Force vs Weight: What's the Difference in Physics?

In everyday language, people say they weigh a certain number of kilograms — but kilograms measure mass, not force. In physics, weight is the gravitational force exerted on an object: W = mg, measured in newtons. Force more generally is any push or pull that can change motion, described by Newton's second law F = ma. Weight is one type of force, always directed toward the center of the gravitating body. Confusing mass with weight causes errors in mechanics problems, engineering loads, and even space science discussions.

Quick comparison

Mass is not force

Mass (kg) is inertia — resistance to acceleration. It does not change when you move to the Moon. Weight (N) depends on local g: on Earth g ≈ 9.8 m/s²; on the Moon about 1.6 m/s². A 70 kg person has mass 70 kg everywhere but weighs about 686 N on Earth and 112 N on the Moon.

Scales in homes often display kg by converting the measured normal force assuming Earth gravity — they infer mass, not direct weight in newtons.

Weight as a contact force in free-body diagrams

On a surface, weight acts downward and the surface pushes up with normal force. When standing still, net vertical force is zero — weight and normal force balance. Weight is not the same as the reading on a scale; a scale measures normal force or tension in supporting strings.

In an elevator accelerating upward, apparent weight increases because normal force exceeds mg. Accelerating downward, apparent weight decreases. True gravitational weight mg is unchanged; the contact force you feel changes.

Other forces in the same problem

Friction, tension, spring force, and applied pushes are forces but not weight. Net force ΣF = ma combines all of them vectorially. A box on a ramp has weight components parallel and perpendicular to the surface plus friction and normal force.

The Force Calculator finds force from mass and acceleration for any net or applied force. The Weight Calculator specifically computes W = mg for gravitational force.

Weightlessness in orbit

Astronauts in orbit are not beyond gravity — they are in free fall around Earth. Gravity still acts; they and their spacecraft accelerate together, so there is no support force and they feel weightless. Mass remains; weight as felt contact force vanishes.

Use cases

• Drawing free-body diagrams for inclined planes (weight components)
• Computing load on a floor or cable (weight in newtons)
• Analyzing elevator apparent weight problems
• Comparing object weights on Earth vs Moon vs Mars
• Distinguishing mass (kg) from force (N) in exam answers
• Calculating net force for acceleration (F = ma, not weight alone)

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