# Drag force of a spherical BB ammo under water

1. Oct 11, 2011

### Xenekaro

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. Oct 11, 2011

### Phrak

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?

3. Oct 11, 2011

### Xenekaro

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 :)

4. Oct 11, 2011

### LawrenceC

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

5. Oct 11, 2011

### SteamKing

Staff Emeritus
cavitation does not mean splash