Physics · Common misconception
Mass and weight: what changes on the Moon?
Keep kilograms and newtons separate, then explore the role of gravitational field strength.
The short answer
Mass is measured in kilograms. Weight is the gravitational force on an object and is measured in newtons. For the GCSE model, calculate weight using W = mg, with g in newtons per kilogram.
Conceptual illustration. Colours distinguish parts of the model.
Where the confusion starts
Changing location does not by itself remove matter from an object. Its mass can remain the same while its weight changes because gravitational field strength is different. A scale displaying kilograms may infer mass from a force measurement and an assumed value of g.
WORK IT THROUGH
Compare a 10 kg object where g = 10 N/kg and where g = 1.6 N/kg.
- At 10 N/kg: W = 10 × 10 = 100 N.
- At 1.6 N/kg: W = 10 × 1.6 = 16 N.
- The object still has mass 10 kg in both situations. The force has changed. Use the g value supplied in a question.
CHECK YOUR UNDERSTANDING
An object has weight 60 N where g = 10 N/kg. What is its mass?
Make a prediction or write an answer before opening the explanation.
Reveal the explanation
6 kg, using m = W/g = 60/10. Give the mass in kilograms, not newtons.
SEE IT. CHANGE IT. EXPLAIN IT.
Take the next step in the lesson.
Use the visual explanation in the lesson, then choose Practice to test the idea with another question.
Spotted something unclear? Send feedback. These are original teaching explanations, not predictions of future exam questions.