Every movement in the Universe is governed by physical laws devised by people such as Newton and Einstein.
Newton’s first law of motion says an object accelerates, slows down or changes direction only when a force is applied.
Newton’s second law of motion says that the acceleration depends on how heavy the object is, and on how hard it is being pushed or pulled.
The greater the force acting on an object, the more it will accelerate.
The heavier an object is — the greater its mass — the less it will be accelerated by a particular force.
Newton’s third law of motion says that when a force pushes or acts one way, an equal force pushes in the opposite direction.
Newton’s third law of motion is summarized as follows: ‘to every action, there is an equal and opposite reaction. Newton’s third law applies everywhere, but you can see it in effect in a rocket in space. In space there is nothing for the rocket to push on. The rocket is propelled by the action and reaction between the hot gases pushed out by its engine and the rocket itself.
You cannot always see the reaction. When you bounce a ball on the ground, it looks as if only the ball bounces, but the Earth recoils too. The Earth’s mass is so great compared to the ball’s that the recoil is tiny.
Einstein’s theory of relativity modifies Newton’s second law of motion under certain circumstances.