GET SET · Space
Get Set: Space · Session 2 · Session Support

Gravity

Why is gravity the weakest force in the universe, and yet the one that built every planet, star and galaxy in it?

45 minutes Session 2 of 12

Got 10 minutes to prep?

Here's all you need to walk in and teach.

Session plan

The full lesson: what pupils will learn, how to run every stage, and all activity instructions and answers.

Open the plan →

Slide deck

Everything you put on the board, in order. Speaker notes included.

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Learner journal

For the plenary. Five-part reflection.

Editable version → Print version →

ITZA pathways

The link you share with the class for the 20 minutes online.

Open ITZA for students →

Main theme

Gravity is the weakest of the four fundamental forces, yet without it there would be no planets, stars or galaxies. Every time a pupil picks something up, their arm beats the pull of the entire Earth.

Teacher tips

Hook

  • Most classes go for A, everyone flying into space. It's worth asking whoever picks it what would actually push them upwards, because there isn't anything. That usually gets them to C on their own rather than being told.
  • Don't let the show of hands settle it. If the room lands unanimously on one option, pick someone and ask them to argue for a different one. The reasoning is what you're after, and a class that agrees too quickly hasn't done any.
  • Flag early that gravity is the weakest of the four forces. Pupils resist this because it doesn't match how it feels, so give them the image now rather than in the debrief: every time you pick something up, your arm beats the pull of the entire planet.

ITZA Platform

  • Pathway 2's distance rule is the bit that gets skimmed. If someone's moving fast, ask what happens to gravity when you double the distance. "Half" is the wrong answer nearly everyone gives first, and it's better caught here than in the activity.
  • Anyone watching the film should explain the equal squeeze in their own words before they move on. Pupils can repeat the phrase without having pictured it, and if they can't say what's doing the squeezing they haven't got it.
  • Early finishers who've drawn the diagram usually draw gravity stopping at some point. Ask them where it ends. It doesn't, it just gets weaker, and that's a better conversation than another task.

Class Activity

  • Each scenario stays up on the board while pupils are in their zone, so they aren't working from memory of a sentence you read once. Use that: get them to reread it before they justify their choice, rather than answering off the first hearing. Most wrong answers come from fixing on one word, distance or mass, and missing the other, not from mishearing either one.
  • Spend the time on the asteroid. It's the only genuine "It Depends" and the two rules pull opposite ways, close but almost no mass. Ask which rule they think wins before you say anything.
  • The 100 km rocket catches people out. Pupils vote Weak because they picture astronauts floating. Gravity there is about 97% of what it is on the ground, and they float because they're falling, not because gravity has gone. Worth having ready, it comes up nearly every time.
  • Push for the number in the debrief. "It gets weaker" is not the same as "it drops to a quarter", and pupils will say the first while believing the second is a half.

Plenary

  • The reflective question is a good one for pupils who find "what did you learn" hard, because it's a scenario rather than a recall. If someone's stuck, ask what would happen when they tried to stand up.
  • Expect answers about heavier objects and nothing about the atmosphere, tides or the Moon's orbit. One prompt fixes it: what else on Earth is being held in place by gravity?
  • The lock-in is long. Read it, but if you only have time for one line, use the last one: gravity is the weakest force in the universe, and at the scale of the cosmos it wins every time.

Misconceptions to watch for

They'll say

"Astronauts float because there's no gravity in space."

Actually

Gravity in low orbit is still close to 97% of what it is on the surface. Astronauts float because they and their spacecraft are falling together, not because gravity has switched off.

They'll say

"Heavier objects fall faster than lighter ones."

Actually

Without air resistance, mass makes no difference to how fast something falls. It's the equal squeeze, gravity accelerates everything at the same rate.

They'll say

"Gravity only pulls things down."

Actually

"Down" just means towards the centre of Earth's mass. Every object with mass pulls on every other object, in every direction, all the time.

They'll ask

"Why isn't an asteroid round like a planet?"

Actually

It takes enough mass for a body's own gravity to overpower its material strength and pull it into a sphere. Small bodies like asteroids never reach that threshold, so they stay lumpy.

Extension tasks

Investigate

Escape velocity. What does it actually take to beat Earth's gravity for good, and why is that speed the same whether you're launching a pea or a rocket?

S2E1 · Beating Gravity →
Reflect

Supermoons. The Moon's orbit isn't a perfect circle, so why does that make some full moons look noticeably bigger and brighter than others?

S2E2 · Super Moon →

Curriculum links

Physics

Gravity as a force, mass versus weight, freefall and orbit, the inverse square relationship with distance.

Maths

Doubling and halving, inverse proportion, ratio, working with very large and very small numbers.

Geography

Tides, Earth's rotation, how the Moon's gravity affects day length and the oceans.

History of science

Newton and the law of universal gravitation, how our understanding of gravity has changed over time.

Philosophy & PSHE

The idea of an invisible force shaping everything at scale, and what changes if a fundamental rule of physics shifts.

English

Justifying a choice under pressure, forming and defending evidence-based answers in the classroom activity.