KS3 · Physics

Gravitational force and weight

On the Moon your school bag would feel lighter, yet not one pencil inside would change. How can something get lighter without losing anything?

Forces · Weight

On Earth, every kilogram gets a 10-newton tug

the mass on Earth: 1/10. Mass 1 kg. Weight on Earth 10 N. Pull on each kilogram (g) 10 N/kg

Mass1 kgWeight on Earth10 NPull on each kilogram (g)10 N/kg

Drag to add mass one kilogram at a time. Watch which readout keeps pace with the mass — and which one never moves at all.

Watch out: The g readout never moves, whatever you drag. g isn't something the object has: it's the pull on each kilogram here on Earth, the same for a feather or a fridge.

Two quantities people mix up

MassvsWeight

Start with the first row. It's the one that matters most.

Focus

What it is

Mass

The amount of material an object contains

Weight

A force: the gravitational force acting on the object

The insight

This is the whole lesson in one row. Mass is how much stuff there is. Weight is how hard gravity pulls on that stuff.

Unit

Mass

Kilograms (kg)

Weight

Newtons (N), like every other force

Is it a force?

Mass

Weight

Yes. It pulls the object towards the centre of the Earth

Where you can feel it

Mass

Nowhere: mass isn't a force, so it doesn't push or pull on anything

Weight

The force needed to hold the object still or lift it, and the force it presses down on a surface with

What can change it

Mass

Only adding or taking away material

Weight

Its mass, and the strength of the gravitational field it is in

More mass, more weight

Reason it through

Why does an object with more mass have a bigger weight?

Link 1 of 3

First link · your turn

Start with the stuff. What does an object with more mass have more of?

Locked — reveal the link above first

Predict, then check

Commit to an answer before you reveal it. Your first instinct is worth testing.

You throw a ball straight up. While it's in the air, touching nothing at all, what happens to the gravitational force on it?

Change the place

What changes when you leave Earth?

A space probe is launched from Earth. It visits the Moon, then flies towards the Sun, and finally heads out into deep space, far from any planet or star.

Which idea is closest to what you think happens to the probe's mass and weight on that journey?

On the Moon it gets lighter, so its mass and its weight both go down.Its weight stays the same everywhere, because weight is just its mass measured in a different unit.Out in deep space there's no gravity to pull on it, so the probe has no mass there.Its mass never changes, but its weight goes down on the Moon, up near the Sun, and to zero in deep space.

How sure are you?

I'm sureI think soI'm guessing

On the Moon it gets lighter, so its mass and its weight both go down.

This is a common misconception

The weight does go down on the Moon, because the Moon's gravitational field is much weaker than Earth's. But the mass can't: nothing has been taken off the probe. It still has exactly the same particles, so it still has exactly the same mass.

Its weight stays the same everywhere, because weight is just its mass measured in a different unit.

This is a common misconception

Weight isn't mass in disguise. It's a force, and it depends on the gravitational field the probe is in. Different planets and stars have different fields, so the same probe weighs less on the Moon and more near the Sun.

Out in deep space there's no gravity to pull on it, so the probe has no mass there.

This is a common misconception

You're right that its weight drops to zero far from any planet or star. But mass isn't a pull. It's the amount of material, and every particle of the probe is still there. Zero weight, same mass.

Its mass never changes, but its weight goes down on the Moon, up near the Sun, and to zero in deep space.

This matches the evidence

Exactly. The probe keeps the same particles the whole way, so its mass is fixed. Its weight follows the gravitational field: much smaller on the Moon, much higher at the Sun, and nothing at all far from any planet or star.

Moving an object changes its weight, never its mass. Mass goes wherever the object goes; weight depends on where it is.

Your turn to calculate

Weigh a bag of apples

A bag of apples has a mass of 750 g. What is its weight on Earth? (g = 10 N/kg)

Write down what you know: m = 750 g and g = 10 N/kg. We want the weight, W.Spot the trap straight away: the mass is in grams, but g is newtons per KILOGRAM.
missing step

Which line is step 2?

Change the mass into kilograms: 750 × 1000 = 750 000 kgChange the mass into kilograms: 750 ÷ 1000 = 0.75 kgKeep the mass as 750, because the unit doesn't matter

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

W = m × g

Your mass is the same everywhere. Your weight depends on where you are.

What you need to know

Gravity pulls objects together. Near Earth it pulls everything towards the centre of the Earth, even things that aren't touching the ground.
Earth is surrounded by a gravitational field. It's weaker further away, fairly constant near the surface, and reaches as far as the Moon and the Sun.
Mass is the amount of material (kg). Weight is the gravitational force on it (N).
Gravitational field strength, g, is the pull on each kilogram. On Earth g = 10 N/kg.
Weight = mass × gravitational field strength: W = m × g. It rearranges to m = W ÷ g and g = W ÷ m.

The big picture

Mass is the amount of material in an object, measured in kilograms, and it never changes when the object moves. Weight is the gravitational force on the object, measured in newtons, and it depends on the gravitational field. On Earth every kilogram is pulled with 10 N, so W = m × g with g = 10 N/kg.

Key points

1Weight is a force: the gravitational force acting on an object, measured in newtons (N).

2Mass is the amount of material in an object, measured in kilograms (kg). 1 kg = 1000 g.

3More mass means more particles, and gravity pulls on each one, so more mass means more weight.

4Mass stays the same wherever an object goes. Weight changes with the gravitational field: less on the Moon, more at the Sun, zero in deep space far from any planet or star.

5On Earth g = 10 N/kg: every kilogram is pulled down with 10 N.

6W = m × g, with W in N, m in kg and g in N/kg. Rearranged: m = W ÷ g, g = W ÷ m.

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Worked example

Problem

A suitcase weighs 150 N on Earth. What is its mass? (g = 10 N/kg)

⚠ Watch out

Saying "I weigh 50 kg". Kilograms measure mass, not weight. On Earth a 50 kg person has a mass of 50 kg and a weight of 500 N. If your answer is a weight, its unit must be newtons.

🧠

Memory hook

Mass is what you've GOT; weight is how hard the planet TUGS on it. On Earth, every kilogram gets a 10-newton tug. Take an astronaut to the Moon and they've got exactly the same mass, but a weaker tug.

✓

Check yourself

Without looking: a 5 kg bag of shopping is taken from Earth to the Moon. What happens to its mass, what happens to its weight, and what does it weigh on Earth?

Flashcards

(15)

What is weight, and what is it measured in?

The gravitational force acting on an object. Like every force, it is measured in newtons (N).

What is mass, and what is it measured in?

The amount of material an object contains, measured in kilograms (kg).

What does gravitational force do?

It pulls objects together. Near Earth it pulls objects towards the centre of the Earth.

Does gravity need contact to pull on something?

No. A person in a plane is still pulled towards the centre of the Earth.

What is a gravitational field?

An invisible zone around a planet, such as Earth, in which any object feels a gravitational force.

How does Earth's gravitational field change as you go further away?

It gets weaker, but it reaches far into space: Earth pulls on the Moon and even on the Sun.

Why does a ball's weight hardly change when you throw it up?

Near Earth's surface the gravitational field is fairly constant, so the force barely changes over that height.

What is gravitational field strength (g)?

The force on every kilogram, measured in newtons per kilogram (N/kg). On Earth, g = 10 N/kg.

Write the equation linking weight, mass and g, with units.

W = m × g: weight (N) = mass (kg) × gravitational field strength (N/kg).

How do you find mass or g from W = m × g?

Mass = weight ÷ g. Gravitational field strength = weight ÷ mass.

How do you change grams into kilograms?

Divide by 1000, because 1 kg = 1000 g.

Why does an object with more mass have a greater weight?

It contains more particles, and each one is pulled by a small gravitational force, so the total pull is bigger.

What happens to an object's mass when it goes to the Moon?

Nothing. It has the same particles, so its mass is the same anywhere.

Where does an object weigh less, more, or nothing at all, compared with Earth?

Less on the Moon, more at the Sun, and nothing in deep space far from any planet or star.

You hold a bag perfectly still. How big is the upward force from your hand?

Equal to the bag's weight. Weight is the force needed to hold an object still or lift it, and the force it presses down on a surface with.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What changes when you leave Earth?

Weigh a bag of apples

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