Determining drag coefficient

[protossadv]

Homework Statement

I made a wind tunnel for a science project and i am trying to determine its windspeed.
I decided to suspend a pingpong ball on a string in the wind tunnel and measure the angle change.
But to do this i need to calculate the drag force and i cannot do this without knowing the drag coefficient of the ball.

Homework Equations

Drag force: 1/2*P*v²*A*C_d
x=v_it + 1/2*at²

The Attempt at a Solution

One idea i have is to drop the same ball from a height and measure the time it takes to hit the ground.
I then use the equation x=v_it + 1/2*at² and solve for a.
Then, using 1/2*P*v²*A*C_d = ma, i solve for C_d, and use this drag coefficient to solve for wind velocity in the wind tunnel.
However, i think i can't simply put ma= drag force becuase this suggests that gravity equals drag force which would mean the ball was at terminal velocity.

How would you solve this?

 

[LowlyPion]

You might find this a useful lecture.
Maybe around 33 min in or so it deals with your interest with air:

https://www.youtube.com/watch?v=9lvNofoUYwI

You may want to consider looking for tables of known coefficients for air at your temperature of interest.

http://en.wikipedia.org/wiki/Drag_coefficient#Drag_coefficient_Cd_examples

 

[protossadv]

for some reason i can't see whatever video or link you posted
can u show me the url?

 

[LowlyPion]

for some reason i can't see whatever video or link you posted
can u show me the url?

It's on Utube. Title: Lec 12 | 8.01 Physics I: Classical Mechanics, Fall 1999



I embedded it in the message. It plays for me just clicking it in the message.

Perhaps you will have better luck at Youtube?