Lesson focus: massThe amount of matter in an object, measured in kilograms (kg); it is the same anywhere, while weight depends on the gravitational field strength. measures inertia and amount of matter, while weightThe gravitational force on an object: weight = mass × gravitational field strength (W = m × g), measured in newtons (N). is the gravitational force on that mass.
Distinguish mass and weight and calculate W = mg in different gravitational fieldThe area around a planet or other mass where objects are pulled towards it..
Build on earlier learning
Forces at a distanceYear 7 introduced gravity as a non-contact force. This lesson quantifies gravitational field strength.
You will be successful when you can…
- Distinguish mass and weight.
- Use W = mg.
- State correct units.
- Explain changes between worlds.
Words that carry the science
- mass
- A measure of inertia and quantity of matter, measured in kilograms.
- weight
- The gravitational force acting on an object, measured in newtons.
- gravitational field
- A region in which a mass experiences gravitational force.
- gravitational field strength
- Force per unit mass at a position, measured in N kg⁻¹.
- newton
- The SI unit of force.
- kilogram
- The SI unit of mass.
Mass and weight are different quantities
01Mass and weight are different quantities
massThe amount of matter in an object, measured in kilograms (kg); it is the same anywhere, while weight depends on the gravitational field strength. is a scalar measured in kilogramThe unit of mass (kg); 1 kg = 1000 g. and relates to inertia. weightThe gravitational force on an object: weight = mass × gravitational field strength (W = m × g), measured in newtons (N). is a vector force directed towards the attracting body’s centre and measured in newtonThe unit of force (N)..
Moving from Earth to the Moon changes weight because gravitational field strengthHow strongly gravity pulls on each kilogram, about 10 N/kg on Earth. changes, but mass remains the same.
02Field strength converts mass into weight
Use W = mg, where W is weight in N, m is mass in kg and g is gravitational field strength in N kg⁻¹. Near Earth’s surface g is approximately 9.8 N kg⁻¹, often rounded only when instructed.
The same numerical value of g can also be written as gravitational acceleration in m s⁻², but the units communicate different interpretations.
03Weight can change across a field
Gravitational field strength decreases with distance from a planet’s centre. “Weightless” orbiting astronauts still experience gravity; they and their spacecraft are in continuous free fall together.
Apparent weight depends on supporting contact forces, whereas gravitational weight remains W = mg.
Weight on other worlds
Mass is the amount of matter in an object and does not change from place to place. Weight is the force of gravity on that mass: W = m × g, where g is the gravitational field strength. Take the same mass to another world and its weight changes; dropped objects also fall faster where g is bigger.
Controls
For Jupiter, Saturn and Neptune, g is given at the cloud tops: they have no solid surface.
Compare Earth and Moon weight
A 60 kg person stands where g is 9.8 N kg⁻¹, then where g is 1.6 N kg⁻¹.
- Earth: W = 60 × 9.8 = 588 N.
- Moon: W = 60 × 1.6 = 96 N.
- Mass remains 60 kg in both places.
- Weight ratio follows the field-strength ratio.
How to make the reasoning visible: The person’s weight changes from 588 N to 96 N, but their mass and inertia remain 60 kg.
Compare gravitational environments
Calculate weight using the stated field strength.
| Mass / kg | g / N kg⁻¹ | Weight / N | Mass change? |
|---|---|---|---|
| 2.0 | 9.8 | 19.6 | none |
| 2.0 | 1.6 | 3.2 | none |
| 75 | 9.8 | 735 | none |
| 75 | 3.7 | 277.5 | none |
Why does the same object have different weights?
Why is weight a vector?
Check your understanding
Answer from memory, then check against the suggested answer.
What unit measures mass?
What unit measures weight?
State the weight equation.
Why is an astronaut in orbit not beyond Earth’s gravity?
Why does a 10 kg object resist acceleration equally on Earth and Moon?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A rover has a mass of 240 kg. On Mars, g = 3.7 N/kg.
Calculate the weight of the rover on Mars. Show your working.
(3)What is the mass of the rover on Earth?
(1)Total for this question: 4 marks
Command word: For calculateA command word: work out a number answer, showing your working and the unit., show equation, substitution, value and unit; then answer the comparison separately.