GCSE Physics revision

Changes in energy

Energy changes in a system, and the ways energy is stored

AQA 4.1.1.2

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Calculate the amount of energy associated with a moving object, a stretched spring and an object raised above ground level.

MOST 🎯🎯

Apply changes in energy knowledge to a relevant numerical or graphical problem and show the working.

SOME 🎯🎯🎯

Analyse an unfamiliar problem involving changes in energy and justify the method or conclusion.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is Gravity?

Gravity is a force of attraction between two objects, and its size depends on the masses of the objects and the distance between them.
Objects with small masses (like apples or buildings) produce a gravitational force so tiny it is negligible.
Even very massive objects like Jupiter exert a tiny gravitational force on nearby Earth objects because they are so far away.
Large objects that are relatively close, such as the Earth or Moon, have a strong enough gravitational influence to significantly affect nearby objects.

1. What is Gravity?AQA

Notes

Gravity is a force of attraction between two objects, and its size depends on the masses of the objects and the distance between them.
Objects with small masses (like apples or buildings) produce a gravitational force so tiny it is negligible.
Even very massive objects like Jupiter exert a tiny gravitational force on nearby Earth objects because they are so far away.
Large objects that are relatively close, such as the Earth or Moon, have a strong enough gravitational influence to significantly affect nearby objects.

Gravitational Fields

A gravitational field is the region of influence surrounding an object where its gravitational force can be felt.
Gravitational field strength, represented by the letter g, measures how strong this field is and is measured in newtons per kilogram (N/kg).
Earth's gravitational field strength is approximately 9.8 N/kg (sometimes rounded to 10
N/kg).
The Moon's gravitational field strength is only 1.6 N/kg because the Moon is much smaller than the Earth.
The Sun's gravitational field strength is around 274 N/kg due to its enormous mass.

2. Gravitational FieldsAQA

Notes

A gravitational field is the region of influence surrounding an object where its gravitational force can be felt.
Gravitational field strength, represented by the letter g, measures how strong this field is and is measured in newtons per kilogram (N/kg).
Earth's gravitational field strength is approximately 9.8 N/kg (sometimes rounded to 10
N/kg).
The Moon's gravitational field strength is only 1.6 N/kg because the Moon is much smaller than the Earth.
The Sun's gravitational field strength is around 274 N/kg due to its enormous mass.

Defining Weight

Weight is defined as the force exerted on an object due to gravity, and is measured in newtons (N).
Weight depends on both the mass of the object and the gravitational field strength acting upon it.
Unlike weight, an object's mass (the amount of matter it contains) remains the same everywhere in the universe.
In everyday language, 'weight' is often used to mean mass, but in physics these are two distinct concepts.

3. Defining WeightAQA

Notes

Weight is defined as the force exerted on an object due to gravity, and is measured in newtons (N).
Weight depends on both the mass of the object and the gravitational field strength acting upon it.
Unlike weight, an object's mass (the amount of matter it contains) remains the same everywhere in the universe.
In everyday language, 'weight' is often used to mean mass, but in physics these are two distinct concepts.

Calculating Weight

Weight is calculated using the formula W = mg , where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg).
For example, a person with a mass of 60 kg on Earth has a weight of 60 × 9.8 = 588 N.
The same person on the Moon would weigh only 60 × 1.6 = 96 N because the Moon's gravitational field strength is much weaker.
This shows that weight changes depending on location, while mass stays constant.

4. Calculating WeightAQA

Notes

Weight is calculated using the formula W = mg , where W is weight (N), m is mass (kg), and g is gravitational field strength (N/kg).
For example, a person with a mass of 60 kg on Earth has a weight of 60 × 9.8 = 588 N.
The same person on the Moon would weigh only 60 × 1.6 = 96 N because the Moon's gravitational field strength is much weaker.
This shows that weight changes depending on location, while mass stays constant.

Mass vs Weight: Key Differences

Mass is a property of an object that describes the amount of matter it contains, measured in kilograms (kg).
Weight is a force caused by gravity acting on a mass, measured in newtons (N).
The distinction between mass and weight is important in physics, even though everyday language often uses the terms interchangeably.

5. Mass vs Weight: Key DifferencesAQA

Notes

Mass is a property of an object that describes the amount of matter it contains, measured in kilograms (kg).
Weight is a force caused by gravity acting on a mass, measured in newtons (N).
The distinction between mass and weight is important in physics, even though everyday language often uses the terms interchangeably.

What is Gravitational Potential Energy?

Gravitational potential energy (GPE) is the energy stored in an object when it is lifted against the force of gravity.
When work is done to lift an object, energy is transferred into the object's gravitational potential energy store.
Gravitational potential energy is a form of energy and is therefore measured in joules (J).

6. What is Gravitational Potential Energy?AQA

Notes

Gravitational potential energy (GPE) is the energy stored in an object when it is lifted against the force of gravity.
When work is done to lift an object, energy is transferred into the object's gravitational potential energy store.
Gravitational potential energy is a form of energy and is therefore measured in joules (J).

Calculating Gravitational Potential Energy

Gravitational potential energy is calculated using the formula Ep = mgh, where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).
For example, an apple of mass 0.1 kg thrown 3 m into the air on Earth gains
0.1 × 9.8 × 3 = 2.94 J of gravitational potential energy.
It is important to convert all values into the correct SI units before substituting them into the formula (e.g. converting grams to kilograms by dividing by 1000).

7. Calculating Gravitational Potential EnergyAQA

Notes

Gravitational potential energy is calculated using the formula Ep = mgh, where m is mass (kg), g is gravitational field strength (N/kg), and h is height (m).
For example, an apple of mass 0.1 kg thrown 3 m into the air on Earth gains
0.1 × 9.8 × 3 = 2.94 J of gravitational potential energy.
It is important to convert all values into the correct SI units before substituting them into the formula (e.g. converting grams to kilograms by dividing by 1000).

Unit Conversions to Watch Out For

Mass must be in kilograms (kg) when using the GPE formula, so grams must be divided by 1000 (e.g. 100 g ÷ 1000 = 0.1 kg).
Height must be in metres (m), gravitational field strength in N/kg, and the resulting GPE will be in joules (J).
Always check units before substituting values into any physics formula to avoid calculation errors.

8. Unit Conversions to Watch Out ForAQA

Notes

Mass must be in kilograms (kg) when using the GPE formula, so grams must be divided by 1000 (e.g. 100 g ÷ 1000 = 0.1 kg).
Height must be in metres (m), gravitational field strength in N/kg, and the resulting GPE will be in joules (J).
Always check units before substituting values into any physics formula to avoid calculation errors.

Gravity in the Solar System

Gravitational field strength varies across different planets and stars depending on their mass and size.
The Sun's immense mass gives it a gravitational field strength of approximately 274
N/kg, far greater than Earth's 9.8 N/kg.
The strong gravitational forces between the Sun and Earth, despite their large separation, keep Earth in orbit around the Sun.

9. Gravity in the Solar SystemAQA

Notes

Gravitational field strength varies across different planets and stars depending on their mass and size.
The Sun's immense mass gives it a gravitational field strength of approximately 274
N/kg, far greater than Earth's 9.8 N/kg.
The strong gravitational forces between the Sun and Earth, despite their large separation, keep Earth in orbit around the Sun.

Types of Energy Changes in Moving Objects

There are three key types of energy changes to understand: kinetic energy (KE) in moving objects, elastic potential energy (EPE) in stretched springs, and gravitational potential energy (GPE) in raised objects.
Energy is transferred between stores when objects move, stretch, or change height, making it essential to identify which energy store is involved in each scenario.

10. Types of Energy Changes in Moving ObjectsAQA

Notes

There are three key types of energy changes to understand: kinetic energy (KE) in moving objects, elastic potential energy (EPE) in stretched springs, and gravitational potential energy (GPE) in raised objects.
Energy is transferred between stores when objects move, stretch, or change height, making it essential to identify which energy store is involved in each scenario.

Kinetic Energy

Kinetic energy is defined as the energy which a body possesses by virtue of being in motion.
The formula for kinetic energy is KE = 1 / 2mv², where mass (m) is in kilograms (kg) and speed (v) is in metres per second (m/s), giving KE in joules (J).
Both mass and speed affect kinetic energy, so a fast-moving object does not necessarily have more KE than a slow-moving one if it has a much smaller mass.
Exam answers may require conversion between joules and kilojoules (divide by 1,000 to convert J to kJ).

11. Kinetic EnergyAQA

Notes

Kinetic energy is defined as the energy which a body possesses by virtue of being in motion.
The formula for kinetic energy is KE = 1 / 2mv², where mass (m) is in kilograms (kg) and speed (v) is in metres per second (m/s), giving KE in joules (J).
Both mass and speed affect kinetic energy, so a fast-moving object does not necessarily have more KE than a slow-moving one if it has a much smaller mass.
Exam answers may require conversion between joules and kilojoules (divide by 1,000 to convert J to kJ).

Worked Example: Kinetic Energy Calculation

To find the KE of a 500 kg object moving at 5 m/s, substitute into KE = 1 / 2 × 500 × 5² = 1 / 2 × 500 × 25 = 6{,}250 J.
Converting to kilojoules gives 6{,}250 J = 6.25 kJ, which is the required unit in this exam question.

Elastic Potential Energy

Elastic potential energy is defined as the energy stored as a result of applying a force to deform an elastic object.
The formula for elastic potential energy is E_e = 1 / 2ke², where k is the spring constant in newtons per metre (N/m) and e is the extension in metres (m).
The limits of proportionality must not be exceeded; if they are, the spring displays plastic behaviour and the formula no longer applies.
When calculating extension, always convert measurements to metres before substituting into the formula.

13. Elastic Potential EnergyAQA

Notes

Elastic potential energy is defined as the energy stored as a result of applying a force to deform an elastic object.
The formula for elastic potential energy is E_e = 1 / 2ke², where k is the spring constant in newtons per metre (N/m) and e is the extension in metres (m).
The limits of proportionality must not be exceeded; if they are, the spring displays plastic behaviour and the formula no longer applies.
When calculating extension, always convert measurements to metres before substituting into the formula.

Worked Example: Elastic Potential Energy Calculation

For a spring with k = 75 N/m stretched from 40 cm to 60 cm, first calculate the extension: e = 0.6 - 0.4 = 0.2 m.
Substituting into the formula gives E_e = 1 / 2 × 75 × 0.2² = 1.5 J.
In some exam questions, you may need to rearrange the formula to find the spring constant k rather than the elastic potential energy.

14. Worked Example: Elastic Potential Energy CalculationAQA

Notes

For a spring with k = 75 N/m stretched from 40 cm to 60 cm, first calculate the extension: e = 0.6 - 0.4 = 0.2 m.
Substituting into the formula gives E_e = 1 / 2 × 75 × 0.2² = 1.5 J.
In some exam questions, you may need to rearrange the formula to find the spring constant k rather than the elastic potential energy.

Gravitational Potential Energy (GPE)

Gravitational potential energy is defined as the energy stored in an object positioned at a height above or below the surface of the Earth.
The formula for GPE is GPE = mgh, where m is mass in kg, g is gravitational field strength in newtons per kilogram (N/kg), and h is height in metres (m).
GPE is directly proportional to both mass and height, so increasing either will increase the GPE of an object.
The value of gravitational field strength (g) will always be provided in the exam question or on the data sheet, so it does not need to be memorised.

15. Gravitational Potential Energy (GPE)AQA

Notes

Gravitational potential energy is defined as the energy stored in an object positioned at a height above or below the surface of the Earth.
The formula for GPE is GPE = mgh, where m is mass in kg, g is gravitational field strength in newtons per kilogram (N/kg), and h is height in metres (m).
GPE is directly proportional to both mass and height, so increasing either will increase the GPE of an object.
The value of gravitational field strength (g) will always be provided in the exam question or on the data sheet, so it does not need to be memorised.

Worked Example: GPE Calculation

For a 70 kg person climbing a 12 m tree with g = 9.8 N/kg: GPE = 70 × 9.8 × 12 = 8{,}232 J = 8.232 kJ, which rounds to 8.2 kJ (1 d.p.).
When an object falls, the loss in GPE is calculated using the same formula, with the distance fallen used as h; a negative value indicates a decrease in GPE.

Conversion Between GPE and Kinetic Energy

As an object falls (ignoring friction and air resistance), gravitational potential energy is continuously converted into kinetic energy.
At the highest point, GPE is at its maximum and KE is zero; just before hitting the ground, GPE is at its minimum and KE is at its maximum.
If friction and air resistance are negligible, the total GPE lost equals the total KE gained: Δ GPE = Δ KE.
Energy can be lost to the surroundings as heat and sound due to friction and air resistance during a fall.

17. Conversion Between GPE and Kinetic EnergyAQA

Notes

As an object falls (ignoring friction and air resistance), gravitational potential energy is continuously converted into kinetic energy.
At the highest point, GPE is at its maximum and KE is zero; just before hitting the ground, GPE is at its minimum and KE is at its maximum.
If friction and air resistance are negligible, the total GPE lost equals the total KE gained: Δ GPE = Δ KE.
Energy can be lost to the surroundings as heat and sound due to friction and air resistance during a fall.

Skydiver Example: GPE and KE Changes

Before jumping, the skydiver has maximum GPE and zero KE as they are stationary at their greatest height.
As the skydiver falls and accelerates, GPE decreases and KE increases, with GPE being converted into KE throughout the fall.
Just before hitting the ground, GPE is at its lowest and KE is at its highest, assuming negligible air resistance.
Upon landing, both GPE and KE become zero, with the energy dissipated as heat and sound.

18. Skydiver Example: GPE and KE ChangesAQA

Notes

Before jumping, the skydiver has maximum GPE and zero KE as they are stationary at their greatest height.
As the skydiver falls and accelerates, GPE decreases and KE increases, with GPE being converted into KE throughout the fall.
Just before hitting the ground, GPE is at its lowest and KE is at its highest, assuming negligible air resistance.
Upon landing, both GPE and KE become zero, with the energy dissipated as heat and sound.