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The answer sheet for the worksheet Gravity, weight and orbits. Each answer comes with the reason, so it can be used to mark or to go over the questions with the class.

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KS3 · Ages 11 to 14 (Years 7 to 9)Answer sheet for teachers

Answers: Gravity, weight and orbits

  1. C: Vesta is a much smaller worldThe smaller the world, the weaker its gravity. Vesta is tiny compared with Earth, so its g is only about 0.25 N/kg.
  2. B: The asteroid orbits the Sun three times for each orbit of JupiterA resonance links two orbits by a simple ratio. In this one, the asteroid completes three orbits in the time Jupiter completes one.
  3. C: Each kilogram is pulled down with a force of 3.7 NGravitational field strength is the force of gravity on each kilogram of mass, so every kilogram on Mars weighs 3.7 N.
  4. A: They move sideways so fast that they keep missing itGravity pulls each asteroid towards the Sun, but its fast sideways motion means it keeps missing, so it travels around the Sun instead.
  5. A: Jupiter's tugs at resonances moved asteroids outThe gaps are the resonances. Asteroids there have their orbits stretched by Jupiter until they leave the belt, which is why resonances act as escape hatches.
  6. True (29.6 N)First find the mass: 80 ÷ 10 = 8 kg. Then W = 8 × 3.7 = 29.6 N on Mars.
  7. True (About 30 times)11 ÷ 0.36 ≈ 30, so rock must move about 30 times faster to escape Earth than to escape Vesta.
  8. 40 times smaller (accept any answer with the same meaning)120 ÷ 3 = 40, because Vesta's g of 0.25 N/kg is 40 times smaller than Earth's 10 N/kg.
  9. 5 kg (accept any answer with the same meaning)Rearrange W = m × g to m = W ÷ g = 8 ÷ 1.6 = 5 kg.
  10. About 125 N (accept any answer with the same meaning)W = m × g = 5 × 25 = 125 N, much more than its 50 N weight on Earth.

Task

Answers will vary. The task comes from the lesson: Find the gravitational field strength on Mercury (3.7 N/kg), Mars (3.7 N/kg) and Jupiter's cloud tops (about 25 N/kg). Calculate the weight of your school bag on each. Then write two sentences explaining why your mass would not change on any of them.

Look for work that uses the key words correctly and gives a reason for each point.

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