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Why is the force required to slide a magnet off a steel plate A LOT less than the force required to directly pull it off?

The force required to pull the magnet can be: 20lb While the force required to slide the magnet can be: 1lb more/less.

Why is that?

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2 Answers 2

When you slide a magnet off a plate, you are doing the same amount of work as when you pull it directly off (slightly more, in fact, because of the friction). However, while it's the same amount of work, it's spread out over a much larger distance, and so requires much less force. When you slide a magnet off a plate, the force decreases gradually, starting as soon as the magnet reaches the edge of the plate. When you pull a magnet off a surface perpendicularly, the force is quite large when the magnet is touching it, but decreases quickly as soon as it is a short distance away.

The basic principle is the same as the one that makes levers, double pulley systems, and switchbacks useful. Since work = force × distance, increasing the distance reduces the force required.

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I don't think so... I did some experiments, having to equal sized magnets attracted to each other, the amount of work done sliding them off is much less. In fact, the force is not as strong as a direct pull. You're right, it could be that by increasing the distance you are doing a lot of work, but I doubt it could ever equal the same amount of work pulling it away from a magnet/steel plate. We need to look at the magnetic force being applied. I think it only acts in one direction maybe? By sliding it the magnetic force is not resistive it stays the same only counter force is friction. –  M. A. A. Aug 1 '13 at 13:57
    
Ignoring friction, pulling a magnet off a plate perpendicularly has to require the same amount of work as pulling it off by sliding it. Otherwise, you could build a perpetual motion machine. –  Peter Shor Aug 1 '13 at 14:05
    
@HYP Peter Shor is using the very specific meaning of "work" that is standard in physics. I think you may be confusing "doing a lot of work" with "applying a large force" which are not the same thing. en.wikipedia.org/wiki/Work_%28physics%29 –  Kyle Aug 1 '13 at 14:57
    
Makes sense. I'll study this some more. –  M. A. A. Aug 1 '13 at 15:47
    
This has to be wrong--the force required to slide the magnet across the surface is something that doesn't depend on the size of the surface. Actually getting the magnet off, however, happens only at the edge. If I double the size of my metal plate, I'll double the "sliding work" without affecting the force. –  Jerry Schirmer Aug 1 '13 at 16:07
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After much studying, and experimentation. I conclude that indeed, the force required to "slide" the magnet's is less than the force required to "pull" them directly off.

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