$A$ sledge of mass $80 \, kg$ moves over the frozen surface of a lake with a velocity of $30 \, m s^{-1}$ and comes to rest in $5 \, s$. Find its negative acceleration and the frictional force between the lake and the sledge.

$(a)$ Two objects of masses $100\, g$ and $200\, g$ are moving along the same line and direction with velocities $2\, m s^{-1}$ and $1\, m s^{-1}$ respectively. They collide and after the collision,the first object moves at a velocity of $1.67\, m s^{-1}$. Determine the velocity of the second object.
$(b)$ If a man jumps out from a boat,the boat moves backwards. Why?

$(a)$ State Newton's first and third laws of motion.
$(b)$ $A$ car of mass $1800 \, kg$ moving with a speed of $10 \, m s^{-1}$ is brought to rest after covering a distance of $50 \, m$. Calculate the force acting on the car.

State whether the following statement is true or false:
Force may or may not produce any motion in a body.

$A$ passenger in a moving train tosses a coin which falls behind him. It means that the motion of the train is

Two balls $A$ and $B$ of masses $m$ and $2m$ are in motion with velocities $2V$ and $V$ respectively. Compare:
$(i)$ their inertia,
$(ii)$ their momentum,and
$(iii)$ the force needed to stop them in the same time.