Newton’s Laws

An inertial frame is a frame of reference in which Newton’s laws are valid. The only way to ascertain whether a particular frame is sufficiently close to being inertial is by comparing calculated results with experimental data. These laws may be stated as follows:

1st Particles with zero resultant force acting on them move with constant velocity in an inertial frame.

2nd The resultant force on a particle is equal to its mass times its acceleration in an inertial frame. In other words, this acceleration is defined as in Eq. (2.3), with the frame F being an inertial frame.

3rd When a particle P exerts a force on another particle Q, Q simultaneously exerts a force on P with the same magnitude but in the opposite direction. This law is often simplified as the sentence: “To every action, there is an equal and opposite reaction.”