# The difference between bv and kv^2 drag

1. Dec 20, 2012

### Hannibal123

1. The problem statement, all variables and given/known data

What is the difference between the two proportionality constants for liner drag
$$F=b*v$$
$$F=k*v^2$$
considering a general object moving trough air? i know that k is given by
$$1/2*ρ*C_w*A$$
where C_w is the drag coefficient

2. Dec 20, 2012

### PhanthomJay

Quadratic drag is typically applicable to objects moving through a medium at relatively high speeds , like heavy falling objects, whereas linear drag is typically applicable to light objects or particles moving through a medium (like a viscous thick fluid) at relatively low speeds. For objects moving through air, generally the quadratic drag equation applies. Similarly, wind forces on structures are proportional to the square of the wind speed....if you double the wind speed, the force or pressure is quadrupled.

3. Dec 20, 2012

### Hannibal123

Thank you. But can the constant in linear drag be descirbed by som factors similar to the constant in qudratic drag? except for sferes moving in fluids....

4. Dec 20, 2012

### PhanthomJay

Oh, i see what you are driving at....see here for the difference betweeen k and b

http://en.wikipedia.org/wiki/Drag_(physics [Broken])

Last edited by a moderator: May 6, 2017
5. Dec 20, 2012

### Staff: Mentor

In the region where the linear drag equation applies, the drag coefficient Cw is inversely proportional to the velocity. More, precisely, the drag coefficient is inversely proportional to the (dimensionless) Reynolds Number (Re), which is equal to ρvL/η, where ρ is the fluid density, η is the fluid viscosity, and L is a characteristic length for the object, such as the diameter of a sphere.

6. Dec 20, 2012

### Hannibal123

Ok thanks. I have made an experiment where i have measured the wind force on an object (a water rocket) at different wind speeds. Plotted into a graph it gives me these values:
http://imgur.com/He0Qd
Or the same values but the with the velocity squared on the horisontal axis
http://imgur.com/0xfMX
Thus obtaining the k value K=0,0032 kg/m
Now based on what you said, it wouldn't make any sense to try and apply this k value to a linear drag right?

7. Dec 20, 2012

Right.