Kicking a football requires less force than kicking a basketball. Why?

Two cars each of mass $1000 \ kg$ are moving in a straight line but in opposite directions. The velocity of each car is $5 \ m s^{-1}$ before the collision during which they stick together. What will be the velocity of the combined cars after collision (in $m s^{-1}$)?

$A$ steam engine of mass $3 \times 10^{4} \text{ kg}$ pulls two wagons,each of mass $2 \times 10^{4} \text{ kg}$,with an acceleration of $0.2 \text{ m s}^{-2}$. Neglecting frictional forces,calculate the:
$(i)$ force exerted by the engine.
$(ii)$ force experienced by each wagon.

Two identical bullets are fired,one by a light rifle and another by a heavy rifle with the same force. Which rifle will hurt the shoulder more and why?

What is the effect of force when it is applied in the following cases?
$(a)$ $A$ balloon is pressed from opposite sides.
$(b)$ $A$ ball is hit with a bat.
$(c)$ $A$ spring is compressed.

Define $1\, kg$ weight and express it in newton.