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A2 Daily A Level Physics question

2025-08-14 OCR A A Level Physics 5.4.3(b) 5.4.4(b)

A satellite is in a stable circular orbit around a planet. A very small, constant resistive force due to the planet's upper atmosphere acts on the satellite, causing its orbital radius to gradually decrease. Which statement correctly describes the changes to the satellite's kinetic energy and its total mechanical energy (kinetic + gravitational potential)?

  1. A Both its kinetic energy and its total mechanical energy increase.
  2. B Its kinetic energy increases, and its total mechanical energy remains constant.
  3. C Its kinetic energy decreases, and its total mechanical energy decreases.
  4. D Its kinetic energy increases, while its total mechanical energy decreases. (correct)

Answer

The correct answer is D.

The resistive force (drag) does negative work on the satellite, as the force opposes the direction of motion. This means energy is removed from the satellite system, so its total mechanical energy must decrease. For a satellite in a circular orbit, the gravitational force provides the centripetal force: GMm/r² = mv²/r. This can be rearranged to show that the kinetic energy is KE = ½mv² = GMm/2r. As the drag causes the orbital radius 'r' to decrease, the kinetic energy must increase, meaning the satellite speeds up. Therefore, the kinetic energy increases while the total mechanical energy decreases.

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