A skateboarder standing still on a flat, frictionless surface pushes against a wall. After pushing, the skateboarder moves away from the wall. Explain, using Newton's laws of motion and the concept of momentum, why the skateboarder moves away from the wall and what happens to the skateboarder's speed once they have left the wall.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A skateboarder with a mass of 60 kg stands stationary on a frictionless surface and pushes against a fixed wall. After the push, the skateboarder rolls away from the wall.
Model answer (5 marks)
The skateboarder and the wall exert equal and opposite forces on each other (Newton’s third law). The force exerted by the wall on the skateboarder is directed away from the wall, so the resultant force on the skateboarder is non‑zero and, by Newton’s second law, the skateboarder accelerates in that direction and gains momentum.
Before the push the total momentum of the system (skateboarder + wall + Earth) is zero because both are at rest. During the push the skateboarder gains momentum in one direction; the wall (and the Earth) gains an equal amount of momentum in the opposite direction, so the total momentum of the system remains zero – momentum is conserved.
Once the skateboarder has left the wall there is no further force acting on them (the surface is frictionless). By Newton’s first law the skateboarder will then continue to move at a constant speed, i.e. their speed remains unchanged after leaving the wall.
Before the push the total momentum of the system (skateboarder + wall + Earth) is zero because both are at rest. During the push the skateboarder gains momentum in one direction; the wall (and the Earth) gains an equal amount of momentum in the opposite direction, so the total momentum of the system remains zero – momentum is conserved.
Once the skateboarder has left the wall there is no further force acting on them (the surface is frictionless). By Newton’s first law the skateboarder will then continue to move at a constant speed, i.e. their speed remains unchanged after leaving the wall.
Examiner tips
- Mention Newton’s third law first to justify the force pair.
- Show that the force on the skateboarder is non‑zero and use F=ma to explain acceleration.
- State that total momentum is zero before the push and remains zero – conservation of momentum.
- Explain that after the push no forces act, so speed stays constant (Newton’s first law).
Common mistakes
- Forgetting to mention the wall’s reaction force or the conservation of momentum.
- Confusing the direction of the force with the direction of motion.
- Assuming the skateboarder slows down after leaving the wall because of a missing force.
Mark scheme (5 marks)
- The skateboarder and wall exert equal and opposite forces on each other (Newton's third law / action-reaction pair)
- The resultant force on the skateboarder causes them to accelerate / gain momentum in the direction away from the wall (Newton's second law)
- Before the push, the total momentum of the system is zero as both the skateboarder and wall are stationary
- The skateboarder gains momentum in one direction; the wall (and Earth) gains equal momentum in the opposite direction, so total momentum is conserved (conservation of momentum)
- Once the skateboarder leaves the wall there is no resultant force (frictionless surface), so by Newton's first law the skateboarder continues at constant speed
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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