1.3Motion, Forces and Energy
MASS AND Weight — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 10 parts Lesson 18 of 91 1 quiz
Lesson summary
1.3 Mass and Weight — IGCSE PHYSICS notes: Define mass and weight, state the difference between them and use W = mg.Mass = a measure of the quantity of matter in an object at rest relative to the observer.Unit — mass is measured in kilograms (kg).1000 g = 1 kg, so divide by 1000 …
- Scalar quantity
- Vector quantity
- Gravitational field strength (g)
- Value on Earth
- Mass never changes
- Weight changes with the strength of the field
Part 1 of 8Mass
Mass
Mass a measure of the quantity of matter in an object at rest relative to the observer.
Unit
- Unit mass is measured in kilograms (kg).
- 1000 g = 1 kg, so divide by 1000 to convert g to kg and multiply by 1000 to convert kg to g.
Scalar quantity
- Scalar quantity mass has magnitude but no direction.
Inertia
- Inertia the greater the mass, the harder it is to speed the object up, slow it down or change its direction.
Part 2 of 8Weight
Weight
Weight the gravitational force acting on an object that has mass.
Unit
- Unit weight is a force, so it is measured in newtons (N).
Vector quantity
- Vector quantity weight has magnitude and direction.
- It acts downwards, towards the centre of the Earth or other planet.
Weight is the effect of a gravitational field on a mass.
- Strong gravitational fields keep objects on the ground, make objects fall and keep satellites in orbit.
Part 3 of 8Gravitational field strength and W = mg
1Gravitational field strength (g)
Gravitational field strength (g) the force per unit mass acting on an object in a gravitational field.
2Equation
- Equation g = W/m, which rearranges to W = mg.
- W is weight in N, m is mass in kg and g is gravitational field strength in N/kg.
3Value on Earth
- Value on Earth g is approximately 9.8 N/kg near the Earth's surface.
4Free fall
- Free fall g is also the acceleration of free fall, so 9.8 N/kg is equivalent to 9.8 m/s2.
Part 4 of 8Mass and weight compared
| Property | Mass | Weight |
|---|---|---|
| Meaning | Quantity of matter | Gravitational force on the mass |
| Unit | kilogram (kg) | newton (N) |
| Type of quantity | Scalar | Vector |
| Depends on location | No, it is the same everywhere | Yes, it changes with g |
| Measured with | Balance | Newtonmeter (spring balance) |
Part 5 of 8Weight in different gravitational fields
1Mass never changes
- Mass never changes an object has the same mass anywhere in the Universe.
2Weight changes with the strength of the field
- Weight changes with the strength of the field it is larger where g is larger.
3Moon
- Moon g is about 1.6 N/kg, so weight is about six times less than on Earth.
4
Comparison for a 60 kg person:
- Earth (g = 9.8 N/kg) gives about 590 N.
- Moon (g = 1.6 N/kg) gives 96 N.
- Mars (g = 3.7 N/kg) gives 222 N.
- Jupiter (g = 25 N/kg) gives 1500 N.
- Deep space (g close to 0) gives almost no weight, although the mass is still 60 kg.
Part 6 of 8Using balances and newtonmeters
Using balances and newtonmeters
A balance compares weights
- A balance compares weights when it balances, the two weights are equal, so the masses are equal because g is the same.
- The reading is mass in kg or g, and balances can be analogue or digital.
A newtonmeter (force meter) consists of a spring and a hook.
- The object hangs from the hook and the reading is its weight in N.
Finding mass from weight
- Finding mass from weight rearrange the equation to m = W/g.
Part 7 of 8Investigating weight and mass
1
Measure the mass of an object on a balance and convert it to kg.
2
Hang the object from a newtonmeter and read its weight in N at eye level.
3
Repeat for several different masses.
4
Plot weight on the y-axis against mass on the x-axis.
5
Draw a line of best fit through the origin; its gradient equals g in N/kg.
Part 8 of 8More from the specification
More from the specification
Mass versus weight
- Mass versus weight mass is the amount of matter in kg and is constant, while weight is the gravitational force on it in newtons, W = mg.
Density
- Density mass per unit volume, calculated as density = mass divided by volume.
Measuring volume
- Measuring volume use a measuring cylinder for liquids and the displacement of water for irregular solids.
Floating
- Floating an object floats when its average density is less than the fluid's, so upthrust balances its weight.
Upthrust
- Upthrust the upward force a fluid exerts on a submerged object, equal to the weight of fluid displaced.
Mass versus weight
- Mass versus weight mass is the amount of matter in kg and is constant, while weight is the gravitational force on it in newtons, W = mg.
Density
- Density mass per unit volume, calculated as density = mass divided by volume.
Measuring volume
- Measuring volume use a measuring cylinder for liquids and the displacement of water for irregular solids.
Floating
- Floating an object floats when its average density is less than the fluid's, so upthrust balances its weight.
Upthrust
- Upthrust the upward force a fluid exerts on a submerged object, equal to the weight of fluid displaced.
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
Density
- Mass per unit volume.
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 | Quantity of matter in an object, a scalar measured in kg. |
| Weight | Gravitational force on an object with mass, a vector measured in N. |
| Gravitational field strength (g) | Force per unit mass in a gravitational field, 9.8 N/kg on Earth. |
| Newton (N) | Unit of force. |
| Newtonmeter | Spring balance that measures weight in N. |
| Inertia | Resistance of a mass to changes in its motion. |
Avoid these mistakes
1
Saying mass and weight are the same thing — mass is the amount of matter in kg, weight is a force in N.
2
Saying mass changes on the Moon — mass stays the same, only the weight decreases because g is smaller.
3
Measuring weight with a balance — weight is measured in newtons with a newtonmeter, and a balance gives mass.
4
Leaving mass in grams in W = mg — convert to kg before calculating.
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