Mass, weight and gravity Cambridge IGCSE Combined Science revision
Not started
Learn it
In plain words
Mass is how much stuff is in something, in kilograms, and it's the same wherever you are. Weight is the pull of gravity on that stuff, in newtons, and it changes if gravity does. On the Moon you'd have the same mass but weigh about a sixth as much.
Three things to know
- Mass is how much matter an object has, in kilograms. It is the same everywhere.
- Weight is the pull of gravity on that mass, in newtons. Weight = mass × gravitational field strength (W = mg).
- On Earth g is about 9.8 N/kg. On the Moon it is about 1.6 N/kg, so the same mass weighs less there.
Worked example
What is the weight of a 3 kg bag on Earth? (g = 9.8 N/kg)
- Weight = mass × g.
- = 3 × 9.8 = 29.4 N.
Tips and tricks
- Mass is in kg, weight is in N. If a question gives grams, change to kilograms first (divide by 1000).
- In everyday talk people say "weight" when they mean mass. In a physics answer, never.
It lands in your notebook with its questions as flashcards.
Mass, weight and gravity: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the unit of weight?
Weight is a force, so it is measured in newtons.
A 3.0 kg bag hangs from a hook. Taking g as 10 N/kg, what is its weight?
W = mg = 3.0 × 10.
An object is taken from the Earth to the Moon. What happens?
The amount of matter is unchanged, but the Moon's gravity pulls on it less.
What does "gravitational field strength" mean?
It is force per unit mass, in N/kg.
On Mars g is 3.7 N/kg. What is the weight of a 20 kg rover there?
W = 20 × 3.7 = 74 N.
An object weighs 120 N where g = 10 N/kg. What is its mass?
m = W ÷ g = 120 ÷ 10.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Mass, weight and gravity
Cambridge IGCSE Combined Science 0653 · 9 marks · papermunch.org
Name ______________________________ Date ______________
Calculate the weight on Earth of a 55 kg student. Take g = 9.8 N/kg.[2]
Show answerHide answer
539 N (540 N to 2 significant figures). W = 55 × 9.8.
A rock weighs 49 N on Earth, where g = 9.8 N/kg. Calculate its mass.[2]
Show answerHide answer
5.0 kg. m = W ÷ g = 49 ÷ 9.8.
An astronaut has a mass of 80 kg. On the Moon g = 1.6 N/kg. State her mass on the Moon and calculate her weight there.[3]
Show answerHide answer
Her mass is still 80 kg. Weight = 80 × 1.6 = 128 N.
Explain why a balance that compares masses gives the same reading on the Moon as on Earth.[2]
Show answerHide answer
Gravity is weaker on the Moon, but it pulls less on both sides of the balance equally, so they still balance.
Answers: Mass, weight and gravity
- 1. 539 N (540 N to 2 significant figures). W = 55 × 9.8.
- 2. 5.0 kg. m = W ÷ g = 49 ÷ 9.8.
- 3. Her mass is still 80 kg. Weight = 80 × 1.6 = 128 N.
- 4. Gravity is weaker on the Moon, but it pulls less on both sides of the balance equally, so they still balance.