$A$ 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.

Suppose a ball of mass $m$ is thrown vertically upward with an initial speed $v$. Its speed decreases continuously until it becomes zero. Thereafter,the ball begins to fall downward and attains the speed $v$ again before striking the ground. It implies that the magnitude of the initial and final momentum of the ball are the same. Yet,it is not an example of the conservation of momentum. Explain why?

$A$ motor car is moving with a velocity of $108 \, km \, h^{-1}$ and it takes $4 \, s$ to stop after the brakes are applied. Find the force exerted by the brakes on the motor car if its mass along with the passengers is $1000 \, kg$.

What force would be needed to produce an acceleration of $4 \, m s^{-2}$ on a ball of mass $6 \, kg$?

Why does friction not roll a ball backward if no external force is pushing or pulling it?

When a force of $40\, N$ is applied on a body,it moves with an acceleration of $5\, m s^{-2}$. Calculate the mass of the body. (in $kg$)