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Destination Space

Gravity

Subject Area
Space Science
Estimated Classroom time
50 minutes
Difficulty
2 of 3 — Moderate

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

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 learner picks something up, their arm beats the pull of the entire Earth.

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Session objectives

  • State that gravity is a universal force generated by anything with mass.
  • Apply the two rules of gravity: more mass means a stronger pull, and more distance means a weaker pull.
  • Explain why planets and stars are spherical, using the idea of the equal squeeze.
  • Describe how the Moon acts as a gravitational brake on Earth’s rotation.
  • Reflect on what would change on Earth without the Moon’s gravitational pull.

Misconceptions to watch for

  • Astronauts float because there’s no gravity in space.

    Gravity in low orbit is still around 90% of what it is on the surface. Astronauts float because they and their spacecraft are falling together, not because gravity has switched off.

  • Heavier objects fall faster than lighter ones.

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

  • Gravity only pulls things down.

    '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.

If a student asks…

Why isn’t an asteroid round like a planet?
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.

Got longer?

Teacher Tips

Hook

  • Most classes choose ‘everyone flying into space’. It’s worth asking “What would actually push you upwards?”, because nothing would. That usually gets them to C on their own rather than being told.
  • It helps not to let the show of hands settle it. If the room lands unanimously on one option, you might 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.
  • Gravity being the weakest of the four forces is worth flagging early. Learners tend to resist this because it doesn’t match how it feels, so you might 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 Pathways

  • Pathway 2’s distance rule is the bit that tends to get skimmed. If someone’s moving fast, it helps to ask “What happens to gravity when you double the distance?” “Half” is the answer nearly everyone gives first, and it’s better caught here than in the activity.
  • Anyone watching the film is best asked to explain the equal squeeze in their own words before they move on. Learners can repeat the phrase without having pictured it, and if they can’t say “What’s doing the squeezing?” it may not have landed yet.

Classroom Activity

  • It helps to encourage learners to re-read each scenario carefully before committing to an answer. Most wrong answers come from fixing on one word, distance or mass, and missing the other, not from mishearing either one.
  • Throughout, it’s worth emphasising that gravity is always relative, determined by distance and mass.
  • Learners could come up with their own relative gravity scenarios to pose to each other and/or the class. This can spark some interesting debate about the relative mass of objects.

Plenary

  • The reflective question is a good one for learners who find “what did you learn” hard, because it’s a scenario rather than a recall. If someone’s stuck, you could ask “What would happen when you tried to stand up?”
  • Expect answers about heavier objects and nothing about the atmosphere, tides or the Moon’s orbit. If that happens, you might ask “What else on Earth is being held in place by gravity?”
Learner journal tip

If using the editable journal, it helps to have each learner make their own copy at the start of the course and save it somewhere easy to find again. Perhaps in a folder on their laptop or in their school drive. It carries across every session, so if the plenary runs short, they can finish Part 2 or the later sections at home without losing earlier reflections.

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 & 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.