# B Question about a bob held by string in a bell jar spinning

1. Oct 13, 2016

### litup

A discussion with some friends: Suppose you have a large wheel, say 10 meters in diameter and a bell jar with a good vacuum inside, the wheel is rotating horizontally to Earth. In the jar is a bob, and it is on a 10 meter string which is connected to the center of the bell jar furthest from center.

The bob is magnetic and held loosely by another magnet fixed to the wheel so there is just enough magnetic force to keep the bob pointed to the center of the wheel,

So with the wheel spinning, I say the bob would have a bias, a force trying to move it off dead center of the bell jar. My buddies say no, there would be no force on the bob and there would be no motion away from dead center of the bell jar.

Of course the whole affair is balanced by masses on the other side of the bell jar so the system doesn't wobble.

So who is right? Me, saying the bob would not want to be dead center because there would be a force pulling it in the opposite direction of motion, or my buddies who say the bob would stay dead center with no bias forces trying to disrupt the pull of the magnet holding the bob inplace in midair center of the bell jar?

2. Oct 13, 2016

### jbriggs444

There are multiple centers of the bell jar and some of them are further from the center of the wheel than others?

3. Oct 13, 2016

### A.T.

Where would that come from?

4. Oct 13, 2016

### litup

Ah, reference to center of wheel, the bob points to the center of the wheel.

5. Oct 13, 2016

### litup

I would have thought since circular motion can be thought of as a continuous acceleration the system would be in an accelerating frame of referance and so the bob would be subject to that acceleration and therefore move to the sidewall of the bell jar opposite the direction of motion. Am I missing something fundamental here?

6. Oct 13, 2016

### A.T.

I don’t know, as I don't fully understand the setup. Can you draw a diagram?

7. Oct 13, 2016

### jbriggs444

So the bell jar is not centered under the wheel? I do not understand the setup either.

8. Oct 13, 2016

### BvU

Your little magnet creates an equilibrium that can never be dead center enough: at some $\omega$ it will not be strong enough to be overcome by the forces required to keep the bob in a circular orbit with an almost zero radius.

But I'm also one of those who don't understand your description of the setup

9. Oct 13, 2016

### litup

Ok, a drawing would make it easier, not sure how to do that though. So there is a big wheel, 5 meter radius, spinning tangentially to Earth's surface (horizontal). On that wheel is a magnet. Over the magnet is a 10 meter long bell jar evacuated, good vacuum inside, centerline of the bell jar pointed to center of circle so it starts at radius 5 meters (edge of wheel) to 15 meters out.
On the inside of the bell jar is a string held at outer end dead center of the jar, say the jar is 500 mm in diameter and the bottom of the jar has a vacuum seal. So the string goes to an iron bob and the bob is held in place in mid air by a magnet strong enough to hold it in mid air but not so strong as to stop at least measurement of forces driving the bob from centerline of the jar.

So the wheel is not rotating at first, steady. So a motor starts it rotating, say to a max of 100 RPM.

During the change in angular momentum getting up to the projected 100 RPM I would expect the bob to have a force applied tending to break the magnetic pull.

But the question is about the steady state 100 RPM part of the system. Of course the system is balanced by equal masses on the opposite side of the wheel from the bell jar to keep it from wobbling in its spin.

So after it reaches the 100 RPM specified, does the bob now no longer have any disturbing forces on it that would tend to try to move the bob away from dead center against the force of the magnet holding the bob to the center line?

I think there still will be a force on the bob even after it reaches steady state RPM. My buddies say no, the bob would remain dead center and the only force on it would be the magnetic force required to hold it aiming at centerline against gravity.

Which view is correct?

10. Oct 13, 2016

### A.T.

Why do you need that jar and the magnet? Can't you just put it into space to have vacuum and zero-g?

11. Oct 13, 2016

### BvU

Is it hard for you to understand why no one can understand this ?
Why would the bob want to move ?

12. Oct 13, 2016

### A.T.

And that force on the bob would be exerted by which body?

13. Oct 13, 2016

### A.T.

Inertial forces in non-inertial frames (if that's what you mean) aren’t "opposite the direction of motion", but rather opposite to the frame's acceleration.

14. Oct 13, 2016

### litup

So the only force on the bob would be the outward force of centripital forces and nothing sideways I think is what you are saying.

So if you reposit this to a train track in an air evacuated tunnel where the track is feeling the exact same curvature of Earth, as if it were on a curve the same as the equator, and further the track is 100% frictionless and you place a car with also 100% frictionless wheels on the round track and the wheels have the same cut but negative so it won't go sideways off the track, you carefully place the car on the track, would the Earth's circular rotation impart a movement to the car?

15. Oct 13, 2016

### jbriggs444

The outward force of the string, the downward force of gravity and the inward force of the magnet. No sideways forces, yes.

So you have train track running around the Earth along a line of latitude other than the equator -- let's say it's on the 45'th parallel. A train is parked at a station. You want to ask whether there is a net force either east or west?

Where do you think that such a hypothetical force would come from?

16. Oct 13, 2016

### A.T.

I'm again not sure if I get what you ask. You sound like those people that were questioning whether the Earth actually rotates, based on the fact that the ground doesn't slide under them.

17. Oct 14, 2016

### litup

I was trying to think of ways you could do simple experiment to prove Earth rotates. On a site I go to there is a science forum and one dude there is a dyed in the wool flat Earther. Just trying to understand the forces available to check proving Earth spins. Obviously Earth spins, I am just trying to come up with unrefutable evidence to blow those idiots out of the water. Mostly any argument doesn't work with those people since they are too programmed or dense to change their views.

18. Oct 14, 2016

### A.T.

Or they just troll you. But if you learn some correct physics in the process of trying to disprove them, then they troll you for a good cause.

19. Oct 14, 2016