Orbits & Gravity

IGCSE Edexcel Physics
8.1–8.6 The Solar System, gravity, orbital types and orbital speed
Key Concepts: Gravity is an attractive force between any two masses. It decreases with distance. Gravity provides the centripetal force for orbits. Planets orbit stars; moons orbit planets; comets have elliptical orbits. Orbital speed: $v = 2\pi r / T$. Closer orbits have shorter periods and higher speeds.

Section A — Gravity and the Solar System

1. State the structure of the Solar System, naming the types of objects that orbit the Sun. [3]
2. Describe how gravitational field strength varies with distance from a massive object. [2]
3. Explain why the gravitational field strength on the Moon's surface is less than on Earth's surface. [2]

Section B — Circular Orbits

4. Explain how gravity keeps a satellite in a circular orbit around Earth. [3]
5. State what happens to the orbital speed and period of a satellite if it moves to a higher orbit (greater radius). [2]
6. Describe differences between the orbits of planets, moons and comets. [3]

Section C — Orbital Speed Calculations

7. Write the equation for orbital speed, defining all symbols. [2]
8. A satellite orbits Earth at a radius of $7.5 \times 10^6\,\text{m}$ with a period of 5800 s. Calculate the orbital speed. [2]
9. A geostationary satellite orbits at radius $4.2 \times 10^7\,\text{m}$ and has a period of 24 hours (86 400 s). Calculate its orbital speed. [2]
10. State one use of geostationary satellites and explain why a 24-hour period is useful for this purpose. [2]

Total marks: 23

Mark Scheme

1. Planets, dwarf planets, moons (natural satellites), asteroids, comets [3 — 1 mark per type, up to 3]
2. Gravitational field strength decreases as distance from the object increases [1]; it follows an inverse-square relationship (halve distance → 4× stronger) [1] [2]
3. The Moon has a smaller mass than Earth [1]; its gravitational field at the surface is weaker, giving a lower value of $g$ [1] [2]
4. Gravity provides a centripetal (inward) force on the satellite [1]; this force continuously changes the direction of the satellite's velocity [1]; the satellite moves in a circle because the inward force balances the tendency of the satellite to travel in a straight line [1] [3]
5. Orbital speed decreases [1]; orbital period increases [1] [2]
6. Planets: nearly circular orbits around the Sun [1]; moons: orbit planets [1]; comets: highly elliptical orbits — very fast near the Sun, slow when far away [1] [3]
7. $v = 2\pi r / T$ [1]; $v$ = orbital speed (m/s), $r$ = orbital radius (m), $T$ = period (s) [1] [2]
8. $v = 2\pi \times 7.5 \times 10^6 / 5800 \approx 8.1 \times 10^3\,\text{m/s}$ [2]
9. $v = 2\pi \times 4.2 \times 10^7 / 86400 \approx 3.1 \times 10^3\,\text{m/s}$ [2]
10. Use: communications satellites / TV broadcasting [1]; a 24-hour period means the satellite stays above the same point on Earth, so a fixed dish can point at it continuously [1] [2]