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Homework Help: Drag force of a spherical BB ammo under water

  1. Oct 11, 2011 #1
    Basically, BB ammos were shot from an airsoft gun into a water filled tank. The experiment was recorded using a video camera. I can calculate the approximate instantaneous velocity of the bullet under water at a given time using Logger Pro.

    1. Relevant equations

    Drag force = 0.5 ρAC0v2

    2. The problem statement, all variables and given/known data

    I have kept the density of water, ρ and the reference area, A constant.
    ρ of water = 997kg/m3
    A = ∏ rsq. ( r = 0.003 m)
    I want to calculate Coefficient of drag of a spherical ammo to find a relationship between velocity and drag coefficient.
    Secondly, the velocity of the BB ammos under water that I calculated using Logger Pro indicates a very low velocity of about 0.7-3m/s (0.2 seconds after under water). The Reynolds number comes to about 0.03

    3. The attempt at a solution
    I have searched online and found that the theoretical drag coefficient of a spherical object should be 0.47.
    The low Reyonlds Number indicates that the drag coefficient is not constant.
    I am facing difficulty because I have two unknown variables, Drag force and Cd.

    Another question: Can the drag force be calculated by Mass * deceleration.
    I know the mass to be 0.12g and can calculate the deceleration of the bullet...

    I am also not sure if Strokes Law may apply to this experiment because the Reynolds number is really low.
    Last edited: Oct 11, 2011
  2. jcsd
  3. Oct 11, 2011 #2
    The context of your question is difficult to decipher.

    Is this just a make-believe homework problem where we are to pretend no cavitation is involved or is it more realistic?

    Was there a real experiment? Was there cavitation?
  4. Oct 11, 2011 #3
    Real experiment. For my Independent research lab for IB Physics HL.

    Yes there was cavitation-if u mean splash. But I am trying to ignore the physics of the bullet before or at the point of striking water. I want to concentrate only at the time when the bullet enters water and experiences drag.

    I am kind of late for submittal so I don't want to ask my teacher. I did most my lab writeup with the assumption that Force of drag is Mass*deceleration. But I just realized that is unlikely. Please help :)
  5. Oct 11, 2011 #4
    I'm skeptical about your Reynolds number.

    In SAE units at a speed of 1 m/s = 3.281 ft/sec:

    Re = rho * D * V/mu = 62.4 * .177/12 * 3.28 * 3600/2.37 = 4586

    Viscosity of saturated water at 70 F = 2.37 lbm/ft-hr
  6. Oct 11, 2011 #5


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    cavitation does not mean splash
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