Object in a test tube

Swatch

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?

Related Introductory Physics Homework Help News on Phys.org

Swatch

What I meant is that the object doesn't accelerate. But then again it changes direction. A direct answer would be appreciated.

Doc Al

Mentor
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.

Swatch

That's what I thought. But wasn't sure because the object isn't in an inertial frame of reference.

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.

Swatch

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.

Doc Al

Mentor
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 $F_{net} = m a_c$.

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving