If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Newton's first law review

Review the key concepts, equations, and skills for Newton's first law of motion, including the difference between mass and weight

Key terms

MassThe amount of matter in an object, independent of its size or any forces acting on the object. Both a property of a physical object and how resistant an object is to acceleration. SI units of start text, k, g, end text.
InertiaThe tendency of an object to remain at rest or remain in motion. This is measured by its mass.
WeightThe force of gravity on an object. Depends on the value of g.
External forceA force acting on an object from the outside, as opposed to forces acting within the object.
F, start subscript, start text, n, e, t, end text, end subscriptThe net force, which is the vector sum of all external forces.

Newton’s first law of motion

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by a net external force.


EquationSymbol breakdownMeaning in words
F, start subscript, g, end subscript, equals, W, equals, m, gF, start subscript, g, end subscript is force due to gravity, W is weight, m is mass, and g is the gravitational field strength (acceleration) which is 9, point, 8, start fraction, start text, m, end text, divided by, start text, s, end text, squared, end fraction on Earth's surface.The weight of an object is a long range force due to gravity. It is directly proportional to its mass and gravitational acceleration g.

Common mistakes and misconceptions

Some students confuse mass and weight. Mass is how much matter an object is made from; it is constant. The weight of an object is the force exerted by gravity on that object, so it depends on the local g value. For example, the value of g on the Moon is much lower than on Earth, so an object weighs less there, but the mass does not change.

Learn more

For more on weight and mass, read our article what is weight?.
To check your understanding and work toward mastering these concepts, check out the exercise on using Newton's first law to find net force and acceleration.

Want to join the conversation?