In a high jump athletic event,the athletes are made to fall on a cushioned bed. Why?

$(a)$ Define inertia. You are provided with three bodies made of wood,rubber,and iron of the same shape and size. Which one of them will have greater inertia? Explain it.
$(b)$ When a striker hits the bottom of the pile of coins placed at the centre of a carom board,only the lowest coin is removed. Give reason for it.

$(a)$ $(i)$ Define momentum. State its $SI$ unit.
$(ii)$ An object of mass $50 \ kg$ is accelerated uniformly from a velocity of $4 \ m/s$ to $8 \ m/s$ in $8 \ s$. Find the magnitude of the force exerted on the object.
$(b)$ State Newton's first law of motion.

Two friends on roller-skates are standing $5\, m$ apart facing each other. One of them throws a ball of $2\, kg$ towards the other,who catches it. How will this activity affect the position of the two? Explain your answer.

$A$ truck of mass $M$ is moved under a force $F$. If the truck is then loaded with an object equal to the mass of the truck and the driving force is halved,then how does the acceleration change?

The velocity versus time graph of a ball of mass $50 \, g$ rolling on a concrete floor is shown in the figure. Calculate the acceleration and the frictional force exerted by the floor on the ball.