AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Changes in energy: E_p = m g h
E_p = m g h is the symbol equation for gravitational potential energy.
Study notes
From the checked teaching notes for this specification point.
The idea
E_p = m g h is the symbol equation for gravitational potential energy. E_p is the energy stored in joules, m is the mass in kilograms, g is the gravitational field strength in newtons per kilogram, and h is the vertical height in metres. For example, a 0.5 kg ball raised 2 m on Earth stores E_p = 0.5 × 9.8 × 2 = 9.8 J. The equation shows that energy stored is directly proportional to mass, gravitational field strength and height. You can rearrange it to make m, g or h the subject. It is used alongside kinetic energy equations to solve problems about falling objects and energy transfers, assuming no energy is lost to friction or air resistance.
What earns the marks
- Identify each symbol in E_p = m g h with its quantity and unit.
- Substitute numerical values into the equation and calculate E_p in joules.
- Rearrange the equation to find m, g or h.
- Use vertical height in metres and mass in kilograms.
- Apply the equation to explain energy transfers when objects fall or are lifted.
- Combine with kinetic energy calculations where appropriate.
Common mistakes and how to put them right
Confusing E_p with kinetic energy.
Put it right by checking whether the object is raised or moving.
Substituting height in centimetres without converting to metres.
Put it right by dividing by 100.
Using mass in grams instead of kilograms.
Put it right by dividing by 1000.
Exam tips
- Learn the symbol equation and the units for each symbol.
- Show your rearrangement before substituting numbers when finding m, g or h.
- Give the final answer with the correct unit, J, and an appropriate number of significant figures.
Try the lesson
The start of the Changes in energy: E_p = m g h lesson, made from checked teaching notes for this specification point.
Have a go first
A book of mass 2.0 kg is lifted onto a shelf 1.5 m above the floor. Using g = 9.8 N/kg, calculate the gravitational potential energy gained by the book.
Listen
E_p = m g h is the symbol equation for gravitational potential energy. E_p is the energy stored in joules, m is the mass in kilograms, g is the gravitational field strength in newtons per kilogram, and h is the vertical height in metres. For example, a 0.5 kg ball raised 2 m on Earth stores E_p = 0.5 × 9.8 × 2 = 9.8 J. The equation shows that energy stored is directly proportional to mass, gravitational field strength and height. You can rearrange it to make m, g or h the subject. It is used alongside kinetic energy equations to solve problems about falling objects and energy transfers, assuming no energy is lost to friction or air resistance. A strong answer earns marks for these points. Identify each symbol in E_p = m g h with its quantity and unit. Substitute numerical values into the equation and calculate E_p in joules. Rearrange the equation to find m, g or h. Use vertical height in metres and mass in kilograms. A common slip: Confusing E_p with kinetic energy. Instead, checking whether the object is raised or moving. Exam tip: Learn the symbol equation and the units for each symbol.
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