Teaching Physics with the Physics Suite

Edward F. Redish

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In the balance

(a) A weightlifter wants to lift 75 pounds with one hand, but he only has a light (but strong) aluminum bar and three steel disks weighing 25 pounds each. He can set them up as in the figure, but then he finds that they are unbalanced if he tries to lift the bar holding it in the middle. Where should he hold the bar so that the weights are balanced? Assume you can ignore the weight of the aluminum bar and that the steel disks are thin compared to the distance L between them. Explain your reasoning
(b) After finishing his workout with the weights, he balances a baseball bat on his finger as shown. His friend comments, “That must be where the center of mass is.” Is the friend right? How do you know?

(c) When the bat is balanced is there more of the bat’s mass to the left of the weightlifter’s finger, more to the right, or is it the same on both sides? How do you know?

(d) Did you use the same physical principles to decide on your answer to (a) as to (c)? If so state what principle you used. If not, are the principles you used consistent? If you apply them to opposite cases do you still get the same answer?

Page last modified December 11, 2010: R28