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16.3 Planets, Satellites and Orbits

GCSE Physics · AQA · Chapter 16: Space

A free GCSE Physics lesson for AQA (8463) from OBA School: gravity and orbits, natural and artificial satellites, motion in a circle and the link between orbital speed and radius (physics only). Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State that gravity provides the force that keeps planets, moons and artificial satellites in orbit
  • (Higher tier) explain why a body moving in a circular orbit at constant speed has a changing velocity and an acceleration towards the centre
  • (Higher tier) explain why a body must move at a particular speed to stay in an orbit of a particular radius, and how speed changes with radius
  • Calculate orbital speed using distance = speed × time for one orbit
  • Compare geostationary and low polar orbits and their uses

Key words

  • Orbit: The curved path of one body around another, such as the Earth around the Sun.
  • Natural satellite: A body that orbits a planet naturally, e.g. the Moon.
  • Artificial satellite: A machine put into orbit by people, e.g. for communications, weather or navigation.
  • Orbital period: The time taken to go round an orbit once.
  • Centripetal force: A resultant force acting towards the centre of a circle, which keeps a body moving in the circle. (Higher tier)
  • Geostationary orbit: An orbit above the equator with a period of 24 hours, so the satellite stays above the same point on the Earth.
  • Polar orbit: A low orbit passing over the North and South Poles, with a period of about 1.5–2 hours.

Gravity keeps things in orbit

In each case a smaller body orbits a much more massive body, and the force that keeps it in orbit is the gravitational attraction between them.

  • Planet (e.g. Earth); Orbits the Sun; Shape of orbit almost circular (a slightly squashed circle, or ellipse)
  • Moon (natural satellite); Orbits a planet; Shape of orbit almost circular
  • Artificial satellite; Orbits the Earth (or another planet); Shape of orbit usually circular
  • Comet; Orbits the Sun; Shape of orbit very elliptical

Without gravity, an orbiting body would carry on in a straight line at constant speed (Newton’s first law). Gravity continually pulls it towards the centre, so it follows a curved path instead.

Moving in a circle (Higher tier)

  • The force of gravity on the planet acts towards the centre of the circle. It is the resultant force and is called a centripetal force.
  • The planet’s velocity is along the tangent to the circle, so it is always at right angles to the force.
  • The force changes the direction of the velocity, so the velocity changes all the time, even though the speed is constant.
  • A change in velocity means the planet is accelerating. The acceleration is in the same direction as the resultant force: towards the centre.
  • Because the force is at right angles to the motion, it does no work on the planet. So the planet’s kinetic energy, and therefore its speed, stays the same.

Exam tip: Speed is a scalar (size only); velocity is a vector (size and direction). In a circular orbit the size stays the same but the direction keeps changing, so velocity changes.

The right speed for each orbit (Higher tier)

Imagine launching a satellite sideways from the top of a very high mountain:

  • For an orbit of a particular radius there is only one speed that keeps a body in that circle.
  • The smaller the radius, the stronger the gravitational force, so the body must move faster to stay in orbit, and it takes less time to go round once.
  • If the speed of an orbiting body changes, the radius of its orbit must change. If a spacecraft fires its engines to speed up, it moves out to a higher orbit, where it then moves more slowly.
  • A satellite in a very low orbit is slowed by drag from the thin upper atmosphere, so it gradually spirals inwards.

Calculating orbital speed

In one orbit a body travels once round a circle, a distance equal to the circumference, 2πr. Using distance = speed × time:

orbital speed = 2 π r ÷ T

r = radius of orbit in metres, m (measured from the centre of the planet) T = orbital period in seconds, s speed in m/s

Worked example: speed of a weather satellite

The satellite is 800 km above the Earth’s surface. Radius of the Earth = 6400 km. Period = 100 minutes.
r = 6400 + 800 = 7200 km = 7.2 × 10⁶ m; T = 100 × 60 = 6000 s
speed = 2 × π × 7.2 × 10⁶ ÷ 6000 = 7540 m/s

Answer: about 7500 m/s (7.5 km/s)

Exam tip: Add the planet’s radius to the height above the surface to get r, and convert km to m and minutes or hours to seconds before you calculate.

Using artificial satellites

  • Geostationary; Height above surface about 36 000 km, above the equator; Period 24 hours, same direction as the Earth spins; Uses communications, satellite TV: a fixed dish can point at the satellite all the time
  • Navigation (GPS); Height above surface about 20 000 km; Period about 12 hours; Uses finding position on the Earth
  • Low polar orbit; Height above surface a few hundred km; Period about 1.5–2 hours; Uses weather, mapping and monitoring: the Earth spins beneath, so the whole surface is scanned each day

A geostationary satellite has the same period as the Earth’s spin, so it stays above the same point on the equator. Low-orbit satellites move much faster and take clearer, closer images.

Common mistakes

  • Saying there is no force on a satellite in orbit, or that an outward force balances gravity. The only force is gravity, acting inwards; it is unbalanced.
  • Saying “constant speed, so no acceleration”. The direction changes, so the velocity changes and the body accelerates towards the centre.
  • Using the height above the surface as the radius. Add the radius of the planet.
  • Thinking a satellite further out moves faster. Larger orbits need lower speeds and take longer.

Revision questions

Try these, then check your answers below each one.

Question 1. Which force keeps the planets in their orbits around the Sun? (Choose from: gravity, friction, magnetism, air resistance.)

Answer: gravity

Working: The gravitational attraction between the Sun and each planet keeps it in orbit.

Question 2. (Higher tier) True or false? A satellite moving at a constant speed in a circular orbit is accelerating.

Answer: True

Working: True. Its direction keeps changing, so its velocity changes. A change in velocity is an acceleration (towards the centre).

Question 3. (Higher tier) In which direction is the acceleration of a planet in a circular orbit? (Choose from: towards the centre of the orbit, along the direction of motion, away from the centre of the orbit, there is no acceleration.)

Answer: towards the centre of the orbit

Working: The resultant force (gravity) acts towards the centre, so the acceleration is towards the centre.

Related lessons

Continue in the full course

This lesson is section 16.3 (AGCPH1603) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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