A student uses a newton-meter to measure the weight of a steel ball bearing on Earth, where the gravitational field strength is 9.8 N/kg. The same steel ball bearing is then taken to the surface of Venus, where the gravitational field strength is 8.9 N/kg. Explain why the weight of the ball bearing on Venus is different from its weight on Earth, 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)

Weight is the gravitational force acting on an object, which depends on the gravitational field strength. The ball bearing has the same mass on Earth and on Venus, because mass is a measure of the amount of matter and does not change with location. On Earth the gravitational field strength is 9.8 N kg⁻¹, so the weight is W=mg. On Venus the field strength is only 8.9 N kg⁻¹, a smaller value, so the same mass experiences a smaller force: W=mg gives a smaller weight. Therefore the weight on Venus is less than on Earth, even though the mass is unchanged.

Examiner tips

  • Use the formula W=mg to link weight, mass and g; mention that mass is constant; explain why g differs between planets.
  • Show the numerical relationship: 9.8 vs 8.9 N kg⁻¹ to justify the change in weight.

Common mistakes

  • Confusing mass with weight and saying mass changes; writing weight is the same because mass is the same; ignoring the difference in gravitational field strength.

Mark scheme (4 marks)

  1. Weight is the gravitational force acting on an object due to a gravitational field / weight depends on gravitational field strength
  2. The gravitational field strength on Venus is different (lower) than on Earth, so the gravitational force on the ball bearing is different (smaller)
  3. Mass is a measure of the amount of matter in the object / mass does not depend on gravitational field strength
  4. Since mass stays the same but gravitational field strength on Venus is smaller, weight on Venus is smaller (W = m × g, so a smaller g gives a smaller weight)

Key terms in this question

weight · mass · gravitational field strength

Related

More Mass and weight questions

▶ Try answering this question with AI marking (free) →