Staying as you are

The idea

Inertia is the tendency of objects to resist change in their state of rest or of uniform motion.

Two balls

One ball lies at rest on the ground. Another rolls steadily in a straight line on a very smooth path.

What do you expect of each ball, if nothing disturbs it?

• Each tends to keep doing what it was doing
• Both stop at once
• Both start speeding up

Rest and steady motion

Press Next step.

A name for the tendency

Objects at rest tend to stay at rest, and objects moving steadily tend to keep moving. Newton called this tendency inertia, and he used the idea to state his first law of motion.

Inertia is the tendency of objects to resist change in their state of rest or of uniform motion.

Notes

Inertia: the tendency of objects to resist change in their state of rest or uniform motion.

Check yourself

What is inertia?

Who used the word inertia for this tendency?

A ball lies at rest. Inertia means that it tends to…

A box moves steadily. Inertia means that it tends to…

• The force of friction on a moving object. Friction is a force that acts against the motion. Inertia is the tendency of objects to resist change in their state.
• The tendency of objects to resist change in their state of rest or of uniform motion — correct. Yes!
• The push that sets an object moving. A push is a force. Inertia is the tendency to resist a change in the state of rest or uniform motion.
• Galileo Galilei. Galileo argued that an object moving on a horizontal plane without any impediments would continue to move indefinitely. The word inertia was used by Newton.
• Nobody, because inertia is a unit. Inertia is a tendency, not a unit. Isaac Newton used the word for it.
• Isaac Newton — correct. Yes!
• stay at rest — correct. Yes!
• start moving by itself. Inertia is the tendency to resist a change in the state of rest.
• speed up on its own. Inertia is the tendency to resist a change in state, so it does not speed up by itself.
• stop immediately. Inertia is the tendency to resist a change in uniform motion.
• keep moving in the same way — correct. Yes!
• turn by itself. Turning would be a change in the motion. Inertia is the tendency to resist such a change.