An athlete always runs some distance before taking a jump. Why?

Two similar vehicles are moving with the same velocity on roads,such that one of them is loaded and the other is empty. Which of the two vehicles will require a larger force to stop it? Give reasons.

Explain the following:
$(i)$ Why does a boatman push the river bank backward with a long bamboo pole when launching his boat into the water?
$(ii)$ Why does a boat tend to leave the shore when passengers are alighting from it? How does the boatman overcome this difficulty?
$(iii)$ Why does a gun recoil when fired?
$(iv)$ How do rockets operate in space?

$(a)$ When a motorcar makes a sharp turn at a high speed,we tend to get thrown to one side. Why?
$(b)$ State Newton's first and third law of motion.
$(c)$ $A$ force of $5 \ N$ gives a body of mass $m$ an acceleration of $10 \ m \ s^{-2}$. Calculate the mass of the body in grams.

An object experiences a net zero external unbalanced force. Is it possible for the object to be moving with a non-$zero$ velocity? If yes, state the conditions that must be placed on the magnitude and direction of velocity. If no, provide a reason.

Quick sand consists of very smooth and tiny grains of sand. What would happen if a man or a heavy animal steps over a patch of such sand in a desert?