Orbits

• The structure of the solar system is determined by orbits - the paths that objects take as they move around each other in space
• Gravity


  Provides the force that creates orbits
• Orbits


  • The planets move around the Sun in almost circular orbits
  • If an object is travelling in a circle it is constantly changing direction, which means it is constantly accelerating
  • This also means it is constantly changing velocity (but NOT changing speed)
  • For an object to accelerate, there must be a force acting on it
  • This force is directed towards the centre of the circle
  • The force would cause the object to just fall towards whatever it was orbiting, but as the object is already moving, it just causes it to change its direction
  • The object keeps accelerating towards what it's orbiting but the instantaneous velocity (which is at a right angle to the acceleration) keeps it travelling in a circle
  • The force that makes this happen is provided by the gravitational force (gravity) between the planet and the Sun (or between the planet and its satellites)
• The planet is 'trying' to move in this direction


  The force is always towards the centre of the circle
• Orbit size


  • The closer you get to a star or planet, the stronger the gravitational force is
  • The stronger the force, the faster the orbiting object needs to travel to remain in orbit (to not crash into the object that it's orbiting)
  • For an object in a stable orbit, if the speed of the object changes, the size (radius) of its orbit must do so too. Faster moving objects will move in a stable orbit with a smaller radius than slower moving ones
• Orbit speed


  • v=30 km/s
  • v=42 km/s