Orbital Motion (OCR Gateway GCSE Physics)

Circular Motion in an Orbit

• Planets travel around the Sun in orbits that are (approximately) circular
• Objects in circular orbit are travelling at a constant speed
• The orbit is a circular path, therefore 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

• The most common example of a non-circular orbit is a comet
• The orbits of comets are very different to those of planets:
  • The orbits are highly elliptical (very stretched circles) or hyperbolic
  • This causes the speed of the comets to change significantly as its 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

  

  Orbital path of a comet

• As the comet approaches the Sun, it loses gravitational potential energy and gains kinetic energy
• This causes the comet to speed up
• This increase in speed causes a slingshot effect, and the body will be flung back out into space again, having passed around the Sun
• As it moves away from the Sun the body will slow down, eventually finishing its orbit and falling back into towards the Sun once more
• In this way, a stable orbit is formed