Physics Q (x post)
 

There are two magnets stuck together through magnetic force but there is a pane of glass seperating them, when I spin the magnet closest to me clockwise, both of the magnets spin and move to the right and down, when I spin them anti-clockwise, they spin and move to the left and down.

The magnets are side by side, and the force applied to make the magnets spin is irrelevant since I have spun them in different ways and from different angles and the result is always the same. They also spin in perfect sync.

http://img260.imageshack.us/img260/3105/image594tb9.jpg

Could someone please summarise the forces involved that make the magnet either shift left or right in both scenarios, I'm pretty sure I already know the answer, but one of my friends disagrees.

Re: Physics Q (x post)
 

Because the force of atraction is strongest locally?

PairTheBoard

Re: Physics Q (x post)
 

I change my answer. It's because of the polarity. They stay alligned with opposite polarities next to each other.

PairTheBoard

Re: Physics Q (x post)
 

Explains nothing, please expand.

Re: Physics Q (x post)
 

I'm really struggling with this because seems that everything balances (except a resultant effects of the spin itself). Going with the perfect spin (magnets perfectly aligned 100% of the time)

Friction should balance throughout the system. The angular velocity at any point on one half of the magnet system = the angular velocity at the reflected point (opposite direction). And friction being the result of Kinetic energy *coefficient of friction

EMF forces would also seem to balance in a similar way (NS::SN alignment so they are attracted to each other through the glass).

The spin seems to be the only reason as it is a tendency in other spinning object systems. IE spin a ball in place, push it forward, it will exibit the same tendency, going in the direction of spin.

A curveball manages to create a high and low pressure zone which moves the ball in the air as it goes to the plate, causing it to break.

Basically, I'm just being really stubborn on my thinking in this situation as to what force is not balanced resulting in the movement.

Re: Physics Q (x post)
 

I would guess, and it's only a guess, that your thumb and forefinger don't apply perfectly equal amounts of force to opposite ends of the magnet, and impart a net acceleration to it.

You can test the theory by spinning ONE magnet on a flat surface, with the same hand motion. Guess: whether one or two magnets, clockwise or counterclockwise, the magnets move the direction your thumb pushed them. (Additional guess: you are right-handed.)

Re: Physics Q (x post)
 

The magnets stay parallel to each other "in sync" because they want to be aligned as such (think about the poles in a magnet - the North of one magnet wants to be near the South of the other and vice versa). This is going to always be true for the pair, so if you spin one, the other will track it.

The second half of the question is about torque and gravity. Before you spin the magnets, they are at equilibrium - there's enough friction between the magnets' coatings and the glass that the attraction between the magnets lets them stick together on the glass. If the glass or coating of the magnet was more slick, then they may just drift down spontaneously.

Once you start spinning them, the magnet pair always moves down once you disturb this equilibrium due to gravity and due to the torque that you apply to the system, combined with the magnetic force field lines that are radiating from the magnet pair. If you could spin them fast enough, you could impart enough torque to pull them upward first, but that may be tough here becasue the magnetic fields are present. This is actually the trickiest thing to think about, but basically the magnetic fields extend out away from the magnets and they serve as a guide for the magnet pair. If you spin it clockwise, you tap into the one that would pull the magnets "right" and if you spin it counterclockwise, you tap into the field line that pulls it "left." Again, since the system starts in a delicate equilbrium, you can tap into either path depending on which way you spin them. The reason that the magnets slip and move down is because frictional forces are always harder to overcome at the beginning, but are easier to maintain movement on - kind of like watching me get off my ass to get a beer on a Sunday afternoon. Once I get off the couch, I can get to the fridge quickly, but that initial momentum is the hardest. Same thing here - once you spin the magnets, there's enough momentum for gravity to overcome the frictional force.

There ya go.

Dr Sucker

Re: Physics Q (x post)
 

How on earth does tapping into one magnet pull them both to one side, and how exactly, am I tapping into one magnet by spinning them either way.

Re: Physics Q (x post)
 

What's really happening is the field lines radiate out from the pair of magnets in a toroidal-like pattern. They go from one end to the other. If you spin it one way, you are starting momentum in this direction and are keying into the fields that have that handedness; if you spin it the other way, you key into the other field lines.

This is the answer, BTW, and would be easier to describe wtih a picture, but I'm not posting one.

Re: Physics Q (x post)
 

There are many explanations in this thread, and in the OOT one, that make no physical sense.

The pane of glass is perpendicular to floor, correct?

The magnets move down when you spin them, but remain stationary otherwise, because frictional forces change when shifting from static to kinetic scenarios. The kinetic frictional force must be less than the static frictional force by definition. In this case, the static frictional force's upper limit is large enough to resist the force of gravity while the kinetic frictional force is not, thus the magnets accelerate downwards (towards the earth) when you spin them.

As the magnets move towards the floor they behave like an object that is spinning on a flat floor with a translational velocity component. This means that while the magnet is spinning there are points on it that are at rest relative to the pane of glass. When this occurs, the more powerful static frictional force takes effect and the spinning magnets then pivot very briefly around that point as opposed to their center of mass. This continually repeats, thus causing the magnets to move down and to the left or right.

The fact that these are magnets makes no difference. The force between the two objects could be totally arbitrary. As long as it acts to pull the two objects together with the same force, the same phenomnon will occur.

However, I think that when you spin the magnets clockwise they should move to the left and down and when you spin them counterclockwise they should move to the right and down. You state the opposite. Are you sure this is what they do?

Try spinning a soccer ball on a flat, smooth floor and then give it a nudge. You will notice a similar effect. Instead of moving in a straight line the ball will tend to continually change directions.

Re: Physics Q (x post)
 

This doesn't have anything to do with magnetostatics as far as I can tell. This is an effect of gravity. The spinning magnets tilt towards earths gravity (down) and put more friction between the surface there, meaning more force on the bottom half of each part of the rotation at the glass/magnet interface, causing it to move right when spun clockwise and left when spun counterclockwise. Just like a wheel.

Re: Physics Q (x post)
 

[ QUOTE ]
This doesn't have anything to do with magnetostatics as far as I can tell. This is an effect of gravity. The spinning magnets tilt towards earths gravity (down) and put more friction between the surface there, meaning more force on the bottom half of each part of the rotation at the glass/magnet interface, causing it to move right when spun clockwise and left when spun counterclockwise. Just like a wheel.

[/ QUOTE ]

I was starting to think my solution was way too simple to be correct, but I believe this is the correct answer.

As the magnet starts to fall towards the earth due to decreased friction, there begins an inbalance in the friction between the lower and upper contact points of the spinning metal upon the glass. The lower part of the glass becomes an area of increased friction due to both the shape of the magnet and the fact that the friction created by the magnet moving downwards affects the bottom part of the spinning metal most. Thus when the magnet is spinning clockwise, the metal at the bottom spinning to the left in a U shape manner pushes the magnets right, just like a wheel would.

This is correct, no?

Re: Physics Q (x post)
 

[ QUOTE ]
[ QUOTE ]
This doesn't have anything to do with magnetostatics as far as I can tell. This is an effect of gravity. The spinning magnets tilt towards earths gravity (down) and put more friction between the surface there, meaning more force on the bottom half of each part of the rotation at the glass/magnet interface, causing it to move right when spun clockwise and left when spun counterclockwise. Just like a wheel.

[/ QUOTE ]

I was starting to think my solution was way too simple to be correct, but I believe this is the correct answer.

As the magnet starts to fall towards the earth due to decreased friction, there begins an inbalance in the friction between the lower and upper contact points of the spinning metal upon the glass. The lower part of the glass becomes an area of increased friction due to both the shape of the magnet and the fact that the friction created by the magnet moving downwards affects the bottom part of the spinning metal most.

[/ QUOTE ]

This last bit is why I was going on about velocity-dependent friction in the other thread. I'm not sure what the argument for increased friction on the lower side is if you don't have such a thing. However, the argument that the shape isn't flat and will lead to tilting downward would mean greater pressure there and thus more friction seems solid. And you're right that if there were more friction on the bottom side, then it would have this effect.

I'm with the people that say that the magnetism is probably a red herring here. I think the only reason it is necessary is to give you something that will keep the frictional force for you. A pretty quick experiment that should address these concerns: take a pane of glass, and lay it parallel to the floor. Set up your magnets on that system and spin them. Without the gravitational asymmetry, by these arguments they shouldn't have the same left-right behavior. (I think the actual dynamics will be complicated because of coupling between different axes of rotation from the weird shape. That is, spinning it will lead to pitching and rolling on the other axes.)

EDIT: For that matter, spinning one of these things by itself on a flat surface should do the trick for debunking the magnetic claims, I think. I think the key thing here is that if it were the magnetic fields, the force wouldn't always be to the right, as the whole thing should be rotationally invariant, and would always be to wherever the "right" is right now.

Re: Physics Q (x post)
 

An addendum to my last post: the rotational invariance will be broken by the Earth's field, but if these are neodymium magnets as I think their field is going to be one hell of a lot stronger. And again, if it were the Earth's field breaking the symmetry then spinning one of these bad boys by itself on a flat surface should exhibit the same behavior.

Re: Physics Q (x post)
 

[ QUOTE ]
For that matter, spinning one of these things by itself on a flat surface should do the trick for debunking the magnetic claims, I think. I think the key thing here is that if it were the magnetic fields, the force wouldn't always be to the right, as the whole thing should be rotationally invariant, and would always be to wherever the "right" is right now.



[/ QUOTE ]

Exactly.

Re: Physics Q (x post)
 

[ QUOTE ]
[ QUOTE ]
This doesn't have anything to do with magnetostatics as far as I can tell. This is an effect of gravity. The spinning magnets tilt towards earths gravity (down) and put more friction between the surface there, meaning more force on the bottom half of each part of the rotation at the glass/magnet interface, causing it to move right when spun clockwise and left when spun counterclockwise. Just like a wheel.

[/ QUOTE ]

I was starting to think my solution was way too simple to be correct, but I believe this is the correct answer.

As the magnet starts to fall towards the earth due to decreased friction, there begins an inbalance in the friction between the lower and upper contact points of the spinning metal upon the glass. The lower part of the glass becomes an area of increased friction due to both the shape of the magnet and the fact that the friction created by the magnet moving downwards affects the bottom part of the spinning metal most. Thus when the magnet is spinning clockwise, the metal at the bottom spinning to the left in a U shape manner pushes the magnets right, just like a wheel would.

This is correct, no?

[/ QUOTE ]

Easy enough to test. OP, can you spin them on a horizontal plane and let us know if they still move in predictable directions?

Re: Physics Q (x post)
 

Sans the horizontal test, I think a good thing to look at is How Much the magnets curve to one side or the other. I can't imagine friction causing that big a curve for relativley dense magnets. If it's a pronounced curving I'd be more inclined to suspect the magnetowatchamacallit effect. Possibly from some kind of lag in the polarity of one magnet catching up to the polarity of the other.

PairTheBoard

Re: Physics Q (x post)
 

[ QUOTE ]
Sans the horizontal test, I think a good thing to look at is How Much the magnets curve to one side or the other. I can't imagine friction causing that big a curve for relativley dense magnets. If it's a pronounced curving I'd be more inclined to suspect the magnetowatchamacallit effect. Possibly from some kind of lag in the polarity of one magnet catching up to the polarity of the other.

[/ QUOTE ]

The speed of light isn't fast enough these days? From the description they're spinning basically in tandem anyhow. And I'm still unclear on what would break the rotational symmetry of the problem. So basically I think friction is a much more compelling answer.

Re: Physics Q (x post)
 

[ QUOTE ]
[ QUOTE ]
Sans the horizontal test, I think a good thing to look at is How Much the magnets curve to one side or the other. I can't imagine friction causing that big a curve for relativley dense magnets. If it's a pronounced curving I'd be more inclined to suspect the magnetowatchamacallit effect. Possibly from some kind of lag in the polarity of one magnet catching up to the polarity of the other.

[/ QUOTE ]

The speed of light isn't fast enough these days? From the description they're spinning basically in tandem anyhow. And I'm still unclear on what would break the rotational symmetry of the problem. So basically I think friction is a much more compelling answer.

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Would you still think that if they curve to the right the same distance as they fall downward? We haven't been told how much they curve yet.

As far as spinning in tandem, I doubt they are so strongly bonded by the magnetic force that they start off spinning as if they were attached to each other. There is bound to be a slight lag at least in the beginning as the magnet on the other side of the glass tries to realign its polarity with the one with force being applied to it. Another test that could be done is to apply equal spinning force to both magnets simulateously and see if they still curve to the right the same way they do when the force is applied to just one magnet. Yes, the magnetic field propagates at the speed of light. That doesn't mean it's force on the non-pushed magnet is equivalent to the physical force being applied to the pushed magnet, or that the nonpushed magnet catches up to the sudden increase in rotational speed of the pushed magnet at a speed of light rate of catch-up. In fact, the non-pushed magnet may never completely catch up it's polarity alignment with the pushed magnet. Yet another thing to measure is whether the rate of magnet curvature is constant. If it's due to friction it should increase to a maximum rate as the magnets fall. If it's due to magnetic imbalance from polarity lag it should decrease as the lag decreases.

PairTheBoard

Re: Physics Q (x post)
 

I just reread the OP and notice he describes the magnets as "shifting" to the right. That doesn't sound like they fall and slowly curve to the right. It sounds like they make a sudden "shift" to the right and then fall downward. If this is correct, it sounds much more likely to be caused by the lag in the back magnet's position of polarity catching up to the front magnet's rotated position of polarity. The effect is most pronounced right at the beginning. Once the polarities get back in opposite synch the effect goes away and the magnets, after shifting to the right drop down more and more straight as their rightward momentum is slowed by friction.

Someone else will have to explain how out of synch polarities act to force the shift to the right.

PairTheBoard

Re: Physics Q (x post)
 

Isn't this just a form of precession?

Wiki - Precession

Re: Physics Q (x post)
 

[ QUOTE ]
Isn't this just a form of precession?

Wiki - Precession

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This may apply to the problem, but I'm not sure how it applies to my theory that the positions of polarity of the two magnets are out of opposite synch at the beginning. The problem I have with my theory is that I don't see why it would imply a bias in one direction or the other. For the clockwise spin, at the top the Back - pole is pulling the Front + pole to the left, while the Front + pole is pulling the back - pole to the right. I don't see any directional bias. And even if there were, the forces at the bottom would be working in just the opposite way.

As far as precession, it seems like you need 3 diminsions for that. Here, the rotation is fixed in 2 dimensions by the glass.

PairTheBoard

Re: Physics Q (x post)
 

There is something more going on here than what has been posted. I have one the magnets here in my office stuck to a metal filing cabinet.
The exact same behavior is exhibited.
If you place it on top of the filing cabinet, it just spins in place.
If you put it on the side and spin it cw, it moves to the left in a circular motion, but then when it gets to the edge of the circle, IT spins BACK and moves to the right, stopping underneath of where it started. It will spin a little more there and then come to a stop (but it is still spinning (sometimes for a few rotations)).
It never moves back up though.
I think it is a precessive type motion but it comes from the combination of magnetic field and gravity for sure. There is a torque applied by gravity as well due to the finite width of the magnet. It may be the combination of these two orthogonal rotational effects that lead to the circular motion.

I don't get how it can move back to the right after it was moving left, the direction of rotation is the same in both cases. However when it is stopped moving left, its momentum is purely down at that point.

Re: Physics Q (x post)
 

[ QUOTE ]
There is something more going on here than what has been posted. I have one the magnets here in my office stuck to a metal filing cabinet.
The exact same behavior is exhibited.
If you place it on top of the filing cabinet, it just spins in place.
If you put it on the side and spin it cw, it moves to the left in a circular motion, but then when it gets to the edge of the circle, IT spins BACK and moves to the right, stopping underneath of where it started. It will spin a little more there and then come to a stop (but it is still spinning (sometimes for a few rotations)).
It never moves back up though.
I think it is a precessive type motion but it comes from the combination of magnetic field and gravity for sure. There is a torque applied by gravity as well due to the finite width of the magnet. It may be the combination of these two orthogonal rotational effects that lead to the circular motion.

I don't get how it can move back to the right after it was moving left, the direction of rotation is the same in both cases. However when it is stopped moving left, its momentum is purely down at that point.

[/ QUOTE ]

I still don't see how gravity acts as torque for precession. It's like a bicycle rolling downhill. The precession principle for the bicycle comes into play when force is applied in a third dimension perpendicular to the plane of the rotating wheels. The bicycle tends not to tip over and rights itself due to precession. With the magnet on the side of the cabinet gravity is acting within the plane of the rotation, like the bicycle rolling downhill.

Seems like you're probably right though and I'm just not seeing it. Gravity clearly has something to do with it or you'd see the same effect on top of the cabinet. The back and forth motion has got to rule out friction as the culprit.

Doesn't a spinning magnet generate an electric current in the metal cabinet? And doesn't that electric current generate a magnetic field of it's own acting back upon the magnet?

PairTheBoard