Its physics at its best. You remember old Newton - right.

Newton's Laws of Motion:

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

II. "Law of Inertia". The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

III. For every action there is an equal and opposite reaction.

This law is exemplified by what happens if we step off a boat onto the bank of a lake: as we move in the direction of the shore, the boat tends to move in the opposite direction (leaving us facedown in the water, if we aren't careful!).

Or :

First law

An object at rest will remain at rest unless acted upon by an external and unbalanced force . An object in motion will remain in motion unless acted upon by an external and unbalanced force.

Second law
The rate of change of the momentum of a body is directly proportional to the net force acting on it, and the direction of the change in momentum takes place in the direction of the net force.

Third law
To every action (force applied) there is an equal but opposite reaction (equal force applied in the opposite direction).

\:D \:D \:\)