NCERT Class-9 Science Solutions: Chapter 11-Work And Energy Part 3

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Question 13:

Can any object have mechanical energy even if its momentum is zero? Explain.

Answer:

Yes, mechanical energy comprises both potential energy and kinetic energy. Momentum is zero which means velocity is zero. Hence, there is no kinetic energy but the object may possess potential energy.

Question 14:

Can any object have momentum even if its mechanical energy is zero? Explain.

Answer:

If a body has momentum it does mean that it is moving. So it possesses kinetic energy.

So if mechanical energy of a body is zero for a body whose momentum is not zero it does mean that

Mechanical energy

If for any moving body then an object can have momentum even if its mechanical energy is zero.

Question 15:

The power of a motor pump is . How much water per minute the pump can raise to a height of ?

Answer:

Given, power of pump

Time

Height

We know,

power work done per unit time.

Work done

Therefore,

Therefore, the pump can raise 1200kg of water in one minute.

Question 16:

The weight of a person on a planet A is about half that on the earth. He can jump up to height on the surface of the earth. How high he can jump on the planet ?

Answer:

Since, weight of the person on planet A is half that on the earth, acceleration due to gravity there, will be that on the earth. Hence he can jump double the height with the same muscular force.

The potential energy of the person will remain the same on the earth and on planet A.

Thus,

Question 17:

The velocity of a body moving in a straight line is increased by applying a constant , for some distance in the direction of the motion. Prove that the increase in the kinetic energy of the body is equal to the work done by the force on the body.

Answer:

This gives,

We can write work done (W) by this force F ,

Question 18:

Is it possible that an object is in the state of accelerated motion due to external force acting on it, but no work is being done by the force? Explain it with an example.

Answer:

Yes, it is possible, if an object is moving in a circular path. Because force is always acting perpendicular to the direction of displacement

In case of vertically projected body, it has acceleration but after some time it will come back again to the initial position

We know that displacement final position initial position

In this case, displacement is zero

Therefore,

Therefore, work done is zero in case of vertically projected body on earth