1.3 Mass and weight
- Syllabus
- 0625–2026–2027
- Topic
- 1.3
- Level
- —
Learning objectives
1.3.1Mass is a measure of the quantity of• State: mass is a measure of the quantity of matter in an object at rest relative to the observer1.3.2Weight is a gravitational force on an• State: weight is a gravitational force on an object that has mass1.3.3Gravitational field strength as force• Define gravitational field strength as force per unit mass; recall/use: g = W/m; know it is equivalent to acceleration of free fall1.3.4Weights (and masses) may be compared• Know: weights (and masses) may be compared using a balance1.3.5Describe, and use the concept of• Describe, and use the concept of, weight as the effect of a gravitational field on a mass
Define mass as quantity of matter
Mass is a measure of the quantity of matter in an object at rest relative to the observer.
| Property | Mass |
|---|---|
| what it measures | quantity of matter |
| common SI unit | kilogram (kg) |
| quantity type | scalar |
| change of location | unchanged when the object moves between places with different gravitational fields |
If matter is added, mass increases; if matter is removed, mass decreases. Compressing an object without losing matter changes its volume but not its mass.
Mass is not a force and is not measured in newtons. The syllabus wording specifies the object at rest relative to the observer; do not replace mass with weight.
Define weight as a gravitational force
Weight is the gravitational force acting on an object that has mass.
| Feature | Mass | Weight |
|---|---|---|
| meaning | quantity of matter | gravitational force on that matter |
| unit | kg | N |
| type | scalar | vector, directed with the gravitational field |
| effect of changing field strength | unchanged | changes |
A force meter or newton meter measures weight directly in newtons. A balance compares masses or weights rather than giving a direct force reading.
Weight is caused by gravity; it is not the same as mass. An object can keep the same mass while its weight changes with location.
Use gravitational field strength
Gravitational field strength g is the gravitational force per unit mass at a location.
g=mW
W=mg
| Symbol | Meaning | Unit |
|---|---|---|
| g | gravitational field strength | N/kg |
| W | weight | N |
| m | mass | kg |
Gravitational field strength is numerically equivalent to the acceleration of free fall at that location: 1 N/kg is equivalent to 1 m/s2.
Select the equation form for the unknown, convert mass to kilograms, substitute the local value of g, and keep weight in newtons.
Do not confuse g with weight. g describes the field at a location; W also depends on the object's mass. Near Earth, use the value stated or required by the question.
Compare mass and weight using a balance
A balance compares an unknown object with known masses. At balance, the two sides have equal turning effects; in the same gravitational field this compares their weights and therefore their masses.
| Step | Action |
|---|---|
| zero | check that the empty balance is level or reads zero |
| compare | place the object on one side and standard masses on the other |
| adjust | add or remove standard masses until the balance is level |
| conclude | the unknown mass equals the total standard mass at balance |
A beam or pan balance compares masses. A spring balance or newton meter responds to force and is used to measure weight in newtons.
A balance does not require you to calculate W=mg when both sides are in the same field: the common field factor cancels. Do not confuse it with a measuring cylinder or force meter.
Explain weight as the effect of a field on mass
A gravitational field acts on mass and produces the force called weight. This cause-and-effect link is summarised by W=mg.
| Change | Mass | Gravitational field strength | Weight |
|---|---|---|---|
| same object moved to a weaker field | unchanged | decreases | decreases |
| same object moved to a stronger field | unchanged | increases | increases |
| more matter at the same location | increases | unchanged | increases |
Weight acts in the direction of the gravitational field. Near a planet, that direction is towards the planet's centre.
An astronaut on the Moon has the same mass as on Earth but less weight because the Moon's gravitational field is weaker.
Being in orbit does not remove mass and does not mean gravity is absent. If a gravitational field acts, the mass has weight even when the object and its surroundings are in free fall.