# Drag Equation

1. Jan 8, 2014

### DarkFalz

For several years i have thought about air (fluid) resistance without getting deeper into the formulas that model its behaviour. Recently, i've decided to take one more step in the subject and learn some of these formulas.

I've learned about the drag equation, and at first glance it seemed like a "pretty" formula, which could answer my questions regarding air resistance in everyday affairs.

Yet, i have decided to try to understand it and its origins, and so i took a look at NASA's website and checked this link https://www.grc.nasa.gov/www/k-12/airplane/momntm.html

I dont know if i am correctly understanding the several steps involved in the derivation of the drag equation. Im familiar with newton's laws and the concept of linear momentum, so i think i understand the first step. It is saying that if a body when moving through a fluid causes its momentum to change from 0 to m*v over a given amount of time T, then a force with average value m*v / T acted upon the moving body throught the period of time T, right?

I am not that much familiar with the second step, but i think i get the point, basically they're determining the amount of the fluid that the body will colide with if it is moving through the fluid with velocity V and has a contact surface A with the fluid.

The point that i am missing, is why the V involved in the first formula (m*V / T), which i assume is the velocity of the fluid, is the same V from the mdot equation, which is related to the velocity of the body passing through the fluid, or so did understand understand.

Thanks for the attention

2. Jan 8, 2014

### A.T.

It's the relative velocity between body and fluid.