Circular Orbits (Edexcel GCSE Physics)

• There are many orbiting objects in our solar system and they each orbit a different type of planetary body

Orbiting Objects or Bodies in Our Solar System Table

• A smaller body or object will orbit a larger body
  • For example, a planet orbiting the Sun

• In order to orbit a body such as a star or a planet, there has to be a force pulling the object towards that body
  • Gravity provides this force

• The gravitational force exerted by the larger body on the orbiting object is always attractive
  • Therefore, the gravitational force always acts towards the centre of the larger body

• The gravitational force will cause the body to move and maintain in a circular path

Gravitational attraction causes the Moon to orbit around the Earth

Orbits 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 the same direction around the Sun

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

The orbits of planets around the Sun

Orbits 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 orbit
  • The greater the speed of the orbit

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

The elliptical orbit of a comet

Artificial Satellites

• A satellite needs to travel at a specific speed to maintain a circular orbit at a particular distance from the object
• If the speed of the satellite is too large:
  • The radius of the orbit will increase and the satellite will spiral into space
  • This is because the gravitational attraction cannot provide enough force to keep it in orbit

• If the speed of the satellite is too low:
  • The radius of the orbit will decrease and the satellite will move towards the object it should be orbiting
  • This is because the gravitational attraction is too strong to maintain a constant orbital radius

Diagram showing how the speed of an artificial satellite affects its orbit

• If an artificial satellite is to change the radius at which it is orbiting then the speed at which it is travelling must change
• To maintain a stable orbit:
  • If the speed increases the radius must increase
  • If the speed decreases the radius must decrease

• Planets travel around the Sun in orbits that are (approximately) circular
  • Objects in circular orbit are travelling at a constant speed but a changing velocity

• In a circular path, the direction in which the object is travelling will be constantly changing direction
  • A change in direction causes a change in velocity

• Acceleration is the rate of change of velocity
  • Therefore, if the object is constantly changing direction then its velocity is constantly changing and so the object in orbit is accelerating

• A resultant force is needed to cause an acceleration
• This resultant force is gravity and it must act at right angles to the instantaneous velocity of the object to create a circular orbit
  • This is always towards the centre of the orbit
  • The instantaneous velocity of the object is the velocity at a given time

The direction of the instantaneous velocity and the gravitational force at different points of the Earth’s orbit around the sun