Newton's First Law of Motion

Law #1-An object in motion will stay in motion unless acted upon by an outside force. An object at rest will stay at rest unless acted upon by an outside force.

Newton's First Law is also commonly referred to as the law of inertia. Inertia is the name for the tendency of an object to stay at rest or in motion unless an outside force acts upon it.

In the picture above, a wagon full of blocks was pushed down a ramp. At the bottom of the ramp, the wagon hit a brick. The brick acted as an outside force and stopped the wagon. The blocks, however, did not have an outside force(they did not hit the brick), so the blocks stayed in motion and flew out of the wagon because there was no outside force to stop them.
Another example is a ball resting on the top of a hill. If the ball is still, it will remain still until an outside force acts upon it and causes it to roll down the hill. The outside force could be wind blowing it, gravity pulling it down, a child kicking it, etc.
Learn more about Newton's First Law of Motion.Watch this video about inertia in a car.