Explain why the total mechanical energy of a skydiver falling through the atmosphere is not conserved, and describe the energy transformation that occurs as the skydiver reaches terminal velocity.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A skydiver jumps from a stationary helicopter and falls through the atmosphere. After some time, the skydiver reaches terminal velocity.
Model answer (4 marks)
1. Air resistance is a non‑conservative force that does negative work on the skydiver.
2. The work done by the drag force converts the skydiver’s mechanical energy into thermal/internal energy (heat) in the air.
3. At terminal velocity the net force is zero, so the drag force equals the weight of the skydiver.
4. Consequently, the loss of gravitational potential energy per unit time equals the rate at which energy is dissipated as heat; the kinetic energy remains constant.
2. The work done by the drag force converts the skydiver’s mechanical energy into thermal/internal energy (heat) in the air.
3. At terminal velocity the net force is zero, so the drag force equals the weight of the skydiver.
4. Consequently, the loss of gravitational potential energy per unit time equals the rate at which energy is dissipated as heat; the kinetic energy remains constant.
Examiner tips
- Use the term ‘non‑conservative force’ and ‘negative work’ to show understanding of energy loss.
- Show the balance of forces at terminal velocity (drag = weight).
- Explain that kinetic energy is constant, so all lost potential energy becomes heat.
Common mistakes
- Confusing the direction of work with the sign of the force; forgetting that drag does negative work.
- Failing to mention that kinetic energy remains constant at terminal velocity.
- Using vague terms like ‘energy is lost’ without specifying conversion to heat.
Mark scheme (4 marks)
- Air resistance (drag) acts on the skydiver, doing negative work on the skydiver / a non-conservative force acts on the skydiver
- The work done by air resistance converts mechanical energy into thermal/internal energy (heat), so total mechanical energy decreases
- At terminal velocity the net force on the skydiver is zero, so the drag force equals the weight / gravitational force
- At terminal velocity, the loss in gravitational potential energy per unit time equals the rate at which energy is dissipated as thermal energy (the kinetic energy remains constant)
Key terms in this question
mechanical energy · terminal velocity
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
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- Decode the mark scheme abbreviations →