A stationary object remains at rest until you apply a force to it. Once you set it in motion, the object continues to move at a constant speed until it strikes another object.

In the same way, a spacecraft far from any source of gravity would need no thrust to keep it moving at a constant speed in a given direction. Once set in motion, it will keep moving forever without propulsion, since there is no friction in space to slow it down. But it does need an additional force—thrust—to change its speed or direction or both.

The more mass an object has, the more force you must apply to make it accelerate—to change its speed or direction or both.

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

To stop or slow down an object, a force must be applied in the direction opposite to that of the object’s motion. Exerting a force results in an equal force in the opposite direction (like the recoil a person feels when firing a gun).

When a spacecraft fires a thruster rocket, the exhaust gas pushes against the thruster and the thruster pushes against the exhaust gas. The gas and rocket move in opposite directions.