Circular Motion
Forces and Motion

Whirling a rubber bung and letting go

Practical Activity for 14-16 PRACTICAL PHYISCS


To show the tangential motion of an object when it is released from circular motion.

Apparatus and Materials

  • Rubber bung or tennis ball
  • String, length 1 m approx

Health & Safety and Technical Notes

Securely attach the rubber bung or ball to the string. Hold the string tightly as you whirl it around. At the point when the bung is released, the tangent to the direction of motion should point in a direction clear of obstacles.

Read our standard health & safety guidance


  1. Swing the bung round in a circle, with a radius of about 50 cm, horizontally above the head.
  2. At the moment when it is nearest to the class, release the string so that it travels tangentially towards the side of the room.

Teaching Notes

  • It is useful to say that the bung will be released when it is nearest to the class and count it down by saying NOW...NOW.... then release it. The class will no doubt duck, convinced that centrifugal force will drive it out to them.. It will travel tangentially out (to a clear space) and not radially out (into the class).
  • Ask: "How does it travel before release?" (In a circle.) "How does it travel after release?" (It continues its motion in a straight line along the tangent.)
Circular Motion
can be analysed using the quantity Centripetal Acceleration
can be described by the relation F=m(v^2)/R
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