$A$ force can be completely described by

$A$ constant force acts on an object of $5 \ kg$ for a period of $2 \ s$. It increases the velocity of the object from $3 \ m s^{-1}$ to $7 \ m s^{-1}$. Find the magnitude of the applied force. Now,if the force were applied for a period of $5 \ s$,what would be the final velocity of the object?

Calculate the ratio of momentum,when
$(i)$ velocity of an object is doubled.
$(ii)$ mass of an object is halved.
$(iii)$ both mass and velocity are increased by three times.

$(a)$ Define momentum. Write its $SI$ unit.
$(b)$ $A$ car and a truck have the same momentum. Whose velocity is more and why?
$(c)$ $A$ bullet of mass $20 \ g$ moving with a speed of $500 \ m s^{-1}$ strikes a wooden block of mass $1 \ kg$ and gets embedded in it. Find the speed with which the block moves along with the bullet.

State the law of conservation of momentum. Derive it by using Newton's third law of motion.

In oil tankers,some space is left at the top while filling them. Explain why.