# Drag coefficient changes with?

#### Erikma

Hello, I need some quick help. Does the drag coefficient of an object change when the volume changes? The mass density remains the same. Thanks!

#### ChipB

PHF Helper
Depending on what level of fluid mechanics you're studying, the answer is: it depends.

The basic model of aerodynamic drag that they first teach is:

Where C_D is the coefficient of drag and is assumed to be a constant for any given shape. The frontal area A is what takes into account the size of the object. So in this model C_D is not affected by the size (volume) of the object, and hence a toy airplane could have the same C_D as a jumbo jet.

But... as you get into more advanced theory involving boundary layers, viscosity, Reynolds Number, etc you find that drag can be influenced by other factors, including the length of the object. So like I said: it depends on what theory you are learning.

#### Erikma

Great! I'm studying mechanics in the first year
at the university so then I guess it's not
supposed to depend on volume.