Write the relation for the collision of two balls $A$ and $B$ and state the related law.

State Newton's first law of motion.

Which would require a greater force $-$ accelerating a $1\, kg$ mass at $10\, m s^{-2}$ or a $2\, kg$ mass at $4\, m s^{-2}$?

In oil tankers,some space is left at the top while filling them. Explain why.

Explain the following:
$(i)$ Why do we jerk wet clothes before spreading them on a line?
$(ii)$ Why does dust fly off when a carpet is hit with a stick?
$(iii)$ Why do fruits fall off the branches in a strong wind?
$(iv)$ Why does a pillion rider fall forward when the driver of a two-wheeler suddenly applies brakes?

$(a)$ Define force and state three possible effects of force on an object.
$(b)$ $A$ motorcycle is moving with a velocity of $90 \, km \, h^{-1}$ and it takes $5 \, s$ to stop after the brakes are applied. Calculate the force exerted by the brakes on the motorcycle if its mass along with the rider is $200 \, kg$.