1.3Motion, Forces and Energy
Mass, Weight and Density — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
10 min read 24 parts Lesson 22 of 91 Unit quiz
Lesson summary
1.3 Mass, Weight and Density (Part 1) — IGCSE PHYSICS notes: Distinguish mass from weight, use W = mg, and compare weights on different planets.Definition — mass is a measure of the quantity of matter in an object.The greater the mass, the harder it is to speed the object up, slo…
- Nature of mass
- Nature of weight
- Why weight matters
- Link to free fall
- Weight equation
- Mass is constant
Part 1 of 8Mass
Definition
- Definition mass is a measure of the quantity of matter in an object.
- The greater the mass, the harder it is to speed the object up, slow it down or change its direction.
Nature of mass
- Nature of mass it is a scalar with magnitude only.
- It is measured in kilograms (kg).
Grams
- Grams mass is sometimes given in grams and must be converted for calculations.
- 1000 g = 1 kg, so divide grams by 1000 to get kilograms.
- Multiply kilograms by 1000 to get grams, so 4.5 kg is 4500 g.
Part 2 of 8Weight
Definition
- Definition weight is the gravitational force acting on an object with mass.
- It is the force on an object placed in a gravitational field.
Nature of weight
- Nature of weight it is a force, so it is a vector measured in newtons (N).
- Weight acts downwards towards the centre of the planet.
Why weight matters
- Why weight matters it keeps objects on the ground, makes dropped objects fall and keeps satellites in orbit.
Part 3 of 8Mass compared with weight
| Feature | Mass | Weight |
|---|---|---|
| Meaning | Amount of matter | Gravitational force on the mass |
| Type | Scalar | Vector |
| Unit | kilogram (kg) | newton (N) |
| Depends on location | No | Yes, changes with g |
| Measured with | Balance | Newton meter (force meter) |
Part 4 of 8Gravitational field strength
Definition
- Definition gravitational field strength is the force per unit mass in a gravitational field.
- g = W ÷ m, measured in newtons per kilogram (N/kg).
Values
- Values near Earth's surface g is about 9.8 N/kg.
- On the Moon g is about 1.6 N/kg.
Link to free fall
- Link to free fall g is also the acceleration of free fall in m/s2.
- The two quantities are equivalent, so 9.8 N/kg equals 9.8 m/s2.
Weight equation
- W = mg.What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg−1 on Earth)
- You are expected to remember g = 9.8 N/kg.
Part 5 of 8Weight on different planets
Mass is constant
- Mass is constant an object's mass is the same anywhere in the Universe.
Weight changes
- Weight changes it depends on the strength of the local gravitational field.
- On the Moon an object weighs about six times less than on Earth.
Effect on a person
- Effect on a person an astronaut keeps the same mass on the Moon but feels much lighter.
Part 6 of 8Measuring mass and weight
Balances
- Balances these compare weights and so measure mass because g is nearly constant across Earth.
- Digital or analogue balances give a reading in kg or g.
- m = W ÷ g.
Newton meters
- Newton meters a force meter has a spring and a hook.
- The object hangs from the hook and the reading is weight in newtons.
Part 7 of 8Equations for weight, mass and field strength
| Find | Equation |
|---|---|
| Weight | W = mg |
| Mass | m = W ÷ g |
| Field strength | g = W ÷ m |
Check the units
- Check the units kilograms for mass, newtons for weight and N/kg for g.
Everyday language
- Everyday language saying "my weight is 60 kg" really refers to mass, and the weight is about 590 N.
Part 8 of 8More from the specification
More from the specification
Mass vs weight distinction
- Mass is the measure of the amount of matter in an object (kg), whereas weight is the gravitational force acting on that mass (W = mg, in N).
Density calculation and measurement
- Density is defined as mass per unit volume (rho = m / V);
- irregular solid volumes are determined using liquid displacement in a measuring cylinder.
Principle of floatation
- An object floats in a fluid if its average density is less than the density of the fluid, displacing its own weight of liquid.
Apparatus
- The equipment needed to carry out an experiment.
Variable
- A factor that can change in an investigation.
Vernier callipers
- An instrument measuring small lengths to 0.1 mm.
Independent variable
- The variable deliberately changed, here time
Dependent variable
- The variable measured, here temperature
Volume
- The amount of space an object occupies.
Liquid
- State with a fixed volume that takes the shape of its container.
Upthrust
- Upward force exerted by a fluid.
Length
- Resistance is directly proportional to the length of a wire.
Floating
- Fb = weight with ρobject < ρfluid.
Key terms
| Term | Meaning |
|---|---|
| Mass | A measure of the quantity of matter in an object, in kilograms. |
| Weight | The gravitational force acting on an object with mass, in newtons. |
| Gravitational field strength | Force per unit mass in a gravitational field, in N/kg. |
| Newton meter | A spring-based instrument for measuring force. |
| Free fall | Motion with only weight acting. |
| Balance | An instrument for comparing weights, used to find mass. |
Avoid these mistakes
1
Using the words mass and weight as if they mean the same — mass is matter in kg, weight is a force in N.
2
Saying mass changes on the Moon — only the weight changes.
3
Forgetting to convert grams to kilograms before using W = mg.
4
Writing weight in kilograms — weight is a force and must be in newtons.
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