Physics GCSE Energy – Conservation of Energy
The conservation of energy
What energy changes happen when a pendulum swings?
Answer
At the top of the swing or start, the pendulum ‘bob’ has 100% gravitational potential energy (GPE). As the ‘bob’ swings down it gains kinetic energy (KE) because it is increasing in speed. When the ‘bob’ is at the bottom of the swing, for a moment in time it has 100% KE and no potential energy (PE). As it starts to swing upwards, the KE decreases and PE starts to increase. Halfway up or down the swing the pendulum has 50% KE and 50% PE.
Please note:
Note: You may be asked why the pendulum stops swinging. This is due to air resistance pushing against the pendulum and also friction at the top of the pendulum. You can call air resistance ‘drag’, but air resistance is more accurate.
Describe the energy changes when a bungee jumper jumps off a bridge.
Answer
In the very first moment the jumper has 100% GPE. As soon as the jumper starts accelerating towards the Earth GPE starts to decrease and KE increases. The rope becomes tight, slowing the jumper down, so KE decreases but as the rope stretches, its own elastic energy increases. This elastic energy continues to increase until the jumper reaches the bottom of the jump or maximum length of the bungee cord. For an instant the KE is zero, GPE is zero and all the energy has been transferred to elastic potential energy. The jumper then starts to move back upwards so KE increases, as does GPE. GPE will not reach its initial value as the jumper would not reach the start height. This is because energy will be dissipated to the surroundings due to air resistance and a small amount of heat loss through friction.