# Work, Energy and Power - Short Question Solution | Physics Grade XI

### Short Question Solution

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#### Kinematics

Introduction to Kinematics

#### Viva Voice Questions # Work, Energy and Power - Short Question Answers

1. A light body and a heavy body have the same momentum. Which one will have greater momentum? Explain.

We know that momentum and K.E. of a body are related as,

K.E. = ½ mv2 = ½ m2v2/m = p2/2m

If a small body and a heavy body have same momentum, the K.E. of the lighter body will be more than the heavier body, Since K.E α 1/m for same momentum.

2. A body is moving with the same velocity v along a circular path of radius r. How much work is done by the centripetal force in revolving the body?

No work is done by the centripetal force because the force and displacement of the body at each point are perpendicular to each other.

Since, W = F. s cosθ,θ= 90, cosθ = 0, W = F. s. 0 = 0

3. Distinguish between conservative and non-conservative forces.

A force is said to be conservative if the work done by or against the force in moving body depends upon only the initial and final positions of the body i.e. the distance between those bodies. If the work done by the body while bringing it into a round circle at same point, then the force applied on it is called conservative force.

A force is said to be non-conservative if work done by or against the force on moving a body from one position to another depends upon the path followed by the body.

4. What are elastic and inelastic collisions? Give examples of each.

Elastic collision: Elastic collision is the mutual interaction between two bodies where their momentum and kinetic energy is conserved. It occurs when conservative force is applied to a body. For example: A collision between any two elementary particles is elastic collision.

Inelastic collision: The collision in which the momentum is conserved but kinetics energy is not conserved is called Inelastic collision.

All the collisions in the physical world are inelastic collisions.

5. In a siphon, water is lifted above its original level during its flow from one container to another. Where does it get the needed potential energy from?

In a siphon, water is lifted above its original level during its flow from one container to another container as shown in the figure. The potential energy needed in this case arises from the air pressure differences between the two containers.

6. How does the K.E. of an object change if its momentum is doubled?

Suppose a body of mass ‘m’ is moving with velocity ‘u’. Then its initial momentum is mu.

Initial kinetic energy = ½ mu2 = ½ m2 u2/ m = p2/2m

When momentum is doubled, p -> 2p,

KE’ = (2p)2/2m

= 4 (p2/2m)

= 4. Initial Kinetic energy

It means when momentum is doubled, Kinetic energy increases by 4 times.

Practice Questions

• The moon is accelerating toward earth. Why isn’t it getting closer to us?
• A man carrying a bucket of water is walking on a level road with a uniform velocity. Does he any work on the bucket while carrying it?
• A stationary mass suddenly explodes into two fragments; one heavy and another light. Which one has greater kinetic energy and why?

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