While playing football,the goalkeeper did not get sufficient time to stop a fast ball shot towards him. Why did he hurt his hand while doing so?

State whether the following statement is true or false:
In any interaction,there are always at least two forces in play.

Look at the diagram below and answer the following questions:
$(i)$ When a force is applied through the free end of the spring balance $A$,the reading on the spring balance $A$ is $20 \ g \ wt$. What will be the reading shown by the spring balance $B$?
$(ii)$ Write the reason for your answer.
$(b)$ If a balloon is filled with air and its mouth is untied,air is released from its mouth in the downward direction. Write the other observations made by you and justify your answer.

Write the relation for the collision of two balls $A$ and $B$ and state the related law.

$A$ man weighing $60 \ kg$ runs along the rails with a velocity of $18 \ km \ h^{-1}$ and jumps into a car of mass $1$ quintal $(100 \ kg)$ standing on the rails. Calculate the velocity with which the car will start travelling along the rails. (in $m \ s^{-1}$)

$(a)$ State Newton's second law of motion. Use this law to find a method to measure force acting on an object.
$(b)$ From a rifle of mass $4 \ kg$,a bullet of mass $50 \ g$ is fired horizontally with an initial velocity of $40 \ m \ s^{-1}$. Calculate the initial recoil velocity of the rifle.