- Inertia: an object at rest tends to stay in rest and an object in motion tends to stay in motion in a straight line at constant speed unless acted upon by an external, unbalanced force.
- The rate of change of momentum of a body is proportional to the resultant force acting on the body and is in the same direction.
- To every action (force applied) there is an equal and opposite reaction (equal force applied in the opposite direction).
The theory was called "special" because it applies the principle of relativity only to inertial frames. Einstein developed general relativity to apply the principle generally, that is, to any frame, and that theory includes the effects of gravity. Special relativity doesn't account for gravity, but it can deal with accelerations.Sources: The entire text of this post is compiled by copying and pasting from relevant Wikipedia articles.