A football is kicked from the ground and travels upward through the air before landing on the ground again. Explain how Newton's first and second laws of motion account for the changes in the football's motion from the moment it leaves the footballer's boot until it hits the ground. You may ignore the effects of air resistance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. When the ball leaves the boot the boot no longer applies a force – the only force is gravity.
2. Newton’s first law states that an object will keep its state of motion unless a net force acts.
3. Because gravity acts downward, a net (unbalanced) force exists on the ball.
4. Newton’s second law (F = ma) tells us that this net force produces an acceleration in the direction of the force.
5. The constant downward acceleration from gravity decelerates the ball while it rises and then accelerates it downward as it falls.
2. Newton’s first law states that an object will keep its state of motion unless a net force acts.
3. Because gravity acts downward, a net (unbalanced) force exists on the ball.
4. Newton’s second law (F = ma) tells us that this net force produces an acceleration in the direction of the force.
5. The constant downward acceleration from gravity decelerates the ball while it rises and then accelerates it downward as it falls.
Examiner tips
- Use the exact wording ‘no longer a contact force from the boot’ to gain the first point. Show the chain: boot force stops → only gravity → net force → acceleration. Mention the direction of acceleration (downward).
Common mistakes
- Forgetting to state that the boot force ceases when the ball leaves. Confusing the direction of acceleration with the direction of velocity. Omitting the reference to Newton’s first law as a prerequisite for the second law.
Mark scheme (5 marks)
- Once the ball leaves the boot there is no longer a contact force from the boot acting on it (the only force acting is gravity/weight)
- Newton's first law: an object remains at rest or moves at constant velocity unless acted on by a resultant force
- Because weight/gravity acts downward there is a resultant (unbalanced) force on the ball
- Newton's second law: a resultant force causes an acceleration in the direction of the force (F = ma)
- The downward resultant force (weight) causes a constant downward acceleration, so the ball decelerates as it rises and accelerates downward as it falls
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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