What's the Difference Between Mass and Weight?

SL and HL (A.2 Forces and momentum): Mass is a scalar property of a body measured in kilograms, whereas weight is the gravitational force acting on that body, measured in newtons. Mass remains constant when location changes, but weight changes with gravitational field strength.

The Reasoning

Mass determines how strongly a body resists acceleration. From Newton's second law, $F=ma$, a larger mass requires a larger resultant force to produce the same acceleration.

Weight is a field force caused by the gravitational interaction between a body and another mass, such as Earth. Near a planet's surface:

$$F_g=mg$$

where $F_g$ is weight in newtons (N), $m$ is mass in kilograms (kg), and $g$ is gravitational field strength in newtons per kilogram ($\text{N kg}^{-1}$). On Earth's surface, the IB data booklet gives .

For example, a $60\,\text{kg}$ student on Earth has weight

$$F_g=60\times9.8=588\,\text{N}.$$

On the Moon, where $g\approx1.6\,\text{N kg}^{-1}$, the same student still has a mass of $60\,\text{kg}$, but weighs approximately $96\,\text{N}$.

A common misconception is that astronauts in orbit have no weight because there is no gravity. Gravity still acts on them; their apparent weight is zero because the astronauts and spacecraft are in continuous free fall together.

Exam Technique

For a Calculate question, show $F_g=mg$, substitute values, and give the answer in newtons. Never give weight in kilograms, and distinguish actual gravitational force from apparent weightlessness.

In an Explain question, state that mass is an intrinsic scalar property, while weight is the gravitational force and therefore depends on $g$. In a free-body diagram, draw weight as an arrow from the body toward the attracting body's centre; do not label it “mass.”