You are here: Home > Practicals > Circular Motion

Circular Motion
AKA: Orbits


A bung on a length of string is whirled in a circle showing how satellites orbit.


This is an old physics demonstration where a mass on a string is whirled overhead in a circular movement. The mass is usually a large double holed bung on a length of strong string. The string should be securely tied to the bung by looping through one of the holes, back through the other hole and tying tight.

Equipment typically required (per demo):

  • Large rubber bung
  • Length of strong string
  • Eye protection (for all)

The bung is often whirled around in a large circle mimicking the way a satellite orbits around the Earth. If the string is allowed to wrap around the finger or hand holding the string, the orbit speed should increase as the bung gets closer to the hand.

Teachers sometimes use this equipment to also show what happens when they let go of the string while the bung is in orbit. The tension in the string may lead you to believe that the outward pull of the bung keeps it in orbit but it is the inward force exerted by the string that maintains the bung's circular motion. If the string is cut or let go, the bung will not travel radially outward but out in a straight line at a tangent.

There is a more complex version of this demonstration involving the same bung and string but this time the string runs through a handle and then attaches to a stack of masses. Using this model, the radius of orbit can be measured and also the speed of the orbit (by working out how many orbits occur in a fixed time). By using these values and the amount of mass connected to the string you may then calculate the centripetal force.



Wear eye protection.

The contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.