In which direction do the following forces act when an object is in motion?
$(i)$ Frictional force
$(ii)$ Gravitational force
$(iii)$ Centripetal force

$A$ large truck and a car,both moving with a velocity of magnitude $v$,have a head-on collision and both of them come to a halt after that. If the collision lasts for $1$ second:
$(i)$ Which vehicle experiences the greater force and why?
$(ii)$ Which vehicle experiences the greater change in momentum and why?
$(iii)$ Which vehicle experiences the greater acceleration and why?

An object of mass $1 \,kg$ travelling in a straight line with a velocity of $10\, m\, s^{-1}$ collides with,and sticks to,a stationary wooden block of mass $5\, kg$. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also,calculate the velocity of the combined object.

$(a)$ State Newton's third law of motion and give two examples to illustrate the law.
$(b)$ Calculate the force required to produce:
$(i)$ an acceleration of $2.5 \, m s^{-2}$ in a body of $4 \, kg$.
$(ii)$ an acceleration of $3 \, m s^{-2}$ in a body of $10 \, kg$.

$A$ bullet of mass $5\, g$ travelling at a speed of $120\, m s^{-1}$ penetrates deeply into a fixed target and is brought to rest in $0.01\, s$. Calculate
$(a)$ the distance of penetration in the target
$(b)$ the average force exerted on the bullet

State whether the following statement is true or false:
The force of friction is smaller in solids than in liquids.