4.4.2—Mass and weight

Mass m is measured in kilograms and remains the same in a given object; weight W=mg is a force in newtons and depends on local gravitational field strength g.

Use mass in F=ma and weight as a force in vertical equations. A scale reading is a contact force and may differ from mg during acceleration.

A 3 kg object weighs about 29.4 N where g=9.8 m s⁻²; its mass is still 3 kg on the Moon.

Mass and weight are not interchangeable, and “weighing zero” in free fall does not mean mass disappears.