This mini-lesson covers Eduqas Topic 10 — Space physics: the Solar System and how the Sun formed, how gravity keeps things in orbit, the stability of orbits and satellites, and the red-shift evidence for an expanding Universe and the Big Bang, plus an extension on the cosmic microwave background.
Make a guess: why doesn't the Moon fall to Earth?
Work through each screen, answer the questions as you go (some are reasoning, some are calculations) and collect ⭐ stars. Items marked Higher tier are for the higher paper only. Press Start when you're ready.
Our Solar System is held together by gravity acting around the Sun — our nearest star. It contains:
How the Sun formed: a huge cloud of dust and gas was pulled together by gravity. As it collapsed it got hotter until hydrogen nuclei began to fuse, releasing energy. The Sun is now stable: the inward pull of gravity (collapse) is balanced by the outward push of expansion caused by the fusion energy.
Zoom out: the Sun is just one of hundreds of billions of stars in our galaxy, the Milky Way — and the Milky Way is just one of billions of galaxies in the Universe.
Tap the word that best describes each member of the Solar System.
A planet, moon or artificial satellite moves in a (near-)circular orbit. Even at constant speed its velocity is always changing, because its direction is always changing. A changing velocity means it is accelerating — so a resultant force must be acting.
That force is gravity, and it always points towards the central body. A force directed to the centre of a circle is called the centripetal force.
Misconception buster: an orbiting body is not "weightless because there's no gravity". It is constantly falling towards the central body — gravity provides the centripetal force — but its sideways (tangential) speed means it keeps missing, so it loops round.
For a stable orbit at a given radius, the body must travel at exactly the right speed so that gravity provides the centripetal force needed to curve it round. This links speed to radius:
That is why Mercury races round the Sun far faster than distant Neptune, and why a low satellite zips round the Earth in about 90 minutes while the far-off Moon takes about a month. The mass of the central body matters too: at the same radius, an orbit round a more massive body must be faster, because its gravity is stronger.
Misconception buster: if an orbiting body speeds up, it can no longer stay at the same radius — to stay stable its orbit radius must change. For a given radius there is only one stable orbital speed.
Make a guess: does red-shift mean we are at the centre?
Elements in a star absorb light at fixed wavelengths, giving a pattern of dark spectral lines — a fingerprint of the chemicals present. If the source is moving away from us, its light waves are stretched, so every line shifts towards the red (longer-wavelength) end. This is red-shift.
Misconception buster: the colour of the star doesn't simply "turn red". Red-shift means the whole spectral-line pattern moves towards longer wavelengths because space between us and the galaxy is stretching the light's wavelength.
Light from distant galaxies is red-shifted, so almost every galaxy is moving away from us. Crucially:
This is exactly what you expect if space itself is expanding — every galaxy gets carried away from every other one. So red-shift is strong evidence that the Universe is expanding, which supports the Big Bang theory: the Universe began about 13.8 billion years ago in an extremely hot, dense state and has been expanding ever since.
Misconception buster: galaxies are not flying through fixed space away from a central explosion. The space between galaxies is stretching, which stretches the light's wavelength — and the more space there is in between (more distant), the faster the recession.
If the Universe really did begin in a hot, dense Big Bang, the radiation released should still be detectable today. It is: the cosmic microwave background (CMB) — faint microwave radiation coming from every direction in space, first detected in 1964.
As the Universe expanded, this radiation was stretched (red-shifted) from short-wavelength, high-energy radiation into the microwaves we detect now. The Big Bang is the only model scientists have that explains the CMB, so it is powerful supporting evidence.
Two pillars of Big Bang evidence:
1. Red-shift of distant galaxies → the Universe is expanding.
2. Cosmic microwave background → leftover radiation from the hot early Universe.
Tap a statement, then tap the box it belongs in.
Solar System: Sun + 8 planets, dwarf planets, moons, asteroids, comets — held by gravity; we sit in the Milky Way galaxy. The Sun formed from dust and gas pulled together by gravity; now fusion energy (expansion) balances gravitational collapse.
Orbits (HT): gravity provides the centripetal force; velocity always changes (direction changes) even at constant speed.
Stable orbits (HT): smaller radius or more massive central body → faster orbital speed; for a given radius there is one stable speed.
Red-shift: distant galaxies' light is stretched to longer wavelengths; more distant → greater red-shift → faster recession.
Expanding Universe + Big Bang: red-shift shows expansion and supports the Big Bang (~13.8 billion years ago).
CMB (extension, not on the Eduqas spec): microwave afterglow from every direction — further Big Bang evidence.
You've covered both parts of Eduqas Topic 10 — the Solar System & orbital motion, and red-shift, the expanding Universe & the Big Bang. Press Finish to see your score.
You've worked through Space Physics for Eduqas GCSE Physics. 🎉
Your stars: 0 / 0
Next: test yourself in the Verify stage.