A pendulum swings back and forth in simple harmonic motion. Describe the energy changes that occur during one complete oscillation of the pendulum, starting from the moment it passes through its equilibrium position moving towards its maximum displacement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A simple pendulum is released from rest at its maximum displacement and swings freely. It passes through its equilibrium (rest) position and continues to its maximum displacement on the other side, before returning.
Model answer (5 marks)
1. At the equilibrium position the pendulum has maximum kinetic energy and zero gravitational potential energy.
2. As it moves from equilibrium towards the maximum displacement, its kinetic energy decreases while its gravitational potential energy increases.
3. At the maximum displacement (amplitude) the pendulum is momentarily stationary, so its kinetic energy is zero and its gravitational potential energy is at a maximum.
4. When it swings back towards equilibrium, the gravitational potential energy is converted back into kinetic energy.
5. Throughout the oscillation the total mechanical energy (kinetic + gravitational potential) remains constant, provided there is no friction or air resistance.
2. As it moves from equilibrium towards the maximum displacement, its kinetic energy decreases while its gravitational potential energy increases.
3. At the maximum displacement (amplitude) the pendulum is momentarily stationary, so its kinetic energy is zero and its gravitational potential energy is at a maximum.
4. When it swings back towards equilibrium, the gravitational potential energy is converted back into kinetic energy.
5. Throughout the oscillation the total mechanical energy (kinetic + gravitational potential) remains constant, provided there is no friction or air resistance.
Examiner tips
- Use the word 'maximum' and 'zero' to show extremes of energy. Show the direction of energy transfer (kinetic → potential → kinetic).
Common mistakes
- Confusing kinetic and potential energy at the equilibrium position. Forgetting to mention that total energy is conserved in an ideal pendulum.
Mark scheme (5 marks)
- At the equilibrium position, the pendulum has maximum kinetic energy
- As the pendulum moves from equilibrium towards maximum displacement, kinetic energy decreases (is converted to gravitational potential energy)
- At maximum displacement (amplitude), the pendulum has maximum gravitational potential energy and zero kinetic energy / it is momentarily stationary
- As the pendulum swings back towards equilibrium, gravitational potential energy is converted back into kinetic energy
- The total energy (kinetic + gravitational potential) remains constant throughout the oscillation (in the absence of energy losses / assuming no friction or air resistance)
Key terms in this question
simple harmonic motion · equilibrium position · maximum displacement
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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