Why does friction not roll a ball backward if no external force is pushing or pulling it?

$(a)$ How much momentum will an object of mass $10 \ kg$ transfer to the floor if it falls from a height of $0.8 \ m$? $(g = 10 \ m s^{-2})$
$(b)$ Explain why it is difficult for a fireman to hold a hose,which ejects a large amount of water at a high velocity.

When a fast-moving horse stops suddenly,a careless rider falls in the forward direction. Give the reason for this.

$A$ book of mass $2 \, kg$ is sliding with a constant velocity of $4 \, m s^{-1}$ on a frictionless horizontal table. What is the force required to keep the object moving with the same velocity (in $, N$)?

State the law of conservation of momentum. How does conservation of momentum take place in each of the following?
$(i)$ $A$ rocket taking off from ground.
$(ii)$ Flying of a jet aeroplane.

The quantitative definition of force is given by