AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Forces › Forces and their interactions
Practise Gravity. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Weight is the force acting on an object due to gravity. You must recall and use W = m g, explain the difference between mass and weight, and know that weight acts at the centre of mass. Expect short calculations and explanations comparing weights on the Earth, the Moon and other planets.
Key facts
W = m g (weight = mass × gravitational field strength)
Units: W in N, m in kg, g in N/kg
g on Earth = 9.8 N/kg
Weight is a force (vector); mass is the amount of matter (scalar)
W ∝ m in a given gravitational field
Weight acts at the centre of mass
Measure weight with a newtonmeter (calibrated spring-balance)
Notes
Mass and weight
Mass is the amount of matter in an object, measured in kilograms (kg). It is the same wherever the object is.
Weight is the force acting on an object due to gravity, measured in newtons (N).
Close to the Earth, the force of gravity is due to the gravitational field around the Earth.
Weight depends on the gravitational field strength (g) at the point where the object is. On the Moon, g is much smaller, so your weight is smaller, but your mass is unchanged.
Calculating weight
diagram
weight = mass × gravitational field strength: W = m g
W in newtons (N), m in kilograms (kg), g in newtons per kilogram (N/kg).
On Earth, g = 9.8 N/kg. Use the value given in the question.
Weight and mass are directly proportional (W ∝ m): in the same place, doubling the mass doubles the weight.
Weight is directly proportional to mass. The gradient is the gravitational field strength.
Convert grams to kilograms first: 250 g = 0.25 kg.
Centre of mass and measuring weight
The weight of an object can be considered to act at a single point, called its centre of mass.
For a uniform, symmetrical object, such as a ruler or a ball, the centre of mass is at its centre.
Weight is measured using a calibrated spring-balance, called a newtonmeter.
How to answer each type of question
Calculate weight
2 marksGrade 4
Check the mass is in kg.
Write W = m g and substitute.
Give the answer in newtons.
Example. A suitcase has a mass of 23 kg. gravitational field strength = 9.8 N/kg Calculate the weight of the suitcase.
Show the model answerHide the model answer
W = 23 × 9.8 (1) W = 225.4 N (1)
Don’t lose marks
Giving a weight in kg. Weight is a force, so its unit is N.
Forgetting to convert grams to kilograms before using W = m g.
Saying an object's mass changes on the Moon. Only its weight changes.
Using g = 10 N/kg when the question gives 9.8 N/kg.
Calling g 'gravity'. Its name is gravitational field strength.
More tips
Memory tricks
Quick check: weight in N is about ten times the mass in kg. A 70 kg person weighs about 700 N (686 N with g = 9.8 N/kg).
The units give the equation away: kg × N/kg = N.
Mass is the matter; weight is the pull on it.
Moon check: g on the Moon is about one-sixth of g on Earth, so weights there are about one-sixth as big.
Exam technique
W = m g is one you must recall: it is not on the equations sheet.
Write the equation, substitute the values, then give the answer with a unit. Each step can earn a mark.
When comparing places, say that g is different so the weight is different, but the mass is the same.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Define weight as the force due to gravityWeight is measured in newtons (N); mass is measured in kilograms (kg).
Grade 4
Calculate weight using W = m gFor example, 60 kg × 9.8 N/kg = 588 N.
Grade 5
Find mass or g from a weightm = W ÷ g and g = W ÷ m.
Grade 5
Describe how to measure weightHang the object from a calibrated spring-balance (a newtonmeter).
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Calculate the weight of the backpack on the Earth. gravitational field strength = 9.8 N/kg Use the equation: weight = mass × gravitational field strength
117.6 N (118 N) N
State the direction in which the weight of the boulder acts.
Towards the centre of the Earth (vertically downwards).
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy6 marks
An astronaut’s backpack has a mass of 12 kg.
(a) Calculate the weight of the backpack on the Earth. gravitational field strength = 9.8 N/kg Use the equation: weight = mass × gravitational field strength[2]
(b) The gravitational field strength on the Moon is 1.6 N/kg. Calculate the weight of the backpack on the Moon.[2]
(c) What is the mass of the backpack on the Moon?[1]
(d) Name the instrument used to measure weight.[1]
Show the answer and mark scheme
(a)Answer: 117.6 N (118 N) N
W = 12 × 9.8
117.6 (N)
(b)Answer: 19.2 N
W = 12 × 1.6
19.2 (N)
(c)Answer: 12 kg
12 (kg)
(d)Answer: A newtonmeter (calibrated spring balance).
newtonmeter / (calibrated) spring balance
Question 2Medium4 marks
A student wants to test whether weight is directly proportional to mass. They have a newtonmeter and a set of 100 g standard slotted masses.
(a) Describe how the student could use this equipment to test whether weight is directly proportional to mass.[3]
(b) The student’s graph is a straight line through the origin with a gradient of 9.8 N/kg. What does this gradient represent?[1]
Show the answer and mark scheme
(a)Answer: Hang increasing numbers of standard masses from the newtonmeter, record the weight each time, and plot weight against mass — a straight line through the origin confirms direct proportion.
record the reading on the newtonmeter for 1, 2, 3 … of the standard masses hung from it, so that the total mass hung each time is known (e.g. 100 g, 200 g, 300 g …)
plot a graph of weight (y-axis) against mass (x-axis)
a straight line through the origin shows that weight is directly proportional to mass