Newton's Laws of Motion (Mechanics) and Einstein's Relativity

Sir Isaac NewtonNewton's Laws of Motion describe only the motion of a body as a whole and are valid only for motions relative to a reference frame:
  1. 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.

  2. 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.

  3. To every action (force applied) there is an equal and opposite reaction (equal force applied in the opposite direction).
Albert EinsteinSpecial relativity overthrows Newtonian notions of absolute space and time by stating that distance and time depend on the observer, and that time and space are perceived differently, depending on the observer. It yields the equivalence of matter and energy, as expressed in the famous equation E=mc2, where c is the speed of light. Special relativity agrees with Newtonian mechanics in their common realm of applicability, in experiments in which all velocities are small compared to the speed of light.

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.

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