Where is newton's law of motion valid?

Answer 1

First Law:

If something isn't moving, it'll stay still, and if it is moving, it'll keep moving, unless a force (gravity, friction, and air resistance among the more common forces acting upon objects) affects it.

Second Law:

F=ma

Basically, the amount of force (power, strength) an object has is found out by multiplying that object's mass (usually in grams) by its acceleration, or speed. This is an important law because it is quantitative--that is, it deals with numbers.

Third Law:

This is best explained with an example: if you stand on one foot and push someone (the action), two things happen: the person you push is moved in the direction you pushed them (the equal reaction) and you are pushed away (the opposite reaction).

Answer 2

They may have been rejected by contrarians, or by people who misunderstand them, but they have not been proven false.

The laws of motion are accepted as applicable for large-scale masses.

Once you get to the quantum level, you need to apply quantum mechanics, which are treated as a special case of the general laws of motion.