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How to Determine the Magnitude of the Average Force From Air Resistance

A cannonball attached to a parachute falls much more slowly than a cannonball alone, but the difference has nothing to do with weight. It stems from air resistance. On the moon, both a feather and a pencil would fall at the same speed. If you need to estimate the force of air resistance, you can do so by using a simple equation, as long as you have the cross-sectional area of your object so you can use it in this equation.

Instructions

- 1
  Look up the drag coefficient for your object by clicking on the link under the Resources section. If your object is not shown, approximate the drag coefficient by using the coefficient for a similar object.
- 2
  Multiply the cross-sectional area by the drag coefficient. If the drag coefficient is 0.6, for example, and the cross-sectional area is 10 meters squared, multiplying the two gives you 6 as a result. Make sure you are using cross-sectional area in meters squared and not some other unit.
- 3
  Multiply your result from the last step by -1/2. If your result was 6, for example, you would have -3 as your answer.
- 4
  Multiply the result from the previous step by the density of air, which is approximately 1.23 kilograms per cubic meter. To continue the example, -3 x 1.23 = -3.69. Make sure, by the way, that you are using units of kilograms per cubic meter for density and not grams per cubic centimeter.
- 5
  Multiply your answer from the previous step by the square of your object's velocity. If the velocity is 10 meters per second, for example, then the square of the velocity is 100, and 100 x -3.69 = -369. Assuming you used all the same units, your answer will be in newtons, the standard unit of force.

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