# Drag and spheres

1. Oct 14, 2011

### visionviper

I maintain a small piece of software for the Airsoft/Paintball community. I include some basic energy and velocity calculations but I have also as of a few versions back started including more "theoretical" calculations like how far a BB will travel and that sort of thing. I want to improve the calculations I am using to be more accurate and take drag into account.

I've taken some physics but we just touched fluid dynamics a little bit. I have been trying to get together the equations and information I will need to translate all of this into code. So far I have been able to find equations for drag coefficients and the drag equation, but honestly I don't really know how to use these.

For calculating my drag coefficient I can easily find things like the mass density of air and so on, but it's the stuff specific to my object that I have problems with. Would someone please explain to me the process putting all the information I have into these equations?

2. Oct 14, 2011

### LostConjugate

What else would you need besides the drag co-efficient and the drag equation? How did you find the co-efficient without knowing stuff specific to your object? All you should need to know is the radius and smooth or non-smooth surface.

3. Oct 14, 2011

### visionviper

Sorry, I must not have made it clear. As far as information I know, I know the properties of the object. They are smooth, I know the diameter. I am just trying to understand how I use all this information to find things like the drag coefficient and then using that to calculate the drag on the BB at any point in time.

Using that I should have no problem converting this into an integral that I will eventually have to find an algebraic representation of.

4. Oct 14, 2011

### RandomGuy88

Calculating the drag coefficient of a sphere cannot be done analytically. It would require CFD.

There is a lot of experimental data out there however. In fact there are charts that give the drag coefficient of a sphere as a function of the Reynolds number. Which for a given object at atmospheric conditions is just a function of velocity. So you could use just one drag coefficient for the entire trajectory or if the velocity changes significantly you could account for the changing drag coefficient by using the experimental data to determine the Cd at the current velocity.