Section 1 of 5One Swing of a Simple Pendulum
1
At the highest point the bob is momentarily still, so its kinetic energy is zero and its gravitational potential energy is at a maximum.
2
As the bob falls, energy transfers from the gravitational potential store to the kinetic store.
3
At the lowest point the speed is greatest, so the kinetic store holds the most energy.
4
As the bob rises on the other side, kinetic energy transfers back to the gravitational potential store.
5
Without air resistance the bob returns to its starting height, and the cycle repeats.
Section 2 of 5Oscillating Systems
Simple pendulum
- A mass swinging on a string exchanges energy between its gravitational potential and kinetic stores.
Mass on a spring
- A mass bouncing on a spring exchanges energy between its elastic potential and kinetic stores.
Bungee jump
- The jumper's energy cycles between gravitational, elastic potential and kinetic stores during each bounce.
Section 3 of 5Ideal and Real Oscillations
Ideal and Real Oscillations
Ideal system
- With no air resistance or friction, the total energy stays constant and the amplitude does not decrease.
Real system
- Air resistance and friction transfer energy to the thermal store of the surroundings, so the amplitude decreases over time.
Dissipation
- The lost energy is dissipated to the surroundings, not destroyed, so the total energy is still conserved.
Total energy
- In an ideal system the total equals the sum of kinetic and gravitational potential energy at every instant.
Section 4 of 5Worked Example
Question
- At the lowest point of its swing, an ideal pendulum bob has 5.0 J of kinetic energy.
- Assuming no air resistance, find its gravitational potential energy at the highest point of its swing.
- At the lowest point all the energy is in the kinetic store, so the total energy is 5.0 J
- At the highest point the bob is momentarily at rest, so Ek = 0
- Total energy is conserved, so Ep at the highest point = 5.0 − 0 = 5.0 J
Answer 5.0 J.
Section 5 of 5Exam Checklist & Tips
2.2.2 – Key Points for the Exam
- Oscillation A repeated back-and-forth motion about a central equilibrium position.
- Equilibrium position The position at which an oscillating object would rest if undisturbed.
- Amplitude The maximum displacement of an oscillating object from its equilibrium position.
- Total energy The sum of all the energy in the stores of an oscillating system at a given moment.
- Dissipation The spreading out of energy to the thermal store of the surroundings, where it becomes hard to use.
Common Mistakes
- Saying the pendulum has maximum kinetic energy at its highest point, when its speed is actually zero there.
- Saying energy is lost or destroyed when a real pendulum slows down, rather than dissipated to the surroundings.
- Claiming the total energy changes during an ideal oscillation, when it stays constant.