(a) A boy, who we

(a) Mass of the boy = 40 Kg

The efficiency of the boy is 28 %.

Therefore, the total energy utilized by him is 28 % of 21 KJ

0.28 × 21 KJ = 5.88 KJ

Now, according to the conservation of energy,

Loss in energy due to climbing = Gain in potential energy

(b) If the balls fall through the same height along a frictionless path, their velocity on reaching the ground will be equal. This is according to the law of conservation of energy.

If they fall from a point at a height ‘h’ from the ground, the potential energy at the point is, PE = mgh

The kinetic energy on reaching the ground is, KE = � mv2

Since, PE = KE

The equation is independent of mass.

Then the ratio of speeds would 1:1:1

(a) We know that - momentum = mass × velocity

A body can only possess velocity if it has kinetic energy and if does not have that energy than velocity = 0, using this value in above mentioned formula

Momentum = mass × 0

Momentum = 0

So, a body cannot have momentum without having energy.

Sometimes even if a body has energy, its momentum can be zero. For example: a brick lying on the roof of a building possesses potential energy but no velocity so momentum will be zero.