Newton's First Law of Motion (Law of Inertia)

Definition

An object at rest will stay at rest, and an object in motion will stay in motion with the same speed and direction, unless acted upon by an external force.

Related terms

Inertial reference frame: A frame of reference where Newton's first law holds true; objects either remain at rest or move with constant velocity unless acted upon by an external force.

Newton's Second Law of Motion:

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

Friction:

A force that opposes the motion between two surfaces in contact, causing objects to slow down or come to a stop.