Newton’s second law of motion states:

The force F acting on a body is the product of its mass m and acceleration a.

or

F=ma

where F and a are vector quantities.

If we set the force to zero we find the acceleration is zero which is why Newton’s second law implies Newton’s first law, which states the principle more clearly.

In astronomy the motion of the stars and planets are well described by Newton’s laws of motion combined with his Universal law of gravitation. Indeed Kepler’s laws can be derived from Newton’s laws of motion and gravitation.

At relativistic speeds (ie where velocity approaches the speed of light) Einstein’s theories must be used to accurately describe motion. In Newtonian physics the speed at which gravity propagates is infinite, whereas in relativity it propogates at the speed of light.