5x30
Therefore, The Total Electricity consumed in april month is 18 units
Long Answer Questions
21. A light and a heavy object have the same momentum. Find out the ratio of their kinetic energies. Which one has a larger kinetic energy?
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Answer
Kinetic energy equation:
Momentum equation:
Kinetic energy equation in terms of momentum:
Kinetic energy of two objects will be same if their mass and momentum are same. Among light and heavy bodies Heavy body has highest momentum. When the momentum is equal light body has more kinetic energy.
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Frictional force
car A moves with a uniform speed, which means engine of car applies a force equal to the frictional force
Power
After collision
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(a)
Initial velocity of the trolley,
Final velocity of the trolleyv
Mass of the trolleym
Distance covered by the trolley before coming to rest,
From Equation 2 as
Force (frictional) acting on the trolley
(b) Since the girl does not move w.r.t. the trolley (as she is sitting on it),work done by the girl
(a) How much work is done by the men in lifting the box?
(b) How much work do they do in just holding it?
(c) Why do they get tired while holding it? (
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Answer
(a)
(b)
While holding box there is no displacement hence workdone is zero
(c) While holding the box they apply muscular force which is equal and opposite to the gravitational force hence they feel tired while holding box.
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Power is the rate of transfer of energy or rate of doing work. Watt is the unit of power and kilowatt is 1000 watts.
Power
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Power
work=force
force
acceleration=velocity/time
Therefore
Power
Here
Power,
velocity,
since time,
displacement,
acceleration,
From equation
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Power of an object which does work at 1 Joules/sec is called watt
1 watt
1 kilowatt= 1000 watts=1000J/sec
mass of car
power for each
speed
power=Force
(i) a butterfly of mass
(ii) a
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Answer
i) Butterfly force
ii) squirrel force
power=25×.5
power