$A$ truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of $400 \, m$ in $20 \, s$. Find the force (in $N$) acting on it if its mass is $7 \, \text{tonnes}$ $(1 \, \text{tonne} = 1000 \, kg)$.

$(a)$ $(i)$ Define momentum. State its $SI$ unit.
$(ii)$ An object of mass $50 \ kg$ is accelerated uniformly from a velocity of $4 \ m/s$ to $8 \ m/s$ in $8 \ s$. Find the magnitude of the force exerted on the object.
$(b)$ State Newton's first law of motion.

$(a)$ Why are the wheels of vehicles provided with mudguards?
$(b)$ $A$ cricket player lowers his hands while catching a ball. Why?
$(c)$ Why is a glass pane of a window shattered when a flying pebble hits it?

$(a)$ State the law of conservation of momentum.
$(b)$ $A$ body of mass $2 \, kg$,initially moving with a velocity of $10 \, m s^{-1}$,collides with another body of mass $5 \, kg$ at rest. After the collision,the velocity of the first body becomes $1 \, m s^{-1}$. Find the velocity of the second body.

Given below are some measuring units of different physical quantities:
$kg\, ms^{-2}, ms^{-2}, kg\, ms^{-1}, ms^{-1}, kg\, m^2 s^{-2}$
Select the units which measure force and momentum respectively.

Two objects,each of mass $1.5\, kg$,are moving in the same straight line but in opposite directions. The velocity of each object is $2.5\, m/s$ before the collision during which they stick together. What will be the velocity (in $m/s$) of the combined object after collision?