UDF to calculate the force of drag of fluid flowing over a cylindrically shaped object

[pkayy121]

Hello. I am new to this website which means i am a new user.

I am in an excel class where we have been creating UDF's (user-defined formulas). I am having the most hardest time understanding and creating the formula. I have one problem that i don't understand at all and i really need help on it. This is the question:

"Write a UDF to calculate the force of drag of fluid flowing over a cylindrically shaped object using the following formula:

Fd= ((1/2)*cd*p*v^2*A)/(gc)

Fd is the force of drag, Cd is the drag coefficient, p is the density of the fluid, v is the velocity, A is the frontal area of the object, and gc is the gravity constant. The program should work for air flowing over an object shaped as a convex Cd=1.2 or concave Cd=2.3.

The program should be a VBA subroutine with input and output in Excel worksheet. Use as input; velocity in mph, shape of the end (concave or convex), length in ft, and radius in inches, with the outpu in lbf. Incorporate a Select Cast statement to handle the shape (concave or convex)."

I am so lost i don't even know where to start >.< Please help!