A spacecraft is travelling through deep space, far from any planet or star, with its engines switched off. A student claims that the spacecraft will gradually slow down and eventually stop. Explain why the student is wrong, using Newton's First Law in your answer.

Pearson Edexcel International GCSE Physics (4PH1) — 1.4 Astronomy · Explain · 4 marks · View as Markdown

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

A spacecraft is moving through deep space at a constant speed of 3000 m/s. Its engines are switched off and there are no significant gravitational forces acting on it.

Model answer (4 marks)

Newton's First Law states that an object continues at constant velocity if the resultant force acting on it is zero.
In deep space there is no air resistance (or friction / resistive force) acting on the spacecraft.
The resultant force on the spacecraft is therefore zero.
Therefore the spacecraft continues to move at a constant velocity and will not slow down.

Examiner tips

  • Use the exact wording of Newton's First Law; mention ‘resultant force is zero’. Show the chain: no forces → zero resultant → constant velocity. Keep answer short and to the point to fit the 4‑mark limit.

Common mistakes

  • Confusing ‘speed’ with ‘velocity’ and ignoring direction. Mentioning forces that do not exist (e.g., air resistance) in space. Failing to state that the resultant force is zero.

Mark scheme (4 marks)

  1. Newton's First Law states that an object continues at constant velocity if the resultant force acting on it is zero
  2. In deep space there is no air resistance (or friction / resistive force) acting on the spacecraft
  3. The resultant force on the spacecraft is therefore zero
  4. Therefore the spacecraft continues to move at a constant velocity and will not slow down

Key terms in this question

Newton's First Law

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