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Year 10
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The energy of an object in a gravitational field (dGPE=m x g x dh)

I can calculate the change in energy of an object because of its movement in a gravitational field.

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Year 10
Edexcel
Foundation

The energy of an object in a gravitational field (dGPE=m x g x dh)

I can calculate the change in energy of an object because of its movement in a gravitational field.

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Lesson details

Key learning points

  1. When an object moves upwards, work is done against the gravitational force.
  2. The gravitational force acting on an object does work to speed it up when it is falling.
  3. Weight = mass x gravitational field strength, W = mg
  4. Change in gravitational potential energy of an object = mass × gravitational field strength × change in height

Keywords

  • Gravitational store - Objects have energy in the gravitational store because of their mass and their height.

  • Work done - Work is done whenever a force makes an object move. The amount of work done is equal to the force multiplied by distance moved in the direction of the force.

  • Weight - Weight = mass × gravitational field strength, measured in newtons.

  • Gravitational field strength - Gravitational field strength, g, is the force per kilogram caused by a gravitational field.

  • Gravitational potential energy - The energy stored in an object due to its height above the ground.

Common misconception

Pupils often think that the amount of energy in the gravitational store depends on the path taken by an object to change its height, or the speed of an object at a particular height.

Teaching should include examples of objects increasing height by the same amount by different routes, and examples of objects at the same height moving at different speeds.

If it is appropriate for your class, this is a good lesson for giving pupils calculations that include values written in standard form.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which of the following properties of an object affects the amount of energy in its gravitational store?
Correct answer: height
Correct answer: mass
speed
temperature
Q2.
Why does a free–falling object accelerate downwards?
energy is created in the kinetic store
energy is lost from the gravitational store
Correct answer: energy is transferred to the kinetic store
Correct answer: energy is transferred from the gravitational store
Q3.
A pupil lifts a box of books off the floor then puts it back down again. How much work is done (compared to the first time) if the box is lifted again to 4× the height?
16× more
Correct answer: 4× more
the same amount
4× less
16× less
Q4.
Which of the following is the unit of weight?
kilograms (kg)
Correct answer: newtons (N)
newtons per kilogram (N/kg)
pounds (lb)
Q5.
How would you calculate the work done lifting a sack of onions onto a table?
work done = mass of the onions × height of the table
work done = mass of the onions ÷ height of the table
Correct answer: work done = weight of the onions × height of the table
work done = weight of the onions ÷ height of the table
Q6.
The gravitational force on Earth is 6× greater than it is on the Moon. Starting with the one with the most energy, put the tools in order of decreasing energy in the gravitational store.
1 - A hammer has a mass of 2 kg and is on Earth.
2 - An axe has a mass of 1.5 kg and is on Earth.
3 - A drill has a mass of 3 kg and is on the Moon.
4 - An electric saw has a mass of 2 kg and is on the Moon.

Exit quiz

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6 Questions

Q1.
How much energy will be transferred to the gravitational store if 2× the amount of work is done lifting a tray of cakes?
the same amount
Correct answer: 2× more
4× more
Q2.
Which of these identical basketballs has the most energy in the gravitational store?
Correct answer: A basketball stuck behind the hoop, at a height of 3.0 m.
A basketball flying through the air at a height of 2.9 m.
A basketball falling from the roof of a sports hall, at a height of 2.8 m.
A basketball that is moving very quickly up into the air, at a height of 2.4 m.
Q3.
What happens to the amount of energy in the gravitational store of a pile of bricks on a table if more bricks are added to double the mass?
The amount of energy in the gravitational store does not change.
Correct answer: The amount of energy in the gravitational store is two times bigger.
The amount of energy in the gravitational store is four times bigger.
Q4.
What is gravitational potential energy?
The type of energy that is in the gravitational store.
The type of energy an object has because it is above the ground.
Correct answer: The amount of energy that is in the gravitational store.
The amount of energy an object has because it is above the ground.
Q5.
What is the equation for gravitational potential energy?
$$ΔGPE = m Δh$$
Correct answer: $$ΔGPE = m g Δh$$
$$ΔGPE = mgΔh^2$$
$$ΔGPE = ½mh^2$$
Q6.
What is the gravitational energy of a discus of mass 2 kg moving at a speed of 8 m/s that is 9 m above the ground? (Gravitational field strength = 10 N/kg)
Correct Answer: 180 J, 180 Joules, 180, 180 j, 180 Joule