Orbital Motion (Edexcel IGCSE Physics)

• The Solar System is made up of many bodies which orbit around other bodies
• The orbiting bodies in the Solar System are shown in the table below:

Table of orbiting bodies in the Solar System

orbiting body	body it orbits
planet	the Sun
moon	planet
comet	the Sun
asteroid	the Sun
artificial satellites	the Earth

 

• Smaller bodies orbit around larger bodies
  • For example, planets orbit the Sun

• Orbital motion is a result of the gravitational force of attraction acting between two bodies
• This gravitational force
  • always acts towards the centre of the larger body
  • causes the orbiting body to move in a circular path

The gravitational force of attraction causes the Moon to orbit around the Earth

Orbital motion of planets

• There are several similarities in the way different planets orbit the Sun:
  • Their orbits are all slightly elliptical (stretched circles) with the Sun at one focus (approximately the centre of the orbit)
  • They all orbit in the same plane
  • They all travel in the same direction around the Sun

• There are also a few differences:
  • They orbit at different distances from the Sun (different orbital radius)
  • They orbit at different speeds
  • They all take different amounts of time to orbit the Sun

• The further away a planet is from the Sun, the slower it travels and therefore the longer it takes to orbit

The planets closest to the Sun have higher orbital speeds, whereas the planets furthest from the Sun have lower orbital speeds

Orbital motion of moons

• Moons orbit planets in a circular path
• Some planets have more than one moon
• The closer the moon is to the planet:
  • the shorter the time it will take to complete each orbit
  • the greater the speed of the orbit

Orbital motion of comets

• The orbits of comets are very different to those of planets:
• Their orbits are highly elliptical (very stretched) or hyperbolic
  • This causes the speed of the comets to change significantly as their distance from the Sun changes
  • Not all comets orbit in the same plane as the planets and some don’t even orbit in the same direction

• As the comet approaches the sun, its speed increases
• As it moves further away from the sun, its speed decreases

Comets follow highly elliptical orbits around the Sun