Inertia

Newton's first law is also called the law of inertia.

Inertia is the tendency of a stationary object to remain at rest, and of a moving object to continue moving with the same velocity in a straight line; that is, objects resist any change in their state of rest or motion.

Everyday examples of inertia

• A passenger standing in a bus is thrown forward when the bus brakes suddenly: his body tends to keep moving forward.
• When a train suddenly moves forward, a small bag lying under a seat appears to slide backward relative to the train, because it tends to stay at rest while the train moves forward under it.
• A card under a coin on a glass: flick the card away quickly and the coin drops into the glass, since it tends to remain at rest.

Measure of inertia. Observation shows that the difficulty of stopping a moving body depends on its mass and its velocity:

• A large truck is hard to stop, a small bicycle is easy, if they move at the same velocity.
• A car moving fast is hard to stop; the same car moving slowly is easy to stop. The two factors together define a new quantity, the momentum.