Teaching Physics with the Physics Suite

Edward F. Redish

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Weighing a non-uniform suitcase

An animal behavior researcher is going on a field trip to observe baboons in Africa for two weeks in the summer and she has packed a large suitcase. She needs to decide whether she can take it on the plane. The airlines have a maximum acceptable mass of baggage of 45 kg and she thinks she might be close to that. She has a bathroom scale that could weigh it, but the suitcase will not fit on the scale. She can put one end on the scale and hold the other up as shown in the picture at the right. Having taken physics, she knows that if the suitcase had its weight distributed uniformly, she would be holding up half the weight and the scale would be holding up half the weight. It would then be easy to get the correct mass from the scale reading. Unfortunately, that is not the case as the suitcase is heavier on one side.

Remembering her physics, she has a flash of insight. She gets a broom handle, puts it under the suitcase, and rolls the suitcase back and forth until she finds the point at which it balances. The balance point is shown in the figure at the right.

a) Where is the center of mass of the suitcase? Why do you say so?

b) The scale in the top figure reads 24 kg. (As a scientist, sher scale reads in kg.) She does not have a meter stick handy so she does not know the length of the suitcase. Does she need to know its length in order to figure out the weight of the suitcase? Explain.

c) If you think she does not need the length to get the correct mass, find the mass. If you think she does need the length, take L = 1 m and find the mass of the suitcase.

Page last modified December 11, 2010: R27