A firefighter standing on a smooth, frictionless platform fires a water hose horizontally. The jet of water travels in one direction and the firefighter and platform begin to move in the opposite direction. Explain why the firefighter and platform move, and describe what happens to the speed of the firefighter and platform as they continue to move while water is being fired.

WJEC A-Level Physics (Wales) — 1.4 Momentum (A-Level only) · Explain · 5 marks · View as Markdown

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

A firefighter stands on a smooth, frictionless platform on wheels. When the firefighter opens the hose and a continuous jet of water is fired horizontally, the firefighter and platform begin to move in the opposite direction to the water.

Model answer (5 marks)

The system is initially at rest, so its total momentum is zero.

The law of conservation of momentum says that the total momentum of a closed system cannot change. Therefore, when the water leaves the hose, the momentum it gains must be balanced by an equal and opposite momentum of the firefighter and platform.

As the jet of water is fired, the firefighter and platform acquire momentum in the opposite direction to the water, so their speed increases. The larger the mass flow rate of water or the higher the speed of the jet, the greater the rate of change of momentum of the firefighter and platform, and the faster they accelerate.

Thus the firefighter and platform move because the water jet carries momentum in one direction, and by conservation of momentum they must move in the opposite direction, gaining speed as long as water is being fired.

Examiner tips

  • Use the phrase "conservation of momentum" and state the system’s initial momentum is zero.
  • Show that the water’s momentum is balanced by the firefighter’s momentum, giving a clear opposite‑direction explanation.
  • Explain that the speed increases because momentum is continually added, linking mass flow rate or jet speed to acceleration.
  • Mention that the acceleration is proportional to the rate of change of momentum (force).

Common mistakes

  • Failing to state that the system’s initial momentum is zero.
  • Confusing the direction of the firefighter’s motion with the direction of the water jet.
  • Not linking the rate of mass flow or jet speed to the rate of change of momentum (acceleration).

Mark scheme (5 marks)

  1. The total momentum of the system before the hose is fired is zero (the system is stationary / at rest).
  2. The law of conservation of momentum states that the total momentum of a closed system remains constant / is conserved.
  3. As the water gains momentum in one direction, the firefighter and platform must gain equal momentum in the opposite direction so that total momentum remains zero.
  4. While water continues to be fired, the firefighter and platform continue to gain momentum / accelerate (their speed increases).
  5. The greater the rate at which water is fired (greater mass of water per second or higher speed of water), the greater the rate of change of momentum of the firefighter and platform / the faster they accelerate.

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