Mass & Weight | CIE AS Physics Revision Notes 2025

What is mass?

Mass is a measure of the amount of matter in an object
Consequently, this is the property of an object that resists change in motion
The greater the mass of a body, the smaller the change produced by an applied force
The SI unit for mass is the kilogram (kg)

Since mass is measured in kilograms in physics, if it is given in grams in a question, make sure you convert to kg by dividing the value by 1000

Weight is the effect of a gravitational field on a mass
Since it is a force, it is measured in newtons (N) and is a vector quantity
The weight of a body is equal to the product of its mass and the acceleration of free fall

$W = m g$

Where:
- W = weight in newtons (N)
- m= mass in kilograms (kg)
- g = acceleration of freefall in metres per second (m s^-2)
The acceleration of freefall, g, on Earth, is 9.81 m s^-2

Gravitational field strength (in N kg^-1) and acceleration due to freefall (in m s^-2) are just two ways of describing the same thing.

They have the same value but have different units depending on the context.

An object in free fall is falling solely under the influence of gravitational attraction
In the absence of air resistance, all bodies near the Earth fall with the same acceleration regardless of their mass

An object’s mass always remains the same, however, its weight will differ depending on the strength of the gravitational field at different locations within the Universe
For example, the gravitational field strength on the Moon is 1.63 N kg^-1, meaning an object’s weight will be about 6 times less than on Earth

Mass vs weight, downloadable AS & A Level Physics revision notes

On the moon, your mass will stay the same but your weight will be much lower

Although you only need to memorise g on Earth, its value on other planets in our solar system is given in the diagram below. Notice how much this varies according to the size of the planet

Gravitational field strength diagram, downloadable AS & A Level Physics revision notes

More massive planets and objects have a greater gravitational field strength

The acceleration due to gravity on the moon is 1/6 of that on Earth. If the weight of a space probe on the moon is 491 N, calculate its mass.

Answer:

Step 1: State the equation for weight

$W = m g$

Step 2: Rearrange to make mass, m, the subject

$m = \frac{W}{g}$

Step 3: Find g for the moon

$g = \frac{g_{E a r t h}}{6} = \frac{9.81}{6} = 1.64 N {kg}^{- 1}$

Step 4: Substitute in the known values

$m = \frac{491}{1.64} = 300 kg$