A space probe travels from Earth to Mars. Explain why the weight of the probe changes during the journey, whilst its mass remains the same.

Cambridge International IGCSE Physics (0625) — 1.3 Mass and weight · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Mass is a measure of the amount of matter in an object and does not change with location.
Weight is the gravitational force acting on the probe and depends on the gravitational field strength.
The gravitational field strength is different on Earth, in deep space and on Mars, so the value of g changes.
Because weight = mass × g, a change in g changes the weight while the mass remains constant.

Examiner tips

Use the definition of mass and weight first, then explain the dependence on g.
Show the equation weight = mass × g to link the concepts.
Mention the different g values on Earth, Mars and in space.
Keep the answer concise and use UK spelling.

Common mistakes

Confusing mass with weight, or saying mass changes.
Giving numerical g values without linking to weight change.
Using vague terms like ‘force’ without specifying gravity.

Mark scheme (4 marks)

Mass is a measure of how much matter is in an object / mass does not depend on location
Weight is the gravitational force acting on the probe / weight depends on gravitational field strength
Gravitational field strength varies between Earth, Mars, and deep space / g differs from planet to planet
Since weight = mass × gravitational field strength, a change in g causes a change in weight whilst mass stays constant

Key terms in this question

mass · weight