AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Changes in energy: The amount of gravitational potential energy gained by an object raised above ground level can be calculated using the equation:
When an object is lifted, work is done against gravity and energy is transferred to the gravitational potential energy store.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
When an object is lifted, work is done against gravity and energy is transferred to the gravitational potential energy store. The gain is calculated using Ep = mgh, where m is mass in kilograms (kg), g is gravitational field strength in newtons per kilogram (N/kg), and h is height in metres (m). On Earth, g is 9.8 N/kg in AQA GCSE questions. For example, lifting a 2 kg mass by 3 m gives Ep = 2 × 9.8 × 3 = 58.8 J. The equation gives the gain in gravitational potential energy, so it assumes the object started at the reference level. If the object is lowered, the same equation gives the energy transferred away from the store.
What earns the marks
- State the equation for gravitational potential energy gain as Ep = mgh.
- Identify m as mass in kilograms, g as gravitational field strength in newtons per kilogram, and h as height in metres.
- Substitute values correctly, for example Ep = 2 kg × 9.8 N/kg × 3 m = 58.8 J.
- Explain that the equation calculates the gain in gravitational potential energy when an object is raised.
- Use the value g = 9.8 N/kg or the value given in the question, and do not round to 10 N/kg unless instructed.
- Recognise that the energy transferred equals the work done against gravity.
Common mistakes and how to put them right
Using mass in grams instead of kilograms.
Put it right by dividing grams by 1000 before substitution.
Using height in centimetres instead of metres.
Put it right by dividing centimetres by 100.
Confusing g with acceleration in m/s² and using the wrong unit.
Put it right by using N/kg for gravitational field strength in this equation.
Exam tips
- Write the equation, then substitute values with units before calculating the final answer.
- Check that the answer is in joules and that the unit is included.
- Always use g = 9.8 N/kg for Earth unless the question explicitly tells you to use a different value.
Try the lesson
The start of the Changes in energy: The amount of gravitational potential energy gained by an object raised above ground level can be calculated using the equation: lesson, made from checked teaching notes for this specification point.
Have a go first
A book of mass 2.5 kg is lifted 3.0 m above the ground. Gravitational field strength is 9.8 N/kg. Which calculation gives the gravitational potential energy gained by the book?
Listen
When an object is lifted, work is done against gravity and energy is transferred to the gravitational potential energy store. The gain is calculated using Ep = mgh, where m is mass in kilograms (kg), g is gravitational field strength in newtons per kilogram (N/kg), and h is height in metres (m). On Earth, g is 9.8 N/kg in AQA GCSE questions. For example, lifting a 2 kg mass by 3 m gives Ep = 2 × 9.8 × 3 = 58.8 J. The equation gives the gain in gravitational potential energy, so it assumes the object started at the reference level. If the object is lowered, the same equation gives the energy transferred away from the store. A strong answer earns marks for these points. State the equation for gravitational potential energy gain as Ep = mgh. Identify m as mass in kilograms, g as gravitational field strength in newtons per kilogram, and h as height in metres. Substitute values correctly, for example Ep = 2 kg × 9.8 N/kg × 3 m = 58.8 J. Explain that the equation calculates the gain in gravitational potential energy when an object is raised. A common slip: Using mass in grams instead of kilograms. Instead, dividing grams by 1000 before substitution. Exam tip: Write the equation, then substitute values with units before calculating the final answer.
Key words
Say what each word means before you turn the card.
Gravitational potential energy
Try answering aloud or in your head before turning the card.
Gravitational field strength
Try answering aloud or in your head before turning the card.
Height
Try answering aloud or in your head before turning the card.
See every definition
- Gravitational potential energy
- The energy gained by an object raised above ground level, calculated using the equation Ep = mgh.
- Gravitational field strength
- A quantity measured in newtons per kilogram, given as 9.8 N/kg on Earth in AQA GCSE questions.
- Height
- A quantity measured in metres in the equation Ep = mgh.
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