Mass is the scalar quantity multiplied by acceleration to give force in Newtonian mechanics.
Newton’s second law is commonly written F = ma, where F is net force, m is mass, and a is acceleration. Mass measures an object’s inertia—its resistance to changes in motion—so the same acceleration requires more force for a more massive object. Conversely, a fixed force produces less acceleration when the mass is larger.
The relationship is more generally expressed as force equals the time derivative of momentum. For an object whose mass remains constant, momentum is mass times velocity, and the equation reduces to F = ma. This distinction matters for variable-mass systems such as rockets, where the simple form must be applied carefully.
Mass is often confused with weight. Mass is an intrinsic property measured in kilograms, while weight is a gravitational force measured in newtons and depends on the local gravitational field. An astronaut’s mass stays essentially the same on the Moon, but their weight is much smaller there.