$A$ $150\, g$ tennis ball coming at a speed of $40\, m/s$ is hit straight back by a bat to a speed of $60\, m/s$. The magnitude of the average force $F$ on the ball,when it is in contact for $5\, ms$,is ........... $N$.

$A$ particle moves in the $x-y$ plane under the action of a force $\overrightarrow{F}$ such that the value of its linear momentum $\overrightarrow{P}$ at any time $t$ is $P_x = 2\cos t$ and $P_y = 2\sin t$. The angle $\theta$ between $\overrightarrow{F}$ and $\overrightarrow{P}$ at a given time $t$ will be $\theta = $ ........... $^o$.

$A$ body is accelerated by applying a force of $30 \, N$. The change in the momentum of the body after $2 \, sec$ is ............ $kg \cdot m/s$.

$A$ wall is hit elastically and normally by $n$ balls per second. All the balls have the same mass $m$ and are moving with the same velocity $u$. The force exerted by the balls on the wall is

Two balls each of mass $250 \ g$ moving in opposite directions each with a speed $16 \ m \ s^{-1}$ collide and rebound with the same speeds. The impulse imparted to one ball due to the other is

$A$ machine gun fires a bullet of mass $35 \ g$ with a speed of $600 \ m/s$. The person holding the gun can exert a maximum force of $147 \ N$ on it. The number of bullets that can be fired from the gun per second is