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SavannahYear 8
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ScienceGravity and light30 min★★★ difficulty

Mass, weight and gravity

Your mass is the same everywhere in the universe; your weight is not. Getting that distinction right, and using W = m x g, is most of this topic.

Part of your national curriculum
  • Physics: space physics: Understand gravity force, weight and mass, and the relationship between them on Earth and other bodies

Lesson overview

What you'll learn in this lesson

Understand gravity force, weight and mass, and the relationship between them on Earth and other bodies

Key learning points

  • • Two different quantities
  • • Using W = m x g
  • • Off Earth
  • • Why gravity acts at all

This lesson at a glance

  • 30 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

gravityweightrelationshipbodiesquantities

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

Mass, weight and gravity

Find out why weight is really a force, and mass is not.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

What actually changes a pendulum's swing?Bending light and splitting white light

In a nutshell

Understand gravity force, weight and mass, and the relationship between them on Earth and other bodies

2

What you already know

Part 2 of 19

Before we start

What you already know

You should know forces are measured in newtons.

Recap: Fuels and energy resources
3

Key words

Part 3 of 19

Key words

Words you'll need today

Mass
The amount of matter in an object, measured in kilograms.
Weight
The force of gravity on an object, measured in newtons.
Gravitational field strength
The force per kilogram, about 10 N/kg on Earth.
Newton
The unit of force.
Free fall
Falling with only gravity acting.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Mass, weight and the equation

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

Two different quantities

Mass is how much matter there is, in kilograms, measured with a balance. Weight is the gravitational force pulling on that mass, in newtons, measured with a newtonmeter. Everyday speech confuses them - a bathroom scale in kilograms is really measuring a force - but in physics answers you must keep them separate.

6

Explore the idea

Part 6 of 19

Learn

Using W = m x g

Weight = mass x gravitational field strength. On Earth g is about 10 N/kg, so a 50 kg person weighs 500 N. Rearranged: mass = weight ÷ g. Always show the equation, then the substitution, then the answer with the correct unit - newtons for weight, kilograms for mass. That is how the marks are given.

7

Quick check

Part 7 of 19

Quick check

Weight is measured in...

8

Quick check

Part 8 of 19

Quick check

A 60 kg person on Earth weighs about...

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Gravity elsewhere

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Off Earth

On the Moon g is about 1.6 N/kg, so a 50 kg astronaut still has a mass of 50 kg but a weight of only about 80 N. On Jupiter g is roughly 25 N/kg, so the same astronaut would weigh about 1250 N. The mass never changed - only the field strength did.

12

Explore the idea

Part 12 of 19

Learn

Why gravity acts at all

Every mass attracts every other mass; the effect is only noticeable when at least one mass is very large. That is why we feel Earth's pull but not a bus's. The same force keeps the Moon in orbit and holds the solar system together, so it is a single idea explaining falling apples and planetary orbits alike.

13

Quick check

Part 13 of 19

Quick check

On the Moon, a 50 kg astronaut's mass is...

14

Quick check

Part 14 of 19

Quick check

If weight is 250 N and g is 10 N/kg, mass is...

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Mass is in kilograms; weight is a force in newtons.
  • Mass does not change on the Moon - weight does.
  • Weightlessness in orbit is not the absence of gravity.
  • g is a field strength in N/kg, not a force by itself.
16

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

____ is the gravitational force pulling on that mass, in newtons, measured with a newtonmeter.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Mass, weight and gravity

6 questions, marked with the reasoning shown. No timer.

  1. 1. Gravitational field strength on Earth is about...

  2. 2. Mass is measured with a...

  3. 3. A 10 kg bag on Jupiter (g ≈ 25 N/kg) weighs...

  4. 4. Weight depends on...

  5. 5. Gravity acts between...

  6. 6. Which quantity never changes when you travel to the Moon?

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

How this is going

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    Understand gravity force, weight and mass, and the relationship between them on Earth and other bodies

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