Give an example to show the conversion of potential energy to kinetic energy when put in use.

Give reasons for the following:

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

Give reasons for the following:

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

Give reasons for the following:

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

Give reasons for the following:

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