Balloon Kebabs

Produced by the Institute of Physics, this Physics to Go video shows a short demonstration that can be used to engage your students. The teacher notes include the equipment needed, tips and contain a full explanation of the physics involved.

Amaze your audience by pushing a kebab skewer into a balloon. The trick is to stop the balloon skin from splitting and bursting. This trick works through an understanding of surface properties.

A balloon is formed by inserting air into a flexible thin rubber sheet. Most of the balloon is stretched evenly, but there are two points where the rubber is least stretched and thus there is the lowest surface tension. These correspond to the tied section and the darker patch at the opposite side of the balloon in fact the darker colour indicates that the balloon is less stretched over that region. Most of the balloon is under high tension, so attempting to push the skewer through just makes the balloon pop. But at the low tension sections it is possible to make a small hole without breaking the overall surface of the balloon.

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• URL: http://stem.org.uk/rx6vu