Paper tricks, soap bubbles and more

Partha Bandyopadhyay

You often need very simple things to demonstrate experiments that seem almost magical. Here are four simple experiments with paper.

paper-tricks Tear the paper into three pieces
Take a paper strip 2-3 cm wide and about 15 cm long. Fold it in the middle and use a pair of scissors to make a cut (about 1 cm in length) approximately in the middle of the folded strip and parallel to its width.

Unfold the strip. There will be two equal cuts situated at equal distances from its ends. Now holding the two ends of the strip between the thumbs and forefingers of your two hands, try to pull it apart into THREE pieces. Try to apply equal force.

Try with smaller and bigger cuts and different positions of the cut. You will always end up with two pieces and not three! Why?

At the microscopic level, the cuts are never equal – no matter how carefully one makes them. The paper is weaker at the bigger cut and starts coming off from there. The cut becomes bigger and the paper even more weak. The process continues till the two pieces come apart. (This vindicates the saying: a chain is only as strong as its weakest link). Perforation in a bank cheque ensures that it comes off just from there. A shallow dent in the middle of a tablet (medicine) serves the same purpose.

Paper falling under gravity
We have all read about Galileo’s famous experiment at the tower of Pisa. Recent findings suggest that he probably did not drop balls from atop the tower but rolled them along inclined planes.

Galileo notwithstanding, astronauts in the 20th century did repeat a similar experiment on the surface of the moon, where there is no air. They dropped a feather and a coin simultaneously and verified that they fell together.

I would like to describe here an easy to do experiment which verifies the same law.

The author is an Associate Professor of Physics (Retd), City College, Kolkata. He can be reached at pbandyo@gmail.com.

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