M-CM 9: Forces in vertical circular motion

PIRA: 1D50.45

Equipment: A coat hanger found at the side of Set 22, Cabinet 7 and a penny found in a milkbox in the drawer of the table where the set-up books are kept.

Procedure: A penny is placed on a coat hanger and rotated in a vertical circle of radius 0.2 m. The minimum speed occurs when the push of the coat hanger on the penny is zero at the highest point. Then g = v²/r, whence v = square root of rg. In the demonstration the value of r is approximately 20 cm = 0.2 m. The minimum speed of the penny is then the square root of (9.8 m/s²)(0.20 m), or 1.4 m/s.

For the demonstration it is best to use the yellow hanger which is less slippery than the ordinary wire hangers. It is a little difficult to balance the coin on the hanger, rotate the hanger about one's finger, and stop the rotation without losing the coin — so the lecturer should do a little practicing before class.