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# Would you weigh less on the Moon?

Year 7 Physics · Forces and motion

<!-- Speaker note: Students distinguish mass (kg) from weight (N), explain gravitational force as depending on mass, and calculate weight = mass × g in different places (AC9S7U04, node 4, a hinge node). About 25 minutes (Engage and Explore 6 min · Explain 10 min · Evaluate 3 min · Typical adds 4 min, Wider about 3 min). -->

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## Today

**We are learning to** mass is how much matter an object has, in kilograms, and it is the same everywhere. Weight is the force of gravity on that mass, in newtons, and depends on where you are: weight = mass × g.

- I can explain the difference between mass and weight, with units.
- I can calculate weight from mass and g.
- I can explain why weight is different on the Moon and other planets.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea; it does not replace weighing masses with a newton meter. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge risk is mass/weight confusion, which breaks F = ma in Year 10. Insist on units every time: kg for mass, N for weight. Board notes: g values are rounded (Earth 9.8, Moon 1.6, Mars 3.7, Jupiter 24.8 N/kg). Jupiter has no solid surface; the rover question says "if it could land". Teacher: In your own words: the 'I weigh 50 kg' paragraph is the evidence. -->

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## Why it matters

> Astronauts on the Moon bounce around like they're on a trampoline. Did they lose something on the way there?

<!-- Speaker note: "Weight" in everyday talk and "weight" in science are different. This lesson sorts that out. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**An astronaut has a mass of 60 kg on Earth. She flies to the Moon. What is her mass there?**

- More than 60 kg
- 0 kg: there's no gravity on the Moon
- 60 kg: mass doesn't change
- About 10 kg

<!-- Speaker note: What happens: Still 60 kg: she is made of the same matter. Her weight (the pull of gravity on her) is what changes: about 96 N on the Moon instead of about 588 N on Earth. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: ask anyone who chose 0 kg why; 'no gravity on the Moon' is common (there is gravity, just less). -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Fweight%2Fstep-2&code=widget%3A+forces%0Aobject%3A+Astronaut%0Amass%3A+10%2C+120%2C+60%0Aplace%3A+Earth%2C+Moon%2C+Mars%0Aforce+W%3A+down%2C+weight%2C+Weight%0Aforce+N%3A+up%2C+support%2C+Support%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/weight/?full=step-2#step-2)

<!-- Speaker note: Try each place. Then change the mass. Watch the weight arrow and the line under the board: **mass × g = weight**. -->

<!-- Speaker note: Teacher: have students say aloud 'mass stays, weight changes' while switching places. -->

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## What happened?

* Still 60 kg: she is made of the same matter. Her weight (the pull of gravity on her) is what changes: about 96 N on the Moon instead of about 588 N on Earth.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Which is which

**Mass or weight?**

Groups: Mass · Weight

Measured in kilograms (kg) · Measured in newtons (N) · The same on Earth and on the Moon · A force, pulling towards the planet · How much matter an object has · Changes if g changes

* Answer: Measured in kilograms (kg) → Mass; Measured in newtons (N) → Weight; The same on Earth and on the Moon → Mass; A force, pulling towards the planet → Weight; How much matter an object has → Mass; Changes if g changes → Weight

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the number checks need units in discussion even though the box takes a number. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Weight on Earth

**A 5 kg bag on Earth (g = 9.8 N/kg). What is its weight, in N?**

* Answer: 49

<!-- Speaker note: Weight = mass × g = 5 × 9.8 = 49 N. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Weight on Mars

**The same 5 kg bag on Mars (g = 3.7 N/kg). Its weight, in N?**

* Answer: 18.5

<!-- Speaker note: 5 × 3.7 = 18.5 N. Same mass, smaller weight. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Why the Moon pulls less

**Why is the astronaut's weight smaller on the Moon?**

- She has less mass there
- The Moon is further from the Sun
- The Moon has much less mass than Earth, so its gravity pulls less (smaller g)
- There is no air on the Moon

* Answer: The Moon has much less mass than Earth, so its gravity pulls less (smaller g)

<!-- Speaker note: Gravity depends on mass. The Moon has about 1/80 of Earth's mass, so its g is only 1.6 N/kg. -->

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## Mix it up

**Looking back at balanced forces: a 2 kg bag hangs still from a spring balance. Its weight is about 20 N. What force does the spring pull up with?**

- About 2 N
- 0 N, because the bag is not moving
- About 20 N: the forces are balanced
- About 40 N, so that it can lift the bag

* Answer: About 20 N: the forces are balanced

<!-- Speaker note: The bag is still, so the forces are balanced: the spring pulls up with a force equal to the weight, about 20 N. What this step is for: Weight is a force, so it follows the same rule as any other force: if nothing is changing, something else balances it. -->

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## Elaborate: a rover on Jupiter

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fphysics-7%2Fweight%2Fstep-5&code=widget%3A+forces%0Aobject%3A+Rover%0Amass%3A+100%2C+1000%2C+900%0Aplace%3A+Earth%2C+Moon%2C+Mars%2C+Jupiter%0Aforce+W%3A+down%2C+weight%2C+Weight%0Aforce+N%3A+up%2C+support%2C+Support%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/physics-7/weight/?full=step-5#step-5)

<!-- Speaker note: What this step is for: A heavier object on a stronger-gravity planet extends weight = mass × g to an unfamiliar case. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Built for Mars

**A rover's wheels are designed for its weight on Mars. It is sent to Jupiter instead (if it could land). What is the problem?**

- Its weight would be over six times bigger, so its wheels and legs might collapse
- Nothing: weight is the same everywhere
- Its mass would be bigger, so it would need more fuel

* Answer: Its weight would be over six times bigger, so its wheels and legs might collapse

<!-- Speaker note: 24.8 ÷ 3.7 ≈ 6.7, so its weight would be nearly seven times bigger. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: do heavy things fall faster?

**On the Moon (no air), an astronaut dropped a hammer and a feather together (Apollo 15, 1971). What happened?**

- They hit the ground together
- The feather floated away
- The hammer landed first

* Answer: They hit the ground together

<!-- Speaker note: The heavier hammer is pulled harder, but it also has more mass to get moving, so both speed up at the same rate. Air resistance is what makes feathers fall slowly on Earth. What this step is for: Refuting the 'heavier falls faster' misconception uses weight and mass together: double the mass, double the pull, but double the mass to move. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Mass** (kg): how much matter. The same everywhere.
- **Weight** (N): the force of gravity on an object. **Weight = mass × g.**
- **g** is the gravitational field strength: 9.8 N/kg on Earth (about 10), 1.6 on the Moon, 3.7 on Mars, 24.8 on Jupiter.
- Gravity depends on mass: the Moon has far less mass than Earth, so it pulls less (smaller g, smaller weight).
- Without air, all objects fall together, whatever their mass.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**A 2 kg rock on Earth (g = 9.8 N/kg). What are its mass and its weight on the Moon (g = 1.6 N/kg)?**

* Mass 2 kg (unchanged); weight 2 × 1.6 = 3.2 N.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: relating force, mass and motion (node 5), a class task in the unit primer. In Year 10, F = ma uses mass, not weight. In class: newton meters and 100 g masses (about 1 N each); the Apollo 15 hammer and feather video. -->