Teaching Physics with the Physics Suite

Edward F. Redish

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Approximating Atwood

In the figure at the right is shown an Atwood's machine with two unequal masses attached by a massless string. The pulley has a mass of 20 g and a radius of 2 cm.

  1. State three approximations that you can make to simplify your calculation of the motion of the blocks. ("Making an approximation" is the process of ignoring a physical effect because you expect it to be small and have little effect on your result if you only care about a few significant figures. If you want more, you may have to include those effects.)
  2. Using your approximations, find the acceleration of block A.
  3. What happens to your result if the two masses are equal. Is the result what you expect? Explain.
  4. If you have suppressed the moment of inertia of the pulley in your calculation in the second part of this problem, set up the equations that would allow you to solve for the acceleration when it is included (but don't solve them).

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Page last modified October 30, 2002: R20