Alright I need some input from people who have a better grasp of physics than myself. I'm working on a research project and wanted to make sure my initial thoughts are correct.

First is how to determine what angle an object will hang at while rotating (in a centrifuge). Would it be correct to simply take the inverse tangent of the 1G force that is acting downward (gravity) over the G forces that are acting outward from the centrifuge due to rotation (tan-1(1G/Gcalc)?

Also if a porous material is hanging in the centrifuge and water is moving through the material would this water flow in a parabolic manner through the sample (compared to the sample)? Since the G forces from rotation depend on the radius, as the flow moved further away from the center of the centrifuge the rotational G forces would increase and the angle of flow (tan-1(1G/Gcalc) would decrease.

If someone knows what I'm talking about and doesn't mind helping I can explain it better. If not I guess I could set up an appointment with a physics professor.

Thanks

10/24/2007 11:43:45 PM

You're forgetting about the third force acting on the object. The buoyant force may or may not be significant depending on what fluid is in the centrifuge (i.e. water or air).

"what angle an object will hang at" depends on what it is attached to. Consider a ping pong ball attached to the wall of a centrifuge (containing water) by a string. Depending on the length of the string and the speed at which the centrifuge is spinning the ball may or may not be totally covered by water. As the centrifuge accelerates and the ball emerges from the water the angle it hangs relative to the wall will change notably because the buoyant force of air is much less than the water.

No one can answer unless you explain your setup.

10/25/2007 12:13:08 AM

The sample hangs in air so I'm neglecting the buoyant force from that.

Here is a picture I took on my phone:


As the centrifuge spins the sample raises (I was holding it up at the time). The sample has ponded water on top of it (in the cylinder) and it flows through the sample as the centrifuge spins. What I'm wondering about is what the angle the sample hangs at (from it's fulcrum) and if the flow path of the water through the sample is non-linear as I assume.

10/25/2007 8:12:55 PM

Just throwin it out there, but I would think if the only forces acting on the liquid to make it percolate 'down' past the porous material would be gravity and the g-forces on by the cfuge. Assuming thats the cylinder that changes angle with radial speed, I can only visualize the water following a linear physical path, and an exponential path mathematically of speed, or force, with respect to time. At t=0, or no motion at all, only 1g downward (max) is forcing the water down. Say it takes 50g's to finally reach as close to measuring horizontal as your equipment allows, since gravity will always pull it slightly down to an extent. At those 50g's 50 are in the x direction, with 1 in the -y direction. At 45 degrees all it takes is 1g in the horizontal to counter the 1g of gravity since all the acceleration goes to both the sample, water, and cylinder. During all this , it is assuming the sample is at the same angle as the cylinder. So you'd be roughly following the path total force from 1g to 2g to 50 or 60g's at just those three points, whatever it takes to get as close to horizontal as possible.

I'm either oversimplifying this or taking it way outta proportion. Draw some FBD's at various angles and see what ya get. As far as what you've given, that's how I'd think of it.

[Edited on October 26, 2007 at 12:43 AM. Reason : .]

10/26/2007 12:42:12 AM