Motion in the UniverseEdexcel IGCSE Physics: Revision notes
Section 1
What is the universe made of?
The universe is a vast collection of billions of galaxies. A galaxy is itself a huge collection of billions of stars, held together by gravity. Our Sun and Solar System are located within the Milky Way galaxy, which is just one of billions of galaxies in the universe.
- Universe → billions of galaxies
- Galaxy → billions of stars
- Our Solar System is inside the Milky Way galaxy
Section 2
Why does gravitational field strength vary?
Gravitational field strength, g, is the force of gravity per unit mass. It varies from place to place because it depends on the mass of the body creating the gravitational field and the distance from its centre.
- A more massive planet produces a stronger gravitational field
- g is different on the Moon (about 1.6 N/kg) compared with Earth (about 9.8 N/kg) because the Moon has much less mass than Earth
- g decreases with increasing distance from the centre of a body
Do not confuse mass and weight — mass stays the same everywhere, but weight (W = mg) changes because g is different on different planets/moons.
Section 3
What keeps objects in orbit?
Gravitational force provides the centripetal force needed to keep objects moving in a curved orbital path. This single force explains many different orbits:
- Gravitational force causes moons to orbit planets
- Gravitational force causes planets to orbit the Sun
- Gravitational force causes artificial satellites to orbit the Earth
- Gravitational force causes comets to orbit the Sun
Without gravity constantly pulling the orbiting body towards the central mass, it would travel off in a straight line instead of curving into an orbit.
Section 4
How do the orbits of comets, moons and planets differ?
| Orbiting body | Shape of orbit | Notes |
|---|---|---|
| Planets | Nearly circular | Orbit the Sun at roughly constant distance |
| Moons | Nearly circular | Orbit their parent planet |
| Comets | Highly elliptical (elongated) | Orbit the Sun; travel very close to the Sun at one point and very far away at another |
Because a comet's orbit is a very elongated ellipse, its distance from the Sun — and therefore its orbital speed — varies hugely during one orbit, unlike the roughly steady speed of planets and moons in their near-circular orbits.
Section 5
How is orbital speed calculated?
For a body moving in a circular orbit, the orbital speed can be found using the relationship between the circumference of the orbit and the time taken to complete one full orbit (the time period):
v = 2πr / T
where v is orbital speed, r is the orbital radius, and T is the time period (time for one complete orbit).
This works because the distance travelled in one orbit is the circumference of the circular path, 2πr, and average speed = distance ÷ time.
A satellite orbits Earth at radius 7,000 km (7 × 10^6 m) with a time period of 6000 s. v = 2π × 7×10^6 / 6000 ≈ 7330 m/s.
Must Know
- The universe contains billions of galaxies; a galaxy contains billions of stars; our Solar System is in the Milky Way
- Gravitational field strength, g, varies with the mass of the body and distance from its centre — it is different on the Moon than on Earth
- Gravitational force causes moons to orbit planets, planets to orbit the Sun, satellites to orbit Earth, and comets to orbit the Sun
- Planet and moon orbits are nearly circular; comet orbits are highly elliptical
- Orbital speed: v = 2πr/T
- A comet's speed varies greatly around its elliptical orbit, unlike a planet's steady speed in its near-circular orbit
That's the notes covered.
Carry on to the next subtopic.