Newton's laws of motion
Newton's laws of motion as a Venn diagram.
A body will remain in a state of rest or uniform motion unless acted upon by a net external force. The first law is also called the "law of inertia" because it enshrines the concept of inertia as developed by Galileo.
The net force on a body is equal to the product of the body's mass and its acceleration (i.e., the rate of change of momentum). Acceleration and force are vectors. In the second law, the directions of the force vector and the acceleration vector are the same.
When two bodies interact, the force on the first due to the second is equal and opposite to the force on the second due to the first. In other words, action and reaction are equal and opposite. (See also D'Alembert's principle).