Forces Lesson 3: Gravity, Mass and Weight

calculate weight and gravitational field strength.
To explain the difference between

mass and weight.

To investigate what happens to mass and weight on different planets.

Learning Objective

Success Criteria

10kg. If he was on Earth, he would weigh more.
No,

he would weigh the same because he is made of the same amount of stuff as he was when he was on the Moon.

What do you think?

when they are near each other.

Gravity is affected by the

mass and the proximity of the objects.

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.

made up of.

The unit of mass is kilograms (kg).
This is

Gary.
Gary has a mass of 80kg.

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?

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 on the moon is 1.6N/kg.

What is Gary’s mass on the moon?


What is Gary’s weight on the moon?

80kg

128N

improve the students’ statements?

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.

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.

On the left-hand side of the equation, gravitational field strength cancels out.

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.

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.

On the left-hand side of the equation, mass cancels out.

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.