Materials required: - a big circular metal sheet, a small circular metal sheet (or a large tin lid and a small tin lid) nail, stone or hammer, string or ru­bber band and a foot square plank
Lay the two lids on the plank a little away from each other. Nail them to the plank and make them to revolve easily. Connect both the lids with a rubber band or string. Does it not look like a wheel? Rotate the big circle once and observe how many revolution the small one makes. Measure the circumference of the small circle. The distance traveled by the small wheel will be as many times this length as its revolution. If it revolved 5 times and the circumference was 20 cm., then the distance will be 5 x 20 = 100 centimeters. There are big and small teeth wheels in various machines. The teeth of one wheel are connected to the teeth of another: When the main wheel moves small wheel also moves. Watches and clocks work on the same principle.

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