Transfer Between KE & GPE

This lesson covers:

  1. Energy transfers between the kinetic energy and gravitational potential energy stores
  2. How to calculate these energy transfers 

KE & GPE


We've covered in previous lessons that energy can be stored in different forms and that it can be transformed from one form to another. One of the transfers you need to know about is the transfer between kinetic energy (KE) and gravitational potential energy (GPE) as an object rises or falls.


The key concepts you need to understand are:

  1. As an object rises, KE is converted to GPE
  2. As an object falls, GPE is converted to KE 
  3. KE lost = GPE gained and KE gained = GPE lost

GPE and KE of a roller coaster


The diagram below shows a roller coaster ride.

Diagram showing the energy transfer of a roller coaster car from gravitational potential energy at point A to kinetic energy at point B.

Energy transfers of a roller coaster car


We can use the equations for kinetic energy (KE) and gravitational potential energy (GPE) to calculate the energy at different points through the ride.


  1. At point A, the GPE of the car is at it's highest value and the KE is zero. 
  2. As the car descends towards point B, the GPE store decreases as energy is transferred mechanically to the KE store of the car. 
  3. The GPE lost is equal to the KE gained. 


A small amount of energy will be lost due to:

  • Friction between the car and the track.
  • Air resistance acting on the car.


These losses are not taken into account in calculations, unless stated otherwise.


Worked example - Calculate the energy at point A


The mass of the roller coaster car is 450 kg, and the car is stationary at point A.

Calculate the car's GPE at point A.

Worked example - Calculate the kinetic energy at point B


Calculate the KE of the rollercoaster car at point B.

electrical / mechanical / elastic / magnetic


As a ball falls through the air, energy is transferred from the ball's gravitational potential energy store to its kinetic energy store. The energy is transferred between the two energy stores via energy transfer.

mechanical

0

/

1

A roller coaster car has 200 kJ of gravitational potential energy and zero kinetic energy when at the top of a 50 m ramp. 


State the gravitational potential energy when the roller coaster is at 25 m above the ground.

200,000 J

0 kJ

200 kJ

100,000 J

0

/

1

A roller coaster car of mass 500 kg is initially stationary. It then descends through a distance of 20 m. 


Calculate the kinetic energy of the roller coaster car after descending 20 m.

100,000 J

0 J

10,000 J

25 J

0

/

1

A ball is thrown vertically upwards. 


Which of the following shows the correct energy stores for the ball at the highest point.

GPE = maximum    and    KE = minimum

GPE = maximum    and    KE = maximum

GPE = minimum    and    KE = maximum

GPE = minimum    and    KE = minimum

0

/

1

A ball of mass 0.2 kg is thrown upwards with a speed of 2 m/s. 


Calculate the maximum height reached by the ball. 

0

/

5