Orbits and Gravity

This lesson covers the role of gravity in orbital motion — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand how gravity keeps planets and moons in orbit, how orbital speed varies with distance, and the difference between geostationary and polar satellites.

---

Gravity and Orbits

What Keeps Objects in Orbit?

Gravity provides the centripetal force needed to keep objects moving in curved paths (orbits). Without gravity, an orbiting object would fly off in a straight line (Newton's first law — an object in motion continues in a straight line unless acted on by a force).

Key principles:

• The Sun's gravitational field keeps the planets in orbit around it.
• A planet's gravitational field keeps its moons in orbit around it.
• The Earth's gravitational field keeps artificial satellites and the Moon in orbit.
• Gravity acts as a centripetal force — it constantly pulls the orbiting object towards the centre (the more massive body), preventing it from flying off in a straight line.

Exam Tip: When asked what keeps a planet in orbit, say: "The gravitational attraction between the planet and the Sun provides the centripetal force needed for circular motion." Do not just say "gravity" — you must link it to centripetal force.

---

Gravitational Field Strength

The gravitational field strength (g) of a body decreases with distance from its centre. This means:

• Objects closer to a massive body experience a stronger gravitational pull.
• Objects further away experience a weaker gravitational pull.

Location	Gravitational Field Strength (N/kg)
Earth's surface	9.8
Moon's surface	1.6
Mars' surface	3.7
Jupiter's surface	24.8
Far from any body	≈ 0

Exam Tip: Gravitational field strength depends on both the mass of the body and the distance from it. A more massive body has a stronger gravitational field; the further you are from a body, the weaker its gravitational field.

---

Circular Orbits

For an object in a roughly circular orbit:

• The speed remains approximately constant (the object moves the same distance in each unit of time).
• However, the velocity is constantly changing because the direction of motion is constantly changing.
• Since velocity is changing (even though speed is not), the object is accelerating — this is centripetal acceleration, directed towards the centre of the orbit.
• The force causing this acceleration is gravity.

Speed vs Velocity

Quantity	Scalar or Vector?	Constant in Circular Orbit?
Speed	Scalar (magnitude only)	Yes — constant
Velocity	Vector (magnitude + direction)	No — direction changes continuously

Exam Tip: This is a common 6-mark question topic. Be clear: in a circular orbit, speed is constant but velocity is not, because velocity includes direction. The changing velocity means the object is accelerating, and the force causing this acceleration is gravity (acting as centripetal force).

---

Orbital Speed and Distance

Orbital speed depends on distance from the central body:

• Objects in closer orbits move faster.
• Objects in further orbits move slower.

This is why:

• Mercury (closest planet to the Sun) has the highest orbital speed (~47 km/s).
• Neptune (furthest planet from the Sun) has the lowest orbital speed (~5.4 km/s).

Planet	Distance from Sun (AU)	Orbital Speed (km/s)
Mercury	0.39	47.4
Venus	0.72	35.0
Earth	1.00	29.8
Mars	1.52	24.1
Jupiter	5.20	13.1
Saturn	9.54	9.7
Uranus	19.2	6.8
Neptune	30.1	5.4

The reason is straightforward: closer objects are in a stronger gravitational field, so they need to travel faster to maintain their orbit. If they moved more slowly, gravity would pull them inward.

---

Artificial Satellites

An artificial satellite is a human-made object placed in orbit around the Earth (or another body). There are two main types of orbit used for artificial satellites:

Geostationary Satellites

Feature	Detail
Orbit	Above the equator
Altitude	Approximately 35,786 km above Earth's surface
Orbital period	Exactly 24 hours (same as Earth's rotation)
Appears to be	Stationary above a fixed point on Earth's surface
Uses	Communications satellites, TV broadcasting, weather monitoring of a fixed region

Because a geostationary satellite orbits at the same rate as the Earth rotates, it always stays above the same point on the equator. This makes it ideal for satellite TV — your dish can point at a fixed position in the sky.

Polar Satellites

Feature	Detail
Orbit	Passes over (or near) the North and South Poles
Altitude	Typically 200–1,000 km above Earth's surface (much lower)
Orbital period	Approximately 90 minutes to 2 hours
Coverage	As the Earth rotates beneath, different strips of the surface are scanned
Uses	Weather observation, environmental monitoring, mapping, military surveillance, scientific research

Because the Earth rotates beneath a polar satellite, the satellite eventually passes over the entire surface of the Earth. This makes polar orbits ideal for mapping and global monitoring.

Comparing Satellite Orbits

Feature	Geostationary	Polar
Altitude	High (~36,000 km)	Low (~200–1,000 km)
Period	24 hours	~90 minutes
Position	Fixed above equator	Passes over poles
Speed	Slower (further out)	Faster (closer in)
Coverage	One fixed region	Entire Earth surface over time
Main uses	Communications, TV	Weather, mapping, surveillance