Name the type of force which acts in the following cases :

(a) A coolie lifts a luggage

(b) A bicycle comes to rest slowly when the cyclist stops pedalling

(c) A stone falls from a roof

(d) A comb rubbed with silk attracts bits of paper

(e) A string hangs with a load

(f) A horse moves a cart

(g) A magnet attracts an iron pin

(h) A boy opens a door

(i) An apple falls from a tree

(j) A man rows a boat

Define the term "weight of a body".

Give an example to illustrate the existence of force of friction.

What is the cause of friction?