Orbital Motion and Gravity
Orbital Motion and Gravity
Gravity is the force that holds the Solar System together, keeps the Moon in orbit around the Earth, and keeps your feet on the ground. Understanding how gravity governs orbital motion is essential for explaining the behaviour of planets, moons, and satellites.
Gravity as a Non-Contact Force
Gravity (or gravitational force) is a force of attraction between any two objects that have mass. It is a non-contact force — it acts at a distance without the objects touching.
Key features:
- Gravity acts between all objects with mass, but is only noticeable when at least one object is very massive (like a planet or star)
- It is always attractive — it pulls objects together (unlike electric or magnetic forces, which can repel)
- The strength of gravity increases with mass and decreases with distance
Gravitational Field Strength
The gravitational field strength (g) at a point is the force per unit mass acting on an object at that point:
weight = mass x gravitational field strength
W = m x g
Where:
- W = weight (newtons, N)
- m = mass (kilograms, kg)
- g = gravitational field strength (N/kg)
On Earth's surface, g is approximately 9.8 N/kg (often rounded to 10 N/kg in calculations).
| Location | g (N/kg) |
|---|---|
| Earth surface | 9.8 |
| Moon surface | 1.6 |
| Mars surface | 3.7 |
| Jupiter surface | 24.8 |
Mass is the amount of matter in an object — it stays the same everywhere. Weight depends on the gravitational field strength and changes depending on location.
Gravitational Fields
A gravitational field is a region around a mass where other masses experience a force. For a planet:
- The field lines point inward towards the centre of the mass (because gravity is attractive)
- Closer to the surface, the field is stronger (lines are closer together)
- Far from the planet, the field weakens
- Close to the surface of a large body, the field is approximately uniform (field lines are parallel)
Circular Orbits
The planets orbit the Sun, and moons orbit planets, in approximately circular paths (actually slightly elliptical, but often treated as circular at GCSE).
For an object to move in a circle, there must be a centripetal force directed towards the centre of the circle. For orbital motion, this centripetal force is provided by gravity.
- The gravitational force on a planet points towards the Sun
- This force is perpendicular to the planet's velocity at every instant
- It continuously changes the direction of the planet's motion (but not its speed, in a circular orbit)
- The planet keeps moving forward due to its velocity, but gravity constantly curves its path into a circle
An orbiting object is in constant free fall towards the body it orbits, but its forward velocity means it keeps missing.
Orbital Speed and Radius
For orbits around the same central body:
- Planets closer to the Sun orbit faster and have shorter orbital periods (Mercury: 88 days; Neptune: 165 years)
- Planets further from the Sun orbit more slowly and have longer orbital periods
This is because:
- Closer planets experience stronger gravity (more centripetal force), requiring a higher speed to maintain a stable orbit
- The circumference of the orbit is also smaller, so there is less distance to cover
The relationship is: for a higher orbit, both the speed decreases AND the distance increases, so the period increases significantly.
Satellites
A satellite is any object that orbits a larger body. This includes:
- Natural satellites — the Moon (orbits Earth), Titan (orbits Saturn)
- Artificial satellites — the International Space Station, GPS satellites, communication satellites, weather satellites
Artificial satellites orbit at different heights depending on their purpose:
| Type | Height | Orbital period | Use |
|---|---|---|---|
| Low Earth orbit (LEO) | 200-2,000 km | ~90 minutes | ISS, Earth observation, some communication |
| Geostationary orbit | ~36,000 km | Exactly 24 hours | TV broadcasting, weather monitoring |
A geostationary satellite orbits above the equator with a period of 24 hours, so it appears to stay above the same point on Earth's surface. This is ideal for communications because satellite dishes can point at a fixed position in the sky.
Comets
Comets have highly elliptical (elongated) orbits:
- When a comet is close to the Sun, gravity is stronger, and the comet moves faster
- When it is far from the Sun, gravity is weaker, and the comet moves slower
- Speed is greatest at the closest approach (perihelion) and least at the furthest point (aphelion)
Exam Tips
- Always state that gravity provides the centripetal force for orbital motion
- Do not say gravity "pulls the planet inward" as if it is moving towards the Sun — it is the continuous change of direction that creates the circular path
- For satellite questions, know the difference between geostationary and low Earth orbit (height, period, uses)
- Weight changes with location; mass does not — always be precise with these terms