Optional Practical: Orbit simulation (Algodoo)


When a body is in a circular orbit round another one the centripetal force required for the circular motion is provided by the gravitational attraction, by equating these terms we can find a value for the speed,v of the orbit.

fraction numerator G M m over denominator r squared end fraction equals fraction numerator m v squared over denominator r end fraction v equals square root of fraction numerator G M over denominator r end fraction end root

Setting up the simulation

In reality the value of G is 6.67 x 10-11 Nm2kg-2 this means that orbits around low mass objects would be very slow, In Algodoo you can vary the gravitational constant to speed things up a bit.

  • Open a new scene and show the grid

  • Draw a circle, double click it and from materials give it a mass of 1000 kg and an attraction of 0.04 Nm2kg-2 (this is G).
  • Place a second smaller circle 10 m to the right of the first one and give it a mass of 0.005 kg
  • Calculate the speed necessary to put the smaller mass in orbit set the velocity using the "velocities" options.

  • Double click the orbiter (pause first) and display the information. You can make this stay in view by dragging the window to one side.

  • Observe the values for KE, PE and total energy as the body orbits.


  • Observe what happens if you change the velocity of the orbiter.
  • Observe the effect of adding a small amount of air resistance (note the changes in energy)

  • Try adjusting the "attraction". Observe the changes in energy, try to keep the orbiter in orbit.
  • Set the large mass to 500 kg and calculate the radius and speed for a circular orbit. Try this out to see if you are right.
  • Make a mini solar system with several "planets".
  • See if you can put a moon in orbit around a planet.
  • Try putting a 500 kg mass in orbit round another 500 kg mass.
  • Cover the surface of a planet with water and observe the tidal effect as a moon orbits.
All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.