Weight and Mass

Mass

Mass is the amount of material in an object, measured in kilograms (kg).

Weight

Weight is the force of gravity affecting the object, measured in newtons (N).

Mass

The amount of material in an object stays constant, as the number of particles does not change in different gravity fields.

An apple contains around an octillion atoms, which is a 1 followed by 27 zeroes!

Defining gravity

Gravity has a lot of terms associated with it that seem interchangeable but are not, which can be confusing!

GRAVITY is the fundamental force of nature that attracts bodies with mass to one another.

GRAVITATIONAL FORCE is the specific force of attraction between those bodies that can be measured in newtons (N). It is the same as weight.

GRAVITATIONAL FIELD STRENGTH is the force per unit mass experienced by an object in a gravitational field, measured in newtons per kilogram (N/kg).

Calculating weight

Weight is equal to the mass of an object, multiplied by the gravitational field strength of the celestial body (e.g. planet or moon) the object is being affected by.

weight
N

gravitational
field strength
N/kg

mass
kg

On Earth, the gravitational field strength is 9.8 N/kg. If your mass is 50 kg, your weight is 50 x 9.8, which is 490 N. Yes, this is different to how we talk about “weighing” yourself on your bathroom scales, so yes, you need to retrain your brain!

Formula Triangle

We can use formula triangles to help visualise equations.

When you are given two values, cover the letter for the unknown value with your thumb, and use what’s left to figure out the equation

Newton metre

We can measure weight using a newton metre, also called a force metre. Weight is a force, and newtons are a unit of force.

_{Remember: when you “weigh” in kilograms, you’re actually measuring mass!}

Constant mass – changing weight

If we take an apple to the moon, or Jupiter, it would have the same mass as on Earth, but a different weight due to the different gravitational field strength (g) on each celestial body:

	Moon	Earth	Jupiter
field strength (N/kg)	1.7	9.8	24.7
mass of apple (kg)	0.15	0.15	0.15
weight of apple (N)	1.7 x 0.15 = 0.26	9.8 x 0.15 = 1.47	24.7 x 0.15 = 3.71

Bigger celestial bodies have more mass, so they have more gravitational field strength. The mass of the apple remains constant no matter where we take it! Assuming it doesn’t rot to nothing on the long journey to Jupiter of course…