$A$ goalkeeper in a game of football pulls his hands backwards after holding the ball shot at the goal. This enables the goalkeeper to

$A$ javelin throw is marked foul if an athlete crosses over the line marked for throw. Explain why the athletes often fail to stop themselves before the line.

Give an example of a force that merely changes the direction of motion of a body.

$(a)$ Define momentum and mention its $SI$ unit.
$(b)$ From the velocity-time graph shown in the figure for a body of mass $5\, kg$,find the force on the body from:
$(i)$ $O$ to $A$ and $(ii)$ $B$ to $C$.
$(c)$ Which would require a greater force: accelerating a $2\, kg$ mass at $5\, m s^{-2}$ or a $4\, kg$ mass at $2\, m s^{-2}$?

Which of the following statements is not correct for an object moving along a straight path in an accelerated motion?

State the law of conservation of momentum. Mention two factors which determine the momentum of a body. $A$ ball of mass $0.4 \ kg$ is thrown vertically upwards with a velocity of $10 \ m s^{-1}$. What will be its initial momentum and what would be its momentum at the highest point of its reach? If this change in momentum occurs in one second, find the negatively accelerating force acting on the ball, when it was going up.