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Torque and Position of Center of Mass Of an Object

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Author: Jerry Silver

The Idea

Experience tells us that objects are more stable if their center of mass is lower to the ground. Based on that, you might think it would be easier to balance a broom with brush side down. This experiment lets you answer whether that is the case.

What You Need

  • meterstick
  • 2 books (physics textbooks are preferred, but English textbooks work almost as well)
  • duct tape
  • alternative: you can do this with an actual broom or any other object that has much of its mass concentrated at only one end. This can be done with modeling clay attached to the end of a broom or pencil.

Center of mass. How to balance a broom.

Method

  1. Insert the meterstick between the two books, so an inch or two of the meterstick protrudes beyond the bottom of the book.
  2. Secure the book to the meterstick.
  3. Predict which end of the meterstick you should support to most easily balance it: the heavy end or the light end?
  4. Support the meterstick on the heavy end.
  5. Try this with the heavy end up and the light end supported by your hand.

Expected Results

One might say that with the mass at the bottom, the meterstick will be more stable. The logic is, like with a drag racer, placing the center of gravity at the lowest point possible results in the greatest stability. The results of trying this, however, reveal the opposite to be the case. It is easier to balance the meterstick with the weight at the top, not the bottom.

Why It Works

The reason for this unexpected behavior is that a small movement at the support end creates a greater torque with most of the weight located at the opposite end. This gives the person trying to balance the meterstick greater control. This principle is used by tightrope walkers who carry a pole with a weight at the end to help establish balance.

Other Things to Try

Skyscrapers are often subjected to vibration when they're exposed to wind. Sometimes, adding mass to the structure can damp down the extent of the swaying. Apply the results of this investigation to determine whether it is more advantageous to add mass to the top floor or to the first floor of a skyscraper. Look up a specific example of how mass was added to the top, rather than the bottom, of the Sears Tower in Chicago.

The Point

Adding mass away from the pivot point increases the torque produced at the other end. This provides a greater degree of control to the end without the weight.

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