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drag of a free falling ball bearing in air.

Postby barbarianboy666 » Wed Dec 05, 2007 11:19 am

i am doing an investigation into the drag acting upon a free falling ball bearing in air. I am using a laptop to calculate the time it takes for the ball bearing to fall. however i am having a problem seeing how i can calculate the difference in time it does and should take for the given ball bearing to fall and how to calculate the coefficient of drag any help would be much appreciated.

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Postby Craig_Bridge » Wed Dec 05, 2007 4:30 pm

Do you have access to an electronic timer with nanosecond resolution and optical beam sensors?

If not, then you probably won't be able to measure the drop time accurately enough to compare to calculated drop time based on zero drag.

This is a case where the experiment and calculations are relatively straight forward and the measurement equipment is non-trivial.

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ball bearing drop experiment

Postby peteryoung » Wed Dec 05, 2007 10:33 pm

I agree with Mr. Bridge: the design of your experiment (length of the free fall distance, the timing equipment, estimating or measuring the inherent measurement errors) will be non-trivial and should be done carefully before starting the experiment.

I suggest doing some basic research on the aerodynamic drag of spheres - a Wikipedia search, for example, will quickly reveal the range of values for spheres' drag coefficients. It's however true that the drag coefficient will depend on the speed and diameter of the sphere (the "Reynolds number" effects at work here).

If possible, reconsider whether or not a ball bearing is the best test article to use. A ball bearing is small but relatively heavy for its size (moderate to high "ballistic coefficient" - another data item to research). What this means is that the ball bearing will respond quicker to gravity ("fall fairly quickly") and not be slowed down much by aerodynamic drag; contrast this to a whiffle ball or pingpong ball, both which have much lower
"ballistic coefficient" values. A slower falling test article will ease your
measurement problems, I should add.

If you are intent on using a ball bearing, you can make a pretty good
estimate of its drop speed by applying Newton's laws, F= ma and integrating to get velocity. This first calculation will neglect aerodynamic drag but should give you an idea of the drop speeds you can expect. A more refined estimate would be to rerun the calculations using the range of drag coefficient values as mentioned above.

A final suggestion: the drop-measurement problems would be made much easier if a "test medium" was chosen which still allows descents of spheres, but has higher viscosity which would slow down the test object.
Water or lightweight vegetable oil in a tall glass column, for example, could be a practical test setup that still allows controlled testing of your hypotheses.

Whew! There's quite a bit of background work I've described here but pre-test background research, and some pre-test experiment planning, will give you good insight into the physics of the problem. Good luck!

Peter Young
Peter Young
Senior Project Leader
The Aerospace Corporation
El Segundo CA

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Postby bradleyshanrock-solberg » Fri Dec 07, 2007 11:26 am

For measuring this sort of thing with cheap tools you need to pick something with a lot of drag (like a parachute) or do it in a more viscious medium than air (such as honey).

As others have stated, measuring drag on a ball bearing in air requires extremely sensitive equipment (as a ball bearing has minimal won't fall much slower in atmosphere than it will fall in a vacuum)

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