A Three-Legged Stool
Date: 06/26/2001 at 11:49:02 From: Teri Brown Subject: Why is a 3-legged stool always steady? I am interested in finding out why a 3-legged stool is always steady, versus a 4-legged stool, which can be wobbly. Thank you for your help!
Date: 06/26/2001 at 14:05:54 From: Doctor Ian Subject: Re: Why is a 3-legged stool always steady? Hi Teri, That's an excellent question! The answer has to do with something called 'degrees of freedom'. Think of it this way: (1) If you hold a cane in the air, you can move it in any direction, twirl it, and so on. Its motion isn't constrained at all. That is, the top of the cane can move freely in three dimensions. (2) If you put (and keep) one end on the ground, now its motion is constrained: you can't lift it, or rotate it... although you can swing the top around in a variety of different arcs. That is, the top of the cane can move freely in two dimensions. (3) If you connect the tops of two canes together and place the other ends on the ground, you can still move the tops, but only along a single (straight) arc, back and forth. That is, the tops of the canes can move freely in one dimension. (4) If you try the same trick with three canes, now you can't move the tops at all. This is basically what's happening with a three- legged stool. The tops of the cans can move in zero dimensions... which is to say, they can't. Each time you add a cane, you remove one dimension in which the top can move freely - that is, each new cane removes one 'degree of freedom'. Now, what happens when you add a fourth cane? Well, now you have too many constraints. This means that there are multiple ways that the stool can 'solve' the problem of which legs to use for support. Wobbling occurs when the stool can't 'decide' which solution to use, or, more precisely, when it's changing its mind about which solution to use. In effect, during the time that the stool is actually wobbling, it's really a two-legged stool, with one degree of freedom - which is the direction of the wobble. In math, we see this kind of behavior in systems of equations. If I have two variables, one equation will give me a whole range of solutions: y = 2x + 3 Solutions include (1,5), (2,7), (3,9) In fact, the range of solutions is just the graph of the equation. If I add another equation, I can have at most one solution: y = 2x + 3 Solution is (2,7) y = 4x - 1 That is, each new equation removes a 'degree of freedom' from the system. With two variables, I need two equations to lock things down. With three variables, I need three equations. And so on. Now what if I add a third equation? y = 2x + 3 y = 4x - 1 No solution y = 4x + 1 There is no pair of (x,y) values that will satisfy all three of these equations simultaneously. Of course, I can _choose_ any two of the equations and get a solution for that pair, and then I could switch to another choice, which would be quite a lot like what happens when a four-legged stool wobbles. So, the 'mathematical' answer to your question is: A 3-legged stool 'solves' a system of three equations in three variables, while a four- legged stool changes its mind about which three of four equations to solve for the same three variables. Does this help? Write back if you'd like to talk about this some more, or if you have any other questions. - Doctor Ian, The Math Forum http://mathforum.org/dr.math/
Date: 09/02/2003 at 16:54:02 From: John Subject: Three-legged stool I found your answer very interesting. If I understand you correctly, you feel that a three-legged stool is more stable than a four-legged stool. Most furniture designers feel that a five-legged stool (especially one on casters) is more stable than a four-legged stool and, I believe, a three-legged stool. For a given leg radius, the consensus seems to be the more legs, the more stability. This concept seems at odds with your analysis. Can you explain this for me?
Date: 09/02/2003 at 23:35:00 From: Doctor Peterson Subject: Re: Three-legged stool Hi, John. There are different kinds of stability! A three-legged stool is guaranteed not to wobble, because the ends of its legs always form a plane. But a little wobble is only an inconvenience. More important for practical purposes, such a stool is LESS stable than one with more legs in the sense that its center of gravity is further inside its base: the more sides a regular polygon has, the greater its apothem (the distance from the center to the middle of an edge). That greater distance means that the sitter can lean farther out in any direction without tipping over. So if you don't mind a little tipping but don't want to fall on your face, or if you have a reasonably even floor, more legs are better. In a barn, I'll take the three-legged stool over the upholstered desk chair any time! If you have any further questions, feel free to write back. - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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