The three laws of Newton

Here, on the table, there are three spheres in state of rest. These spheres will remain still unless an external force acting upon them. The three spheres are still but if we apply an external force upon them they won't keep their state. The external force has been applied to the three spheres and their state of motion has changed so their position has changed too. In this example, this body represents a pulling force on a 50Kg mass. The body is accelerating the object in the direction of the pulling force. Mass and force are directly proporcional so if we double the mass and we want to accelerate the object at the same velocity we will have to double also the force. If the two bodies apply the same force, we will see that acceleration and mass are indirectly proporcional so the acceleration will be different in the two cases. In conclusion, we can see that acceleration depend on mass. In this example we have a rocket, the rocket to take off have to apply a force upon the Earth. This is the action force. The reaction force make the rocket take off because this force is equal and opposite than the action one.