Sliding a Magnet Attracted to a Steel Plate

Q:
When a magnet is attracted to a large piece of steel, why is the sliding force substantially less maybe less, than the direct pull force from the steel plate?
- Moe (age 20)
FortCollins,CO
A:

It has to do with the coefficient of friction between the magnet surface and the steel plate.  As a simple example suppose you have a one kilogram weight resting on a table.   The force required to lift it is F = mg where g is the acceleration due to gravity.   However, the force required to slide it is μ*F where μ is the coefficient of friction.   For many, if not most surfaces, μ is less that one so the sliding force is usually less than the attractive force.  The same principle principle applies to the magnet case.

LeeH

(published on 08/01/2013)

Follow-Up #1: Work done when removing a magnet from a steel plate

Q:
The work done by sliding a magnet off a steel plate, and the work done by pulling a magnet off a steel plate. Is the work equal? Or sliding is less(Ignoring friction). I believe by sliding the magnetic force does not act.
- Moe (age 20)
Fort Collins,CO
A:

Hello Moe,

There is no way you can remove the magnet to a large distance from the plate without doing work.  It would contradict the first law of thermodynamics.  Since you can obtain energy by harnessing the force when the magnet is attracted to the plate, you will have to pay that back when you remove the magnet.  Sliding the magnet sideways against friction will take up even more energy.  Perhaps you were thinking of sliding the magnet to the edge of a finite plate and somehow removing it with less force.  It doesn't work.

LeeH

(published on 08/06/2013)

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