Waves and space · GCSE Physics
The solar system
GCSE Physics only (not Combined Science): teacher-written revision on the solar system, gravity and orbits, planets, moons, satellites, and why mass is unchanged while weight depends on g.
Physics only, not Combined. Gravity holds the solar system. Bigger mass, bigger pull; bigger distance, much smaller pull. Orbits are free-fall with a sideways velocity.
The important bits
What you need to know
- 1
This topic is GCSE Physics (Triple) on AQA-style specifications, not Combined Science. Combined still uses gravity and g, but not the space chapter as a whole.
- 2
The solar system: the Sun (a star), planets, dwarf planets, moons, asteroids and comets, all held by gravitational attraction.
- 3
Weight on other planets changes because g changes; mass does not. W = mg still applies, with a different g.
- 4
Orbital motion is continual free-fall: gravity provides the centripetal force that changes the direction of velocity. Speed can be constant while velocity changes.
- 5
If a satellite speeds up it tends to move to a higher orbit where gravity is weaker. A stable circular orbit needs the right speed for that radius.
- 6
Natural satellites are moons. Artificial satellites may be in polar orbits (mapping, weather) or geostationary orbits (communications, same point above the equator, 24-hour period).
- 7
Comets have very elliptical orbits: fast and close when near the Sun, slow and far at the other end. Gravity is still the force; the distance changes, so the speed changes.
- 8
There is gravity in space. Astronauts feel weightless because they are falling with their spacecraft, not because g is zero.
Quotations worth analysing
Short evidence. Real method.
“Space physics is assessed on GCSE Physics, not on Combined Science.”
If you sit Combined, do not spend scarce revision time here unless your teacher has included extra material. If you sit Physics, this chapter is required.
“Gravity provides the force that keeps planets and satellites in orbit.”
No extra “orbital force” is needed. Gravity pulls inwards; the object’s velocity is sideways, so it keeps missing the planet.
“Mass is constant; weight depends on gravitational field strength.”
The same 10 kg mass weighs about 98 N on Earth and about 16 N on the Moon. Inertia does not change.
Go deeper
Orbits are gravity plus a sideways velocity
If you could throw a ball sideways fast enough, the ground would curve away as fast as the ball fell — that is an orbit, not a magic floating force. Gravity still acts, so the satellite is accelerating even if its speed is constant, because direction is changing. A higher orbit has a longer period and a lower orbital speed. Students often write that there is no gravity in space. There is; astronauts feel weightless because they are falling at the same rate as their spacecraft. Mass of the planet sets g at the surface. That is why you would weigh less on Mars but your mass, and your inertia, would be unchanged. For Physics-only exams, name geostationary versus polar: 24-hour period over the equator for dishes that do not have to track, versus a sweep over the poles for whole-Earth imaging.
Go deeper
The inventory of the solar system is a comparison, not a list to chant
The Sun is a main-sequence star, far more massive than anything else in the system, so it sits at the gravitational centre. Planets orbit it; moons orbit planets. Asteroids are rocky and mostly between Mars and Jupiter; comets are icy with stretched orbits that grow tails when the Sun sublimes their ice. Dwarf planets such as Pluto orbit the Sun but have not cleared their neighbourhood. In six-mark questions, compare two objects: a comet versus a planet (orbit shape, composition, tail), or a geostationary satellite versus the Moon (period, use, height). “There are eight planets” without gravity will not score. Always return to the force that holds the system together and to the fact that this whole page is Triple Physics, not Combined.
See the idea in action
A 70 kg student has weight 70 × 9.8 = 686 N on Earth. On Mars, g ≈ 3.7 N/kg, so weight = 70 × 3.7 = 259 N. Mass is still 70 kg. A geostationary satellite has a period of 24 h = 86 400 s and stays above the same point on the equator so a TV dish need not turn. A polar-orbit weather satellite might have a period of about 90 minutes and a much lower orbit; it is not geostationary. Both stay in orbit because gravity pulls them into free-fall around Earth while they have sufficient sideways speed. There is no “zero gravity” at either height.
Exam technique
Turn knowledge into marks
Write “GCSE Physics only” in your head before you revise this. Use W = mg with the local g. Explain orbits as gravity plus sideways velocity, not as the absence of forces. Name the job of a polar versus a geostationary satellite.
Common mistakes
Do not give these marks away
- 01
Saying there is no gravity in space, or that mass changes on the Moon.
- 02
Revising this chapter for Combined Science as if it were required.
- 03
Claiming geostationary satellites sit above the UK or have a 90-minute period.
Why does an astronaut in orbit feel weightless?
AThere is no gravity beyond the atmosphere
BThe astronaut’s mass has become zero
CThe astronaut and spacecraft are in free-fall together; gravity still acts
DMagnetic fields cancel weight
Show the answer
The astronaut and spacecraft are in free-fall together; gravity still acts. Orbit is continuous falling with enough sideways speed to miss the Earth. g is not zero; the cabin falls with the person, so they do not press on the floor.
Quick questions
If this is the bit you searched
Is the solar system on Combined Science?
Usually not. On AQA-style routes it sits in GCSE Physics (Triple). Combined still includes gravity, weight and g, but not this space chapter.
What is a geostationary satellite?
An equatorial satellite with a 24-hour period, so it stays above the same point on Earth. Used for communications and TV.
Do you weigh less on a high mountain because you are closer to space?
g is slightly smaller further from Earth’s centre, so weight is slightly smaller. Mass is unchanged. Orbiting “weightlessness” is free-fall, a different effect.
What force keeps the Earth in orbit around the Sun?
The Sun’s gravitational attraction. It acts as the centripetal force that continually changes the Earth’s direction.