Cambridge IGCSE Physics 0625
1.3 Mass and weight
Quantity of matter, gravitational force, gravitational field strength and balances.
2026-2028 syllabus
Syllabus checklist
Core
- State that mass measures 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 g = W ÷ m, and know that it is equivalent to free-fall acceleration.
- Know that weights and masses may be compared using a balance.
Supplement
- Describe and use the concept of weight as the effect of a gravitational field on a mass.
Key language
Definitions
Mass
A measure of the quantity of matter in an object at rest relative to the observer.
Weight
The gravitational force acting on an object that has mass.
Gravitational field strength
Gravitational force per unit mass.
Balance
An instrument used to compare masses or weights.
Core
Mass, weight and field strength
Mass
Mass measures the quantity of matter in an object. Its SI unit is the kilogram (kg). Mass is a scalar quantity: it has magnitude but no direction.
The mass of an object does not change when the object is moved from Earth to the Moon or to another planet.
Weight
Weight is the gravitational force acting on an object. Its SI unit is the newton (N), because weight is a force. Weight acts towards the centre of the planet or other body producing the gravitational field.
Weight can change with location because gravitational field strength can change. An astronaut has the same mass on Earth and the Moon but weighs less on the Moon.
Gravitational field strength
Gravitational field strength, g, is the gravitational force acting on each kilogram of mass. Its unit is N/kg.
Near Earth's surface, g is approximately 9.8 N/kg. Gravitational field strength is numerically equivalent to the acceleration of free fall, so 9.8 N/kg is equivalent to 9.8 m/s2.
Mass compared with weight
| Mass | Weight |
|---|---|
| Quantity of matter | Gravitational force on a mass |
| Measured in kilograms (kg) | Measured in newtons (N) |
| Does not change with location | Changes when gravitational field strength changes |
| Scalar quantity | Force with a direction towards the attracting body |
| Compared using a balance | Measured directly using a force meter or spring balance |
Measurement method
Comparing mass with a balance
Apparatus
A beam balance, an unknown object and known standard masses.
Method
-
1
Check that the empty balance is level. Adjust the zero setting if necessary.
-
2
Place the unknown object in one pan.
-
3
Add known masses to the other pan until the beam is level.
-
4
Add the known masses. Their total equals the mass of the unknown object.
Supplement only
Weight as the effect of a field
A mass placed in a gravitational field experiences a force. This force is its weight. The size of the weight depends on both the mass and the gravitational field strength:
For a fixed mass, a stronger gravitational field produces a greater weight. For a fixed gravitational field strength, a greater mass produces a greater weight. This is why weight is directly proportional to mass at one location.
Worked example
A 60 kg person is in a gravitational field of strength 1.6 N/kg.
W = mg = 60 × 1.6
W = 96 N
Core and Extended
Equations
Gravitational field strength
- g: gravitational field strength (N/kg)
- W: weight (N)
- m: mass (kg)
Weight
- Weight is directly proportional to mass for a fixed g.
- Use mass in kg to obtain weight in N.
Cambridge-style practice
Practice questions
State the difference between mass and weight.
- Mass measures the quantity of matter in an object.
- Weight is the gravitational force acting on the object.
- Mass is measured in kg; weight is measured in N.
An object has a weight of 49 N and a mass of 5.0 kg. Calculate the gravitational field strength.
g = W ÷ m
g = 49 ÷ 5.0
g = 9.8 N/kg
Explain why a beam balance can compare masses.
- Both objects are in the same gravitational field.
- When the beam is level, their weights are equal.
- Because g is the same on both sides, their masses are equal.
An astronaut travels from Earth to the Moon. State what happens to the astronaut's mass and weight.
The astronaut's mass stays the same. The astronaut's weight decreases because the Moon has a smaller gravitational field strength.
A rover has a mass of 200 kg on a planet where g = 3.7 N/kg. Calculate its weight.
W = mg
W = 200 × 3.7
W = 740 N
An object weighs 19 N where g = 3.8 N/kg. Calculate its mass.
m = W ÷ g
m = 19 ÷ 3.8
m = 5.0 kg
Explain why weight is directly proportional to mass at one location.
W = mg. At one location, g is constant. Therefore, multiplying the mass by a fixed value gives the weight, so doubling the mass doubles the weight.
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