Object in a test tube

  • Thread starter Swatch
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  • #1
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An object is put in a test tube . The test tube is put in a centrifugal machine where it spins around. Would it be correct to say that the net force on the object is zero since it is not moving. I know the test tube has a net inward force that makes it change direction. But the object wants to fly out of the tube but is stopped by the bottom of the test tube.

Am I totally wrong here?
 

Answers and Replies

  • #2
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it is not moving
What?????????
 
  • #3
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What I meant is that the object doesn't accelerate. But then again it changes direction. A direct answer would be appreciated.
 
  • #4
Doc Al
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centripetal acceleration

An object spinning in a circle is most definitely accelerating! It is accelerating towards the center of the circle; this is usually called centripetal acceleration. And, since it is accelerating, there must be a net force pulling the object towards the center.
 
  • #5
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That's what I thought. But wasn't sure because the object isn't in an inertial frame of reference.
 
  • #6
MalleusScientiarum
If an object isn't in an inertial frame of reference then pretty much by definition you will observe that a force is acting on it.
 
  • #7
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O.K. I think I understand this but the real problem I don't understand, here it comes.

An incompressible fluid with density rho is in a horizontal test tube of inner cross-sectional area A. The test tube spins in a horizontal circle with angular speed w. Gravitational forces are negligeble. An object of volume V and density RHOob has its center of mass at a distance Rcmob from the axis. Show that the net horizontal force on the object is rhoVw^2Rcm, where Rcm is the distance from the axis to the center of mass of the displaced fluid. I have already derived expressions for the pressure at a distance from the surface of the water, and for the pressure difference for a volume element of thickness dr. It seems to me that the net force on the object is the mass*acceleration of the displaced fluid volume.

I don't know how to treat this problem. I see the force as the mass*acceleration of the object + displaced water.


Could anyone please help me.
 
  • #8
Doc Al
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Swatch said:
It seems to me that the net force on the object is the mass*acceleration of the displaced fluid volume.
The net force on any object is its mass time its acceleration. This object is being centripetally accelerated, so [itex]F_{net} = m a_c[/itex].
 

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