Name the physical quantity whose $SI$ unit is "newton".

What type of force is acting in the cases given above?

Prove that $1 \text{ N} = 10^{5} \text{ dynes}$.

$A$ bullet of mass $10 \, g$ is fired with a velocity of $400 \, m s^{-1}$ from a gun of mass $4 \, kg$. What is the recoil velocity of the gun (in $, m s^{-1}$)?

The qualitative definition of force is given by

$(a)$ Two objects of masses $100\, g$ and $200\, g$ are moving along the same line and direction with velocities $2\, m s^{-1}$ and $1\, m s^{-1}$ respectively. They collide and after the collision,the first object moves at a velocity of $1.67\, m s^{-1}$. Determine the velocity of the second object.
$(b)$ If a man jumps out from a boat,the boat moves backwards. Why?