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# Re: Incorrect force direction

- To: <femm@egroups.com>
- Subject: Re: Incorrect force direction
- From: "Rob Tickle" <tickle@aem.umn.edu>
- Date: Wed, 3 Jan 2001 22:51:51 -0600
- Importance: Normal

    This was an interesting problem to me because it showed me how my intuition
    could be misled by the geometry of the problem. One would think by looking
    at the problem (I did anyway) that there could be a centering force. It's
    obvious that the magnet would experience an attractive force before it gets
    fully between the bars - calculation with femm agrees with this if you move
    the magnet a bit to the left. I think the position it is in for this problem
    is right where it hits the fuzzy zone where that force is disappearing. To
    convince myself of this, I grabbed the magnitude and normal components of B
    along the right and left side contours where the force line integral is done
    and put them in a spreadsheet. Then I calculated the x-component of the
    force from the stress tensor for right and left sides and added them up. The
    graph shows the right contour contribution (red) and the left contour
    contribution (grey) with the total x component of force in black. It
    illustrates what David mentioned in the previous post about two large but
    opposite sign contributions summing to zero (or close to it). 2 notes: this
    illustrates force up to a multiplicative constant since I didn't multiply by
    path lengths. Also, I didn't include the x-component of force from the top
    and bottom contours, since they are of equal magnitude but opposite sign,
    and I assumed from the symmetry of the problem that they would cancel.
    Based on this result though, I could see how the answer might be down in the
    noise so to speak.

    I tend to trust this result, but I would be interested in seeing some pics
    of Greg's actual setup and magnets. I know the NdFeB magnets are highly
    anisotropic and very strong; one possible source of stray force to look out
    for would be if the magnetization direction of the magnet were not
    completely perpendicular to the bar surfaces. Does the actual setup show a
    true centering force in the center of the bar; i.e. does the force direction
    switch signs as you roll past the middle?

    Rob

Attachment: `jpg00000.jpg`
Description: JPEG image

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    - From: "Steven Stretz" <stretzsj@execpc.com>

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