Newton's Laws of Motion

The first law describes how an object moves when no force is acting on it. A stationary object remains at rest until you apply a force to it. Once moving, the object continues to move at a constant speed until another external force is applied. 

This can best be demonstrated in space. A spacecraft far from any source of gravity or drag (friction) would need no thrust to keep it moving at a constant speed. Once set in motion, it would keep moving without propulsion, since there is no friction in space to slow down.

The second law describes how force and acceleration are related. The more mass an object has, the more force you must apply to make it accelerate. This also applies if you want to slow an object down or to change its direction. 

The more you want a spacecraft to accelerate, the more force you need to apply. To reach a certain speed you need to apply a small force for a long time or a large force for a short time.

The third law states that every action has an equal and opposite reaction. Exerting a force, such as speeding something up, results in an equal force in the opposite direction.

When a spacecraft ignites a rocket, the expanding gas pushes against the rocket and the rocket pushes against the expanding gas. The gas and rocket therefore each move in opposite directions.