A metal plate of mass $100 \mathrm{g}$ is balanced in mid-air by throwing $100$ balls per second, each of mass $5 \mathrm{g}$, vertically upwards from below. The balls rebound with the same speed with which they strike the plate. Find the velocity with which they strike the plate.

A bullet of mass $0.05 \mathrm{kg}$ moving with a speed of $80{ \mathrm{ms}}^{-1}$ enters a wooden block and is stopped after a distance of $0.40 m$. The average resistance force exerted by the block on the bullet is

A particle of mass $2 \mathrm{kg}$ is initially at rest. A force acts on it whose magnitude changes with time. The force-time graph is shown below  The velocity of the particle after $10 s$ is

A body of mass $8 \mathrm{kg}$ is suspended through two light springs $X$ and $Y$ connected in series as shown in figure. The readings of $X$ and $Y$ are: