A boy sitting in a railroad car moving at constant velocity throws a ball straight up into the air, according to the person sitting next to him. Where would the ball fall if the train accelerates forward while the ball is in the air?

The time period of a particle $P$ moving along a circular path of radius $1 \mathrm{cm}$ is $\pi s.$  Another particle $Q$ starts from rest at a point $A$ and travelling along a straight line path reaches the point $B$ in $2 \mathrm{s}$. If $AB=12 \mathrm{cm}$ and the particles $P$ and $Q$ are respectively at $D$ and $B$ at an instant shown in the figure, the acceleration of particle $Q$ with respect to particle $P$ is _______$\mathrm{cm} {\mathrm{s}}^{-2}$. (Points $A\mathit{,}B\mathit{,}C\mathit{,}D$ and $O$ lie along the same straight line)

A swimmer wants to cross a river from point $A$ to point $B$. Line AB makes an angle of $30°$ with the flow of the river. The magnitude of the velocity of the swimmer is the same as that of the river. The angle $\theta$ with the line $AB$ should be _______$°$, so that the swimmer reaches point $B$.