KE, GPE & Elastic Energy
- An object can have one of the following forms of energy:
- Kinetic energy when it is in motion (KE)
- Gravitational potential energy when it is at a height above the Earth's surface (PE)
- Elastic energy when it is being stretched or compressed (EPE)
- Kinetic energy is defined as:
The energy an object possesses due to its motion
- This means that any object that is moving has kinetic energy
- If an object speeds up, it gains more kinetic energy
- If an object slows down, it loses kinetic energy
- Gravitational potential energy is defined as:
The energy an object possesses due to its position
- This means:
- If an object is lifted up, it gains gravitational potential energy
- If an object falls, it loses gravitational potential energy
- An example of an energy transfer between gravitational potential and kinetic energy is a ball dropped from a height
- Before it is dropped the ball has zero kinetic energy because it is stationary but has maximum gravitational potential energy
- As it is dropped the gravitational potential energy is transferred into kinetic energy
- As the ball loses gravitational potential energy, it gains kinetic energy
Energy Transfer on a Ball
As it falls the ball loses gravitational potential energy and gains kinetic energy
- An example of an energy transfer between kinetic, gravitational potential and elastic energy is the oscillation of a mass on a spring
- The total energy of the mass-spring system will remain constant
- However, there will be different amounts of each energy form transferred as it moves through its different positions
KE, PE and EPE in a Spring
Energy changes take place when a spring is stretched and compressed about its equilibrium point at the orange line
- At position A:
- The spring has some elastic energy since it is slightly compressed
- The spring has zero kinetic energy because it is stationary
- The spring has a maximum amount of gravitational potential energy because the mass is at its highest point
- At position B:
- The spring has some elastic energy since it is slightly stretched
- The spring has maximum kinetic energy because it passes through its resting position at maximum speed
- The spring has some gravitational potential energy since the mass is still above its lowest point in the oscillation
- At position C:
- The spring has maximum elastic energy because it is at its maximum extension
- The spring has zero kinetic energy since it is stationary
- The gravitational potential energy is at a minimum because it is at its lowest point in the oscillation
Worked example
The diagram below shows a student before and after a bungee jump.
Describe the energy conversions taking place during the jump.
Answer:
Step 1: Consider the energy on the platform before the jump
- Before the jump, the bungee jumper has:
- Maximum gravitational potential energy because he is at the maximum height
- Zero kinetic energy because he is not moving
- Zero elastic energy because the bungee chord is unstretched
Step 2: Consider the energy conversion as the bungee jumper falls before the chord extends
- During the fall and before the chord extends, the bungee jumper has:
- Decreasing gravitational potential energy because he is losing height
- Increasing kinetic energy because he is gaining speed
- Zero elastic energy because the chord is unstretched
- The energy conversions are: gravitational potential energy → kinetic energy
Step 3: Consider the energy conversion as the bungee jumper falls whilst the chord is extended
- During the fall and as the chord extends, the bungee jumper has:
- Decreasing gravitational potential energy because he is losing height
- Decreasing kinetic energy because he is being slowed by the chord as it stretches
- Increasing elastic energy because the chord is being stretched
- The energy conversions are: gravitational potential energy → kinetic energy → elastic energy
Step 4: Consider the energy transfer of the jumper at the end of the jump
- At the bottom of the jump, the bungee jumper has:
- Minimum gravitational potential energy because he is at the lowest point of the jump
- Zero kinetic energy because he is not moving
- Maximum elastic energy because the chord is fully stretched
Exam Tip
In an energy transfer question, think carefully about which energy type has the maximum and minimum values and where. Then work from there to determine the transfers taking place between them. You only need to know about the three energy transfers described here, so don't worry about any others you may come across.
