**Q.1. Can any object have mechanical energy even if its momentum is zero? Explain.**

Yes, mechanical energy comprises of both potential energy and kinetic energy. Zero momentum means that velocity is zero. Hence, there it no kinetic energy but the object may possess potential energy.

**Q.2. The potential energy of a body is 39600J. How high is the body if its mass is 20kg?**

The potential energy of a body = mgh

h = PE/mg = 39600j/20kg x9.8m/s2=198m

**Q.3. A force of 20 N displaces a body through a distance of 1 m at an angle of 60° from its own direction. Calculate the amount of work done.**

Here, force F = 20 N, displacement, s = 1 m. Angle between force and displacement 60°.

Work done,W =Fscos0 =20 X 1 X cos60°=20X 1 X 1/2 = 10J.

A man of 50 kg jumps up to a height of 1.2 m. What is his potential energy at the highest point?

The potential energy of man = mgh = = 50 + 10 X 1.2 J = 600 J.

**Q.4. How much work is done by a force of 10 N in moving an object through a distance of 4 m in the direction of the force.**

Work done Force x Displacement =F.s = (10 N) x (4 rn) = 40 joule or 40J.

**Q.5. 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?

Linear momentum of 1st object = p1=m1v1

Linear momentum of 2nd object = p2=m2v2

Given, p1 > p2 ---------------------------------(i)

Þ m1v1 > m2v2

But, m1<m2 (A light and a heavy object) Þ v1 > v2 ------------(ii)

Ke = ½ mv2 = ½ m vx v =1/2 pv

From (i)and (ii) p1v1 > p2v2 Þ ½ p1v1 > ½ p2v2 Þ KE1> KE2

**Q.6. What is power? How do you differentiate kilowatt from kilowatt hour?**

Power is the rate of doing work. Kilowatt is the unit of power and kilowatt hour is the unit of energy.

**Q.7. A rocket is moving up with a velocity v. If the velocity of this rocket is suddenly tripled, what will be the ratio of two kinetic energies?**

Initial KE/Final KE = ( ½ mu2) /( ½ mv2) = ( ½ mu2) /{ ½ m(3v2)} =1:9

**Q.8. Calculate the work done in lifting 200 kg of water through a vertical height of 6 m.**

Work done in lifting a body = Weight of body X vertical distance

The work done in lifting = W = mgh = 200 kg x 10m/s2 x6 m = 1200J

**Q.9. Give one example each of potential energy (i) due to position (ii) due to shape.**

(i) Potential energy due to position: Water stored in dam has potential energy.

(ii) Potential energy due to shape: In a toy car, the wound spring possesses potential energy, and as the spring is released, its potential energy changes into kinetic energy due to which the car moves.

**Q.10. What kind of energy transformation takes place when a body is dropped from a certain height?**

When a body falls, its potential energy gradually gets converted into kinetic energy. On reaching the ground, the whole of the potential energy of the body gets converted into kinetic energy

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