# Drag Coefficient out by a factor of 10, can anyone help?

1. Mar 1, 2009

### Euan2007

Drag Coefficient out by a factor of 10, can anyone help??

Im doing an experiment on the drag of a sphere and am trying to work out the drag coefficient. From looking at data online this should be around 0.5 (its a smooth sphere). Using a wind tunnel I measured the drag for various sizes of spheres and wind speeds. An example of one of my results is

Drag - 0.68N
Wind Speed - 7.8m/s

Then, using the formula

Drag Coefficent = (2 x Drag) / Area x Wind Speed^2 x Density of Air

Having measured the room temperature to be 22 degrees celcius I took the density of air to be 1.204kg/m^3 and the projected frontal area was calculated using pi x r^2

But this equals 5.039!!!! so could anyone point out where im going wrong?? Im 90% sure all of my values are correct. Thanks in advance for any help :)

2. Mar 2, 2009

### Weirdbro

Re: Drag Coefficient out by a factor of 10, can anyone help??

Are you absolutely sure about the calculated drag force? Your equation is correct, although I get 4.293 when I plug your data into it (probably rounding differences). I'm not terribly familiar with wind tunnels, but try checking the units of the readings that you're getting from it.