Inertia

Inertia is a property of all matter. An object that is sitting still wants to keep sitting still. An object that is moving wants to keep moving. It takes a force to change what an object is doing. Heavier objects have more inertia, which means they are harder to start moving or stop.

Inertia at Rest

A book on a table stays on the table until something pushes it. A soccer ball on the grass stays still until someone kicks it. This is inertia at rest. Objects do not start moving on their own. They need a force, like a push or a pull, to get them going. The heavier the object, the more force you need.

Inertia in Motion

A hockey puck sliding on ice keeps sliding until friction or a wall stops it. A ball thrown in space would travel forever because there is nothing to stop it. This is inertia in motion. That is why seatbelts are important. When a car stops suddenly, your body's inertia wants to keep moving forward.

Fun Facts

• You can pull a tablecloth out from under dishes if you pull fast enough, because the dishes' inertia keeps them in place.
• A fully loaded freight train can take over a mile to stop because it has so much inertia.
• Isaac Newton described inertia as his first law of motion.

Did You Know?

In outer space, there is almost no friction to slow things down. If you threw a baseball in space, it would keep traveling at the same speed in the same direction forever, unless something got in its way!