Summary and Main Ideas

Newton’s first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia. Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an object’s mass. Mass is the quantity of matter in a substance.

Summary

Newton’s first law of motion states that a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force. This is also known as the law of inertia.
Inertia is the tendency of an object to remain at rest or remain in motion. Inertia is related to an object’s mass.
Mass is the quantity of matter in a substance.

Glossary

inertia

the tendency of an object to remain at rest or remain in motion

law of inertia

see Newton’s first law of motion

mass

the quantity of matter in a substance; measured in kilograms

Newton’s first law of motion

a body at rest remains at rest, or, if in motion, remains in motion at a constant velocity unless acted on by a net external force; also known as the law of inertia