Orbital motion

• Circular Motion in an Orbit

  • Planets travel around the Sun in orbits that are circular
  • Objects in circular orbit are travelling at a constant speed
  • The orbit is a circular path, therefore the direction the object is travelling constantly changes direction
  • Change in direction causes a change in velocity
  • Acceleration is the rate of change of velocity
  • A resultant force is needed to cause an acceleration
  • The resultant force is gravity and it must act at right angles to the instantaneous velocity of the object creating a circular orbit towards the centre of the orbit
• Non-Circular Orbits

  • 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