Consider a dumbbell made of two equal masses, m. The dumbbell spins around, with its center pivoted at the end of a stick (see Figure). If the speed of the masses is v, then the energy of the system is 2?m. Treated as a whole, the system is at rest. Therefore, the mass of the system must be 2?m. (Imagine enclosing it in a box, so that you can??t see what is going on inside.) Convince yourself that the system does indeed behave like a mass of M = 2?m, by pushing on the stick (when the dumbbell is in the ??transverse?? position shown in the figure) and showing that F ? dp/dt = Ma.

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