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Name: Danny
Status: Student
Grade: 9-12
Location: N/A
Country: United States
Date: April 2008


Question:
I am doing an experiment in order to prove that the drag coefficient (cd) of a sphere is 0.5. The equation for Cd = 2Fd / v^2A where Fd is the force of drag, is the density of the fluid . (Air is 1.293 kg/m3 at 0°C and 1 atmosphere), v is the velocity of the object, A is the cross-sectional area, and Cd is the drag coefficient (around 0,5 for spheres).

I am planning to have the velocity and the cross-sectional area has my variables but do not know how I could find the Force Of Drag experimentally. Any ideas on how I could do this? I already have a wind tunnel.



Replies:
I think you are missing a density term in your equation. It should be in the denominator next to velocity and area.

Make sure to calculate your Reynolds number -- your drag equation only applies to systems with Re>1000. For example, a 1cm sphere traveling at 10cm/sec would have too low a Reynolds number.

I suppose one easy way to calculate drag is to suspend your sphere from a string, measure the angle by which it is deflected, and do a force balance calculation. If you do not like that idea, and if your wind tunnel is large enough (and has uniform velocity from top to bottom), you could drop your ball and measure how far down-wind it is deflected, and then calculate an average velocity from the drop-time and the distance. You can then calculate the force required to accelerate the ball.

Hope this helps,
Burr Zimmerman



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