A pendulum bob is pulled to one side and released. It swings down to the lowest point of its path and continues upward to the other side. Explain the energy transfers that occur from the moment the bob is released until it reaches the highest point on the other side, and explain why the bob does not rise to exactly the same height as the starting position.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A pendulum consists of a small metal bob attached to a string. The bob is pulled to one side, raising it above its lowest position, and then released from rest.
Model answer (4 marks)
1. When the bob is released it has maximum GPE and zero KE.
2. As it swings down, GPE is converted into KE, so KE increases while GPE decreases.
3. At the lowest point all the initial GPE has become KE.
4. As it swings up on the other side, KE is converted back into GPE, but some energy is lost as heat and sound from air resistance and pivot friction, so the GPE at the top is less than the initial GPE and the bob does not reach the original height.
2. As it swings down, GPE is converted into KE, so KE increases while GPE decreases.
3. At the lowest point all the initial GPE has become KE.
4. As it swings up on the other side, KE is converted back into GPE, but some energy is lost as heat and sound from air resistance and pivot friction, so the GPE at the top is less than the initial GPE and the bob does not reach the original height.
Examiner tips
- Use the correct terms GPE, KE and waste energy; show the energy flow direction; mention air resistance/friction; explain why height is lower
Common mistakes
- Confusing GPE with KE; forgetting to mention energy loss; writing that the bob reaches the same height
Mark scheme (4 marks)
- As the bob swings downward, gravitational potential energy (GPE) is transferred to kinetic energy (KE).
- As the bob swings upward on the other side, kinetic energy (KE) is transferred back to gravitational potential energy (GPE).
- Some energy is transferred to the surroundings as heat (and sound) due to air resistance / friction at the pivot — this is a waste energy transfer.
- Because energy has been dissipated to the surroundings, less GPE is stored at the top of the other side, so the bob does not reach the original height — total energy in the system has decreased.
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Conservation of energy questions
- A cyclist rides along a flat road and then brakes to a complete stop. Explain wh…
- A student uses an electric kettle to heat water for a hot drink. The kettle is d…
- A battery-powered toy car is switched on and accelerates across a smooth floor. …
- A roller coaster car starts from rest at the top of a hill and rolls down to the…