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IB Physics SL D 1 3 Point Mass Approximation Questions

Practise IB Physics SL D.1.3 by deciding when extended bodies can be treated as point masses in Newtonian gravitation.

Syllabus
First assessment 2025
Course
Physics SL
Level
SL

Exam points

  • State the conditions that justify treating an extended body as a point mass.
  • Explain why a spherical uniform body can be modelled from its centre outside the body.
  • Apply the approximation to satellite and planetary force situations.

Learn this topic

Study D.1.3—Point-mass approximation conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

IB Physics SL D 1 3 Point Mass Approximation Questions question 1

[Maximum number: 1]

Ion-thrust engines can power spacecraft. In this type of engine, ions are created in a chamber and expelled from the spacecraft. The spacecraft is in outer space when the propulsion system is turned on. The spacecraft starts from rest.

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The mass of ions ejected each second is 6.6×10−6 kg6.6 \times 10^{-6} \mathrm{~kg} and the speed of each ion is 5.2×104 m s−15.2 \times 10^{4} \mathrm{~m} \mathrm{~s}^{-1}. The initial total mass of the spacecraft and its fuel is 740 kg . Assume that the ions travel away from the spacecraft parallel to its direction of motion.

On arrival at the planet, the spacecraft goes into orbit as it comes into the gravitational field of the planet.

Newton's law of gravitation applies to point masses. Suggest why the law can be applied to a satellite orbiting a spherical planet of uniform density.

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