Drag Force Equation?

1. Jan 27, 2014

xphysics

1. The problem statement, all variables and given/known data
Hi, i have a peculiar question after watching MIT lecture:http://ocw.mit.edu/courses/physics/...echanics-fall-1999/video-lectures/lecture-12/

What is the difference between the equation representing the drag force for a sphere: C(sub1)rv+C(sub2)r^(2)v^(2) and this drag force equation: (1/2)pv^(2)C(sub d)A

2. Relevant equations

3. The attempt at a solution

2. Jan 28, 2014

lightgrav

viscous resistance is proportional to velocity (1st term), drag resistance is proportional to v² (2nd term).
at very slow speeds, the viscous term is larger ... at high speeds, the drag term dominates.

3. Jan 28, 2014

xphysics

But why are there 2 different equation? Why didn't prof. WL just use the drag force one?

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4. Jan 28, 2014

haruspex

It is very well explained in the lecture that C1rv+C2r2v2 is the general equation, but if you are in a regime where one of those terms is very much larger than the other then you can omit the smaller term. If that doesn't answer your question, please specify the section of the video (minutes from start) that's puzzling you.

5. Jan 28, 2014

xphysics

I completely understand the lecture it's just that the equation represents the total resistive force on the object and I have a question on how is that equation(from the lecture) is different from the drag equation(google it) since they both shows the resistive force(if I'm correct)

6. Jan 28, 2014

haruspex

Perhaps you're not reading the fine print. E.g. http://en.wikipedia.org/wiki/Drag_equation: [Broken]
http://en.wikipedia.org/wiki/Drag_%28physics%29: [Broken]
where the Reynolds number depends on the speed (linearly). I.e. the linear term of the full equation has been hidden inside the drag coefficient.

Last edited by a moderator: May 6, 2017