What type of force is weight in Newtonian mechanics?

In Newtonian mechanics, weight is a gravitational force. It is the force with which a celestial body, such as Earth, attracts an object; near Earth’s surface, its magnitude is commonly written W = mg, where m is mass and g is gravitational acceleration.

Weight is measured in newtons because it is a force, while mass is measured in kilograms. A 1-kilogram object weighs about 9.8 newtons at Earth’s surface, but its weight would be roughly one-sixth as large on the Moon because lunar gravity is weaker.

A common mix-up is treating mass and weight as interchangeable. Mass is an object’s inertia and remains essentially constant when it moves between worlds; weight changes with the local gravitational field. Some physics contexts also use “weight” for the support force measured by a scale, which explains why an orbiting astronaut can feel weightless while still being acted on by Earth’s gravity. In the standard introductory Newtonian definition, however, weight is the gravitational force on the object.