Question

Give reasons for the following :

(a) No work is done if a man is pushing against a wall.

(b) The hammer drives a nail into the wood only when it is lifted up and then struck.

(c) A horse and a dog are running at the same speed. Which one of them has more kinetic energy than the other

(d) A teacher moving around in the class is doing work but a child standing and reading a book is not doing any work?

Answer 1

(a) As wall does not move from its placc.-distance moved is zero. Hence, no work is done.
(b) On lifting the Hammer, its potential energy is stored in the hammer on striking the nail with a hammer this energy is used in driving the nail
into the wood.
(c) A horse has more mass than a dog. As both are running at the same speed.
M₁ of the horse is greater than M₂ of dog
∴ K.E. of the horse is more than K.E. of dog
(d) A child reading a book while standing is not moving from its place i.e. displacement is zero. Hence the product of force and displacement ¡s zero
W=F × S
W = mg × O = O
Hence, the child is not doing any work. Whereas the teacher is moving from its place is doing work.