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IGCSE Physics Notes Mass and Weight

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IGCSE Physics Notes Mass and Weight

CORE:

State that mass is a measure of the quantity of matter in an object at rest relative to the observer
State that weight is a gravitational force on an object that has mass
Define gravitational field strength as force per unit mass; recall and use the equation g = W m and know that this is equivalent to the acceleration of free fall
Know that weights (and masses) may be compared using a balance

EXTENDED:

Describe, and use the concept of, weight as the effect of a gravitational field on a mass

Revision Notes

Video preview of our IGCSE Physics Revision notes by Cambridge Experts

IGCSE Physics / Mass, Weight and Gravitational Field Strength Practice

Core + Extended • Exam-Oriented Notes • Detailed Reveal Answers

How to use this page: Learn the key ideas first, then practise the exam-style questions. Click Reveal Answer to see full mark scheme style solutions, proper steps, and examiner hints.

1Key Definitions

Mass
Mass is a measure of the quantity of matter in an object at rest relative to the observer.

Weight
Weight is the gravitational force acting on an object that has mass.

Gravitational field strength
Gravitational field strength is the force per unit mass.

Balance
Weights and masses may be compared using a balance.

Important Equations

W = m × g

g = W ÷ m

Meaning of symbols:
W = weight in newtons (N)
m = mass in kilograms (kg)
g = gravitational field strength in N/kg

Very important idea: Gravitational field strength g is equivalent to the acceleration of free fall.

2Mass and Weight – Do Not Confuse Them

Mass	Weight
Amount of matter in an object	Gravitational force acting on the object
Measured in kg	Measured in N
Does not change from place to place	Changes if gravitational field strength changes
Compared using a balance	Measured as a force

Examiner tip: If an object goes from Earth to the Moon, its mass stays the same, but its weight changes.

3Core Facts You Must Know

Mass
Quantity of matter in an object.

Weight
Gravitational force on a mass.

g
Force per unit mass.

On Earth
g is often taken as 10 N/kg in exam questions.

On the Moon
g is much smaller, so weight is smaller.

Balance
Used to compare masses and weights.

4Worked Example – Astronaut Question

Question 1

An astronaut has a mass of 65 kg on Earth, where the gravitational field strength is 10 N/kg.

(a) Calculate the astronaut’s weight on Earth. [2]

Reveal Answer

Step 1: Use the equation W = m × g.

Step 2: Substitute the values:
W = 65 × 10

Step 3: W = 650 N

Mark scheme build-up:
1 mark for using mg or W = m × g
1 mark for the correct answer 650 N

(b) Complete the following sentence.

The astronaut’s weight on Earth is the ................................................................. force between the astronaut and ................................................................. . [1]

Reveal Answer

The astronaut’s weight on Earth is the gravitational or attractive force between the astronaut and the Earth.

Mark scheme build-up:
Accept gravitational / attractive and the Earth.

(i) State the astronaut’s mass on the Moon. [1]

Reveal Answer

The astronaut’s mass does not change when going from Earth to the Moon.

Answer: 65 kg

Mark scheme build-up:
1 mark for 65 kg

(ii) Calculate the weight of the astronaut on the Moon. [2]

Reveal Answer

Step 1: Use W = m × g.

Step 2: Substitute the Moon values:
W = 65 × 1.6

Step 3: W = 104 N

In some questions, if g = 1.6 N/kg is rounded or simplified, you may sometimes see 100 N accepted.

Mark scheme build-up:
Accept 104 N or sometimes 100 N
Correct follow-through from part (i) also gains credit.

5More Exam-Style Practice

Question 2

A rock has a mass of 12 kg. The gravitational field strength on a planet is 8 N/kg. Calculate the weight of the rock. [2]

Reveal Answer

Step 1: W = m × g

Step 2: W = 12 × 8

Step 3: W = 96 N

Question 3

An object weighs 45 N on a planet where the gravitational field strength is 9 N/kg. Calculate the mass of the object. [3]

Reveal Answer

Step 1: Use g = W ÷ m rearranged as m = W ÷ g.

Step 2: m = 45 ÷ 9

Step 3: m = 5 kg

Mark scheme hint: For full marks, show the rearranged formula before substituting.

Question 4

State why the mass of an astronaut stays the same on Earth and on the Moon, but the weight changes. [2]

Reveal Answer

Mass is the amount of matter in the astronaut, so it does not change.

Weight is the gravitational force on the astronaut, so it changes because the gravitational field strength is different on Earth and on the Moon.

Question 5

Explain what is meant by saying that weight is the effect of a gravitational field on a mass. [3]

Reveal Answer

A mass placed in a gravitational field experiences a force of attraction.

This force is called weight.

The stronger the gravitational field strength, the greater the weight of the same mass.

Extended idea: Weight depends on both the mass and the gravitational field strength.

6Balance and Comparison Questions

Question 6

State how masses may be compared in the laboratory. [1]

Reveal Answer

Masses may be compared using a balance.

Question 7

A student says, “A balance compares weight, not mass.” Explain why a balance can still be used to compare masses. [2]

Reveal Answer

A balance compares the effects of gravity on both sides.

Since both sides are in the same gravitational field, equal weights mean equal masses.

7Common Mistakes

Mistake 1:
Writing weight in kg.
Correct: Weight is a force, so its unit is N.

Mistake 2:
Saying mass changes on the Moon.
Correct: Mass stays the same; weight changes.

Mistake 3:
Forgetting the formula.
Correct: W = m × g

Mistake 4:
Mixing up g and W.
Correct: g is gravitational field strength, W is weight.

8Rapid Revision Summary

Mass
Quantity of matter.

Weight
Gravitational force.

Equation
W = m × g

Unit of mass
kg

Unit of weight
N

Unit of g
N/kg

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