A skateboarder starts from rest at the top of a slope. She rolls down the slope, reaching a speed of 8 m/s after 4 seconds of uniform acceleration. She then travels along a flat section of track at a constant speed of 8 m/s for 10 seconds before gradually decelerating to a stop. Describe the motion of the skateboarder in each of the three stages of her journey, and explain what the constant speed stage tells us about the forces acting on the skateboarder during that stage.

Edexcel A-Level Physics (9PH0) — 2.1 Motion · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

Stage 1 – The skateboarder starts from rest and accelerates uniformly down the slope, reaching 8 m s⁻¹ after 4 s.
Stage 2 – She then moves along the flat track at a constant speed of 8 m s⁻¹ for 10 s.
Stage 3 – Finally she slows down, decelerating until she comes to a stop.
The constant‑speed stage shows that the forces acting on her are balanced; the resultant force is zero, so there is no acceleration (Newton’s First Law).

Examiner tips

  • Use the word ‘accelerates’ for stage 1, ‘constant speed’ for stage 2, ‘decelerates’ for stage 3. Mention the zero resultant force and Newton’s First Law for the constant‑speed stage. Keep each stage concise – one sentence each – to hit the 5 marks.

Common mistakes

  • Confusing ‘uniform acceleration’ with ‘constant speed’ in stage 1. Forgetting to state that the resultant force is zero during the constant‑speed stage. Using vague terms like ‘moves’ instead of ‘accelerates’ or ‘decelerates’.

Mark scheme (5 marks)

  1. Stage 1: the skateboarder accelerates uniformly (from rest / starting at 0 m/s, reaching 8 m/s in 4 s)
  2. Stage 2: the skateboarder travels at constant/steady speed (8 m/s for 10 s)
  3. Stage 3: the skateboarder decelerates (speed decreases / slows down to a stop)
  4. At constant speed, the forces on the skateboarder are balanced / the resultant force is zero
  5. Because there is no resultant force, there is no acceleration (Newton's First Law / an object at constant velocity has zero resultant force)

Key terms in this question

uniform acceleration · constant speed

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