A ball is dropped from rest and falls vertically downwards through the air, eventually reaching terminal velocity. Describe how the kinetic energy and the gravitational potential energy of the ball change from the moment it is released until it reaches terminal velocity, and explain why the ball's total mechanical energy decreases during this fall.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Kinetic energy increases as the ball speeds up.
Gravitational potential energy decreases as the ball loses height.
Some of the lost potential energy is transferred to the air as heat and sound.
Because energy is taken away by air resistance, the increase in kinetic energy is less than the decrease in potential energy, so the ball’s total mechanical energy falls.
Gravitational potential energy decreases as the ball loses height.
Some of the lost potential energy is transferred to the air as heat and sound.
Because energy is taken away by air resistance, the increase in kinetic energy is less than the decrease in potential energy, so the ball’s total mechanical energy falls.
Examiner tips
- Use the terms kinetic energy, gravitational potential energy and total mechanical energy. Show the direction of change (increase/decrease) for each. Explain that air resistance is a non‑conservative force that removes energy from the system.
Common mistakes
- Saying the ball’s kinetic energy stays constant. Claiming the total mechanical energy stays the same. Forgetting to mention the role of air resistance or heat.
Mark scheme (4 marks)
- Kinetic energy increases as the ball falls (and speeds up)
- Gravitational potential energy decreases as the ball falls (loses height)
- Some energy is transferred to the surroundings / lost as thermal (heat) energy due to air resistance / friction with the air
- Total mechanical energy decreases because energy is not conserved within the ball–Earth system / energy is transferred away from the ball by air resistance (so the decrease in GPE is greater than the increase in KE)
Key terms in this question
kinetic energy · gravitational potential energy · terminal velocity · total mechanical energy
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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