Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Whirling a rubber bung and letting go


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.



a Swing the bung round in a circle, with a radius of about 50 cm, horizontally above the head. 

b 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

1 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).  

2 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.) 


Related experiments

CO2 puck on a glass plate

The motion of the Moon around the Earth

Whirling a rubber bung on a string

Fine beam tube


Cookie Settings