P8.24SD0 · Section P8: Astrophysics

Motion in the universe

Key learnings

Structure of the universe and the light year
Learn the nested structure from large to small: the universe contains galaxies; a galaxy is a very large collection of stars held by gravity; a solar system is a star plus everything orbiting it; and our Sun is one star in the Milky Way galaxy. Astronomers use the light year as a unit of distance (not time) — the distance light travels in one year. The Sun lies about 29 000 light years from the galactic centre; that is the radius of its orbit, not the galaxy's diameter.
Gravitational field strength g and weight
Gravitational field strength g is the force of gravity on each kilogram of mass (g = W/m); its unit is the newton per kilogram (N/kg). g is larger for a more massive body, so it varies between bodies: about 9.8 N/kg at Earth's surface but only 1.6 N/kg on the Moon, which has less mass. Weight is W = mg, so a 10 kg bag has the same mass everywhere but weighs 10 × 9.8 = 98 N on Earth and 10 × 1.6 = 16 N on the Moon. g differs because the bodies have different masses.
Gravity provides the orbital force
Every orbit is kept going by a gravitational force pulling the orbiting body towards the larger central body. This inward force is the centripetal force that curves the path into an orbit. Planets, moons and satellites follow near-circular orbits at constant speed; comets follow highly elliptical orbits, fastest when closest to the Sun and slowest when furthest away. For a near-circular orbit, $v = 2\pi r/T$ , with $2\pi r$ the circumference and T the orbital period.

Common mistakes

Drawn from real examiner reports.

Unit of g is N/kg, not N

The unit of gravitational field strength is the newton per kilogram (N/kg), because g is a force per unit mass (g = W/m). Writing just the newton (N) — the unit of the force or weight — loses the mark. Keep three ideas apart: gravitational force (N, causes the orbit), gravitational field strength g (N/kg, force per kilogram) and weight (N, = mg).

Orbital force is gravity, not energy

When asked what force keeps a moon or planet in orbit, the answer is the gravitational force (gravity), acting towards the central body. Candidates instead wrote "gravitational potential energy" or "gravitational field strength", which are not forces and earn no credit. Name the force itself — gravity — as the cause of the orbit.

June 2024 Paper 1P Q1(a)(ii): gravitational potential energy or gravitational field strength written where the gravitational force was required — not credited.

State both sides of an orbit difference

A "difference between two orbits" needs both bodies described for the mark. Writing "the comet's orbit is elliptical" without adding "whereas the planet's orbit is approximately circular" is incomplete. Pair every statement: circular vs elliptical, constant speed vs changing speed, short period vs long period.

June 2024 Paper 1P Q1(b): comet elliptical stated without the planet being circular — an incomplete difference.

What orbits what

Be precise about what orbits what: comets and planets orbit the Sun (a star); moons and artificial satellites orbit planets. The most popular wrong answer for "an object that orbits the Earth" was a comet — comets orbit the Sun — and some wrongly said asteroids orbit the Earth. A comet does not orbit the Earth.

June 2024 Paper 1P Q1(a)(iii): comet or asteroid wrongly given as objects that orbit the Earth.

Force arrow points to the central body

In a diagram the gravitational force arrow must point towards the central body — from a moon towards its planet, from a star towards the galactic centre. Drawing it towards the orbiting body, outward, or flipped 180 degrees loses the mark. The force is always the inward, centripetal pull that curves the orbit.

Orbital radius is not a diameter

A distance quoted "from the centre" (for example 29 000 light years to the galactic centre) is already the orbital radius r — do not halve it before using $v = 2\pi r/T$ . Only halve a value that is given as a diameter. Mixing up radius and diameter changes the answer by a factor of two.

Sig figs and powers of ten in orbit sums

When using v = 2πr/T with values in standard form, keep the powers of ten organised and give the answer to the number of significant figures asked for (often 2 s.f.). Marks were lost for wrong significant figures, and for power-of-ten errors when dividing by a number in standard form. Always add the unit m/s.

November 2024 Paper 1P Q1(c): answers not given to the required 2 significant figures, with power-of-ten errors dividing a standard-form value.

Exam tips

Use v = 2πr/T carefully
Write $v = 2\pi r/T$ , substitute r and T in standard form, work out the powers of ten, then round to the significant figures requested (often 2 s.f.) and add the unit m/s. Check whether a quoted distance is already the radius before substituting.
Give paired orbit comparisons
For "difference between orbits" questions, give a paired comparison naming both bodies — circular vs elliptical, constant vs changing speed, short vs long period. A one-sided statement about only one body does not earn the comparison mark.
Name the gravitational force
For "what keeps it in orbit" questions, name the gravitational force (gravity), acting towards the central body, as the cause of the orbit. Do not answer with gravitational potential energy or gravitational field strength — those are not forces.
Keep orbit diagrams simple and 2D
Draw a simple 2D orbit (not 3D), put the central body clearly at the centre of a near-circular orbit, and draw the force arrow pointing inward towards that central body. Label both bodies, and use the marks available to judge how many separate points are needed.

Revision notes