Mass and Weight

Forces Lesson 3:
Gravity, Mass and Weight

Learning Objective
To investigate what happens to mass and weight on different planets.
Success Criteria
• To accurately measure weight using a newton meter.
• To calculate weight and gravitational field strength.
• To explain the difference between mass and weight.
The Man on the Moon
The Man on the Moon weighs No, he would weigh the same
10kg. If he was on Earth, he because he is made of the same
would weigh more. amount of stuff as he was when
he was on the Moon.
What do you think?

Gravity
Gravity is a force exerted by one object on another when they are near each other.
Gravity is affected by the mass and the proximity of the objects.
A gravitational force acts between this pineapple and this mouse.
There is a larger gravitational force between this elephant and this lorry because they have a
bigger mass.
There is a larger gravitational force between this elephant and this lorry because they are
closer together.
Gravity
The Earth has a large mass compared with
everything that is on Earth.
The Earth’s gravitational field strength is

larger than our own, so we don’t notice the
gravitational force that our own bodies exert.
On Earth, everything is pulled to the Earth’s

centre.
The Earth has a gravitational field strength

of 10 newtons per kilogram (N/kg).
This means every kilogram on Earth has a

force of 10 newtons acting on it.
Mass
Mass is the amount of matter (stuff) an object is made up of.
The unit of mass is kilograms (kg).
This is Gary.
Gary has a mass of 80kg.
Weight
He is standing on Earth which has a gravitational field strength of 10N/kg.
This means every kilogram of Gary has a force of 10 newtons acting on it.
The total amount of force acting on Gary is his

weight.
What is Gary’s weight?
800N
Since weight is a force its unit is newtons.

Weight
To calculate weight we use the equation:
weight = mass × gravitational field strength
The gravitational field strength on Earth is 10N/kg.
80kg × 10N/kg = 800N
Gary flies to the moon. The gravitational field strength

on the moon is 1.6N/kg.
What is Gary’s mass on the moon?

80kg
What is Gary’s weight on the moon?

128N
The Man on the Moon
The Man on the Moon weighs No, he would weigh the same
10kg. If he was on Earth, he because he is made of the same
would weigh more. amount of stuff as he was when
he was on the Moon.
What do you think now?
Can you improve the students’ statements?

Measuring Weight
The gravitational field strength on Earth is 10N/kg.
Use a newton meter to measure the weight of the container you

have been given. This is the weight of the container on Earth.
Calculate the mass of the container.

To do this we need to rearrange the equation we used before.
Rearranging Equations
mass × gravitational field strength = weight
We need to make mass the subject of this equation.
Mass is currently multiplied by gravitational field strength, so we need to divide it by

gravitational field strength to make it the subject.

gravitational field strength gravitational field strength
mass × gravitational field strength = weight On the left-hand side of the

equation, gravitational field
gravitational field strength gravitational field strength strength cancels out.
So we are left with:

mass = weight
gravitational field strength
Measuring Weight
Imagine the same container is taken on a tour of the solar system. The mass will not change
as the container still contains the same amount of ‘stuff’.
Move to the next planet and measure the weight of the container on that planet.
Record your measurements in the table.
Using your measurements and the mass you calculated before, calculate the gravitational
field strength on each planet.
To do this we need to rearrange the equation in a different way.

Rearranging Equations
We need to make gravitational field strength the subject.
Gravitational field strength is currently multiplied by mass, so we need to divide it by mass

to make it the subject.

mass mass
mass × gravitational field strength = weight On the left-hand side of the

equation, mass cancels out.
mass mass
So we are left with:

gravitational field strength = weight
mass
Measuring Weight
Gravitational Field Strength
Planet Weight (N) Mass (kg) (N/kg)
Earth 2 0.2 10
Mercury 0.74 0.2 3.7
Venus 1.78 0.2 8.9
Mars 0.74 0.2 3.7
Jupiter 5 0.2 25
Saturn 2.08 0.2 10.4
Uranus 1.78 0.2 8.9
Neptune 2.24 0.2 11.2

Use this table to help you complete the
worksheet
Planet Gravitational Field Strength
(N/kg)
Earth 10
3.7
Mercury
8.9
Venus
Mars 3.7
Jupiter 25
Saturn 10.4
Uranus 8.9
Neptune 11.2
Mass vs Weight
Mass Weight
Sort the statements below into the correct columns.
The total amount of force acting on an object due to gravity.

The amount of matter an object is made up of.
Measured in newtons (N).
Measured in kilograms (kg).
The value does not change when an object’s location changes.

The value does change when an object’s location changes.
Mass vs Weight
Mass Weight
The amount of matter an object is The total amount of force acting on an

made up of. object due to gravity.
Measured in kilograms (kg). Measured in newtons (N).
The value does not change when an The value does change when an
object’s location changes. object’s location changes.

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Forces Lesson 3:

Gravity, Mass and Weight

What do you think?

Gravity is affected by the mass and the proximity of the objects.

A gravitational force acts between this pineapple and this mouse.

The Earth’s gravitational field strength is

On Earth, everything is pulled to the Earth’s

The Earth has a gravitational field strength

This means every kilogram on Earth has a

The unit of mass is kilograms (kg).

He is standing on Earth which has a gravitational field strength of 10N/kg.

The total amount of force acting on Gary is his

What is Gary’s weight?

Since weight is a force its unit is newtons.

weight = mass × gravitational field strength

Gary has a mass of 80kg.

The gravitational field strength on Earth is 10N/kg.

80kg × 10N/kg = 800N

Gary flies to the moon. The gravitational field strength

What is Gary’s mass on the moon?

What is Gary’s weight on the moon?

What do you think now?

Can you improve the students’ statements?

Use a newton meter to measure the weight of the container you

Calculate the mass of the container.

We need to make mass the subject of this equation.

Mass is currently multiplied by gravitational field strength, so we need to divide it by

mass × gravitational field strength = weight

mass × gravitational field strength = weight On the left-hand side of the

So we are left with:

Record your measurements in the table.

To do this we need to rearrange the equation in a different way.

We need to make gravitational field strength the subject.

Gravitational field strength is currently multiplied by mass, so we need to divide it by mass

mass × gravitational field strength = weight

mass × gravitational field strength = weight On the left-hand side of the

So we are left with:

Mercury 0.74 0.2 3.7

Venus 1.78 0.2 8.9

Mars 0.74 0.2 3.7

Saturn 2.08 0.2 10.4

Uranus 1.78 0.2 8.9

Neptune 2.24 0.2 11.2

Sort the statements below into the correct columns.

The total amount of force acting on an object due to gravity.

The value does not change when an object’s location changes.

The amount of matter an object is The total amount of force acting on an

Measured in kilograms (kg). Measured in newtons (N).

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