# NCERT Physics Class 11 Exemplar Ch 6 Work Energy and Power Part 5

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## VSA

Q 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?

Ans: Yes, No

Q 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?

Ans: To prevent elevator from falling freely under gravity.

Q 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

(b) Gravitational force?

Ans: (a) Positive, (b) Negative 6.25

Work done against gravity in moving along horizontal road is zero.

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

Ans: No, because resistive force of air also acts on the body which is a non-conservative force. So the gain in KE would be smaller than the loss in PE.

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

Ans: No, because resistive force of air also acts on the body which is a non-conservative force. So the gain in KE would be smaller than the loss in PE.

Q 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.

Ans:

No, work done over each closed path is necessarily zero only if all the forces acting on the system are conservative.

Q 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.

Ans:

(b) Total linear momentum.

While balls are in contact, there may be deformation which means elastic potential energy which came from part of KE. Momentum is always conserved.

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

Ans:

Power:

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

Ans:

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

Ans:

A charged particle moving in a uniform magnetic field.

Q 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?

Ans:

Work done = change in KE both bodies had same KE and hence same amount of work is needed to be done. Since force applied is same, they would come to rest within the same distance.

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

(a) Point B?

(b) Point C?

(c) Point X?

Ans:

(a) Straight line: vertical, downward

(b) Parabolic path with vertex at C.

(c) Parabolic path with vertex higher than C.

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