A book on a table stays on the table until you push it. A moving car keeps moving until you brake. That resistance to changes in motion is inertia. It is not a force. It is a property of matter. The more mass an object has, the more inertia it has. A heavy truck is harder to start and harder to stop than a bicycle. Inertia is the reason you lurch forward when a bus stops suddenly. Your body keeps moving even though the bus has stopped.

Inertia is the basis of Newton's first law of motion. An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force. The law is also called the law of inertia. It was Galileo who first described it, and Newton who made it a law. Inertia is not the same as momentum, though the two are related. Momentum is mass times velocity. Inertia is the resistance to change in motion, which depends on mass. A massive object at rest has inertia but no momentum. A massive object in motion has both.

Inertia facts

• Definition. Resistance to change in motion.
• Depends on. Mass.
• Newton's first law. Objects resist changes in motion.
• Not a force. A property of matter.
• Examples. Seat belts, lurching forward when a bus stops.

Inertia is not the same as gravity. Gravity pulls objects toward each other. Inertia resists changes in motion.