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Homework Help: Determining drag coefficient

  1. May 7, 2009 #1
    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations
    Drag force: 1/2*P*v2*A*Cd
    x=vit + 1/2*at2

    3. 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=vit + 1/2*at2 and solve for a.
    Then, using 1/2*P*v2*A*Cd = ma, i solve for Cd, and use this drag coefficient to solve for wind velocity in the wind tunnel.
    However, i think i cant 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?
  2. jcsd
  3. May 7, 2009 #2


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  4. May 7, 2009 #3
    for some reason i cant see whatever video or link you posted
    can u show me the url?
  5. May 7, 2009 #4


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    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?
    Last edited by a moderator: Sep 25, 2014
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