# Rotating a marble in a glass tube ?

1. Aug 2, 2010

### cragar

So if i have a marble in a glass tube and i rotate the tube around in a circle . and the marble is at a distance r from the center of rotation . is the reason the marble goes towards the outside of the tube because when the glass tube starts to rotate the marble wants to go tangent to the circle so it is allowed to go further back in the tube when it rotates. and the end of the glass tube keeps it their . Is this what gives rise to the fictitious force .

2. Aug 2, 2010

### DaveC426913

I think you need to elaborate. Describe the experiment. Ask your question. Make them two distinct steps.

Also, normally it wouldn't be a big deal but, since your message is already confusing, some attention to correct punctuation would go a long way.

3. Aug 3, 2010

### cragar

the marble is at rest when we start to rotate the tube . so wouldn't the marble
want to go into a straight line .

4. Aug 3, 2010

### arydberg

If we lay a test tube on a table and place a marble in it then glue an vertical axis ( a vertical wire) on to the open end of the tube and rotate the tube around the wire the marble will moved to the closed end of the test tube. It is centrifugal force that does this. If the tube is suddenly broken then the marble will travel in a straight line determined by it's position when the tube broke. Also yes if the tube is intact it is the marble trying to move in a straight line that causes it to move to the closed end of the test tube.

5. Aug 3, 2010

### DaveC426913

You haven't even specified which axis the tube is rotating through, so I can't even begin to imagine the setup.

6. Aug 3, 2010

### cragar

we rotate the glass tube as if we were swinging a golf club , the golf club being the glass tube , and the marble is half way down the tube , halfway from the point of rotation
to the edge of the circle . and we rotate it with a constant angular velocity .

7. Aug 3, 2010

### DaveC426913

Really?? Here I was guessing that the tube was rotating through its long axis. :grumpy:

Look, I've already put more effort into this thread than you have. If you're satisfied with the answers you're getting then power power to ya. :grumpy: :grumpy:

I'm done here.

8. Aug 3, 2010

### rcgldr

Assuming the tube is frictionless, it's my guess that until the marble collides with the end of the tube, there's no radial (centripetal) component of force, just tangental. When the marble intiallly collides with the end of the tube, it may bounce, depending on how elastic the collision is. Once the marble has stabilized at the end of the tube, then the end of the tube produces only a radial component of force, and no tangental component (assuming constant angular velocity). The newton third law pair of forces at this time would be a centripetal force exerted by the end of the tube onto the marble, coexistant with the reactive force exerted by the marble onto the end of the tube. Those forces combine to produce a compressive deformation at the point of contact between marble and end of tube. That reactive force is only called "fictitious", if that force is observed in an accelerating frame of reference, such as a rotating frame of reference (for example a frame of reference based on the end of the tube).

Last edited: Aug 3, 2010
9. Aug 3, 2010

### cragar

thanks rcgldr , that makes sense , and thanks to everyone else who also gave input .

10. Aug 3, 2010

### arydberg

The axis of rotation is the wire.

11. Aug 3, 2010

### DaveC426913

There is no wire.

12. Aug 4, 2010

### arydberg

Maybe we should start over again.

13. Aug 4, 2010

### DaveC426913

This is what I've been trying to say, yes.

14. Aug 4, 2010

### cragar

i got the answer i was looking for , but you guys can continue if you want .

15. Aug 8, 2010

### Acut

If I'm not wrong, this is precisely how Walter Lewin (that MIT professor) explained, in one of his lectures, the origin of the centrifugal force. It was a brilliant lecture!

16. Aug 9, 2010

### cragar

yes you are correct , he is a very smart man

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