Newton's First Law of Motion

What is Newton’s first Law of Motion?

Inertia

What is the idea behind inertia?

An object at motion will stay in motion, unless it is affected by an external force. An object at rest will stay at rest, unless it is affected by an external force.

How do seatbelts save lives (during a crash), in terms of inertia?

An object at motion will stay at motion, unless it is affected by an external force. When a car crashes, the car stops the the humans (objects) inside continue moving due to inertia. They will not stop moving until they come into contact with something else, essentially the windscreen. Seatbelts stop humans from going forward.

If a ball is on a wagon and the wagon is pulled forward what will happen to the ball?

It will not move with the wagon but instead stay still. It will then drop to the ground due to gravity.