Newton's First Law

Statement. Every object continues in its state of rest, or of uniform motion in a straight line (constant velocity), unless an external force acts on it.

This inability of a body to change, on its own, its own state of rest, motion, or direction is called inertia; mass is the quantitative measure of an object's inertia. There are three distinguishable types of inertia, depending on exactly what change is being resisted.

1. Inertia of rest — resistance to starting to move. When a stationary bus suddenly accelerates forward, a standing passenger's body (still at rest, by inertia) is left behind relative to the bus floor, and this is felt as a sudden backward push.
2. Inertia of motion — resistance to stopping (or slowing). When a moving bus suddenly brakes, the passenger's body (still moving, by inertia) continues forward relative to the now-slower bus, felt as a sudden forward push.
3. Inertia of direction — resistance to a change of direction. A stone whirled on a string, if the string suddenly snaps, does not continue curving — it flies off along the straight tangent to the circle at the point of release, because there is no longer any force bending its path.