1. For massive objects (such as celestial objects)
2. For objects and particles travelling at speeds closer to the speed of light.
3. Sometimes for the motions in quantum realm.
Newton's first Law
An object will continue to be at rest or travel with uniform velocity, unless an external, unbalanced force acts on it.
In other words, for an object which is at rest or moving with constant velocity, the state of motion can only be changed by an external resultant force.
Newton's second Law
The rate of change of momentum is directly proportional to the net force acting on the same direction.
F = (mv-mu)/t
F = ma
Here, F - Resultant force
m - mass
v- final velocity
u - initial velocity
a - acceleration
t - time
Newton's third Law
If an unbalanced force is exerted on B by A, B will also exert an equal but opposite force on A.
In other words, every action has an equal, but opposite reaction.
For further information, search the web search bar for "Newton's laws of motion" or go to Newton's laws of motion - Wikipedia
