The Law of Inertia!!

What is inertia you ask? Well an object in motion possesses an inertia that causes it to remain in motion, with its same velocity and direction unless acted on by an external force. For example, a block of wood on a table. If you push that block of wood, it will come to rest if you stop pushing. Why? Because of Friction.
There are two opposite forces acting on this block of wood; the pushing force and friction.

If the force of friction was not acting on it, an object could be pushed across a table preserving a constant speed forever, even after it is no longer being pushed. The only way this object could be stopped or the constant speed interrupted is if an outside force acted on it.