Gravitational Potential Energy (AQA GCSE Physics)
Revision Note
Author
Katie MExpertise
Physics
What is Gravitational Potential Energy?
- Energy in the gravitational store of an object is defined as:
The energy an object has due to its height in a gravitational field
- This means:
- If an object is lifted up, energy is transferred to its gravitational potential store
- If an object falls, energy will be transferred away from its gravitational potential store
Gravitational Potential Energy Equation
- The gravitational potential energy, Ep, of an object can be calculated using the equation:
Ep = m × g × h
- Where:
- Ep = gravitational potential energy, in joules (J)
- m = mass, in kilograms (kg)
- g = gravitational field strength in newtons per kilogram (N/kg)
- h = height in metres (m)
Gravitational Field Strength
- The gravitational field strength (g) on the Earth is approximately 9.8 N/kg
- The gravitational field strength on the surface of the Moon is less than on the Earth
- This means it would be easier to lift a mass on the Moon than on the Earth
- The gravitational field strength on the surface of the gas giants (eg. Jupiter and Saturn) is more than on the Earth
- This means it would be harder to lift a mass on the gas giants than on the Earth
Some values for g on the different objects in the Solar System
Worked example
A man of mass 70 kg climbs a flight of stairs that is 3 m higher than the floor. Gravitational field strength is approximately 9.8 N/kg. Calculate the energy transferred to the man's gravitational potential energy store.
Step 1: List the known quantities
-
-
- Mass of the man, m = 70 kg
- Gravitational field strength, g = 9.8 N/kg
- Height, Δh = 3 m
-
Step 2: Write down the equation for gravitational potential energy
ΔEP = mgΔh
Step 3: Calculate the gravitational potential energy
ΔEP = 70 × 9.8 × 3
ΔEP = 2058 J
Exam Tip
When doing calculations involving gravitational field strength, g, always use the value of 9.8 N/kg unless you are told otherwise in your exam question. You will be expected to remember the value of g for your exam!
When teaching this topic, I find that it aids students understanding to bring it to life with a demonstration. On my desk I have three masses disguised in coffee cans, each says 1 kg on the can and its location, Earth, Moon or Jupiter. The can labelled Earth contains a 1 kg mass, however, the can labelled Moon actually contains a 100 g mass and the Jupiter can contains a 2.5 kg mass.
We discuss the fact that gravitational field strength is different on different planets. To demonstrate this, I get a student, to raise the 1 kg Earth can as high as possible, and then to put it down. The student then lifts up the Moon can (100 g mass) which simulates lifting that same 1 kg on the moon, where the gravitational field strength is less than on Earth. They then lift the Jupiter can (2.5 kg mass) simulating lifting that same 1 kg mass on Jupiter, where the gravitational field strength is greater than on Earth. I usually find that all the students want to have a go at this, and they remember vividly how the same mass has different weights due to the different gravitational field strengths at different locations.
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