It is dangerous to move out of moving bus. Why?

Find the retardation with which a train moving at $108 \mathrm{km}/\mathrm{hr}$ is brought to rest in $12$ seconds.

Define Newton's first law and Inertia.

Newton's laws are universal.

Give scientific reasons.
Even though the magnitude of action force and reaction force are equal and their directions are opposite, their effects do not get cancelled.

A body moves with constant velocity of $20 \mathrm{m} {\mathrm{s}}^{-1}$. The mass of body is $2 \mathrm{kg}$. The unbalanced force acting on the body is _____ $\mathrm{N}$.