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# Re: [femm] Re: Incorrect force direction

- To: <femm@egroups.com>
- Subject: Re: [femm] Re: Incorrect force direction
- From: "Steven Stretz" <stretzsj@execpc.com>
- Date: Thu, 4 Jan 2001 21:14:51 -0600
- References: <NHEBIHNJBDNGFALCCCCIKECPCAAA.tickle@aem.umn.edu>

    I agree, this was a great problem. I, hopefully others, learned something.
    Thanks
    ----- Original Message -----
    From: "Rob Tickle" <tickle@aem.umn.edu>
    To: <femm@egroups.com>
    Sent: Wednesday, January 03, 2001 10:51 PM
    Subject: [femm] Re: Incorrect force direction

    > 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
    >
    >
    >

- References:
  - Re: Incorrect force direction
    - From: "Rob Tickle" <tickle@aem.umn.edu>

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