

Finding Force of Drag
Name: Danny
Status: Student
Grade: 912
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 crosssectional area, and Cd is the drag
coefficient (around 0,5 for spheres).
I am
planning to have the velocity and the
crosssectional 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 downwind it is deflected, and then calculate an
average velocity from the droptime and the distance. You can then calculate
the force required to accelerate the ball.
Hope this helps,
Burr Zimmerman
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Update: June 2012

