(a) What is the d

(a) When no external force acts on either body involved in a straight-line collision then law of conservation of momentum holds true.

According to the law, momentum before and after collision is conserved so their difference will be zero.

(b) (i) A flying airplane has kinetic energy as well as potential energy.

(As it is on a certain height and moving)

(ii) A man climbing a hill has kinetic energy as well as potential energy

(As it posses both height and velocity