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Re: [Phys-l] Statics conundrum



OK - What am I missing here?

F1 and F2 are applied at the ends.

F3 is applied, say, L/4 from the left end.

F1+F2+F3=mg where m is the mass of the rigid bar.

Moments from right end
F1 + F3*3L/4 - mgL/2 = 0

Moments from left end
F3*L/4+F2*L-mgL/2 = 0

Solve to get F1=mg/4, F3=mg/3, and F2=5mg/12

Why are you restricting the solution to only one moment equation?

Bob at PC

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of Bob Sciamanda [treborsci@verizon.net]
Sent: Sunday, March 14, 2010 5:02 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: [Phys-l] Statics conundrum

Here is a common elementary statics problem:

A perfectly rigid and uniform beam of given weight and length (W and L) is suported by two men, exerting upward vertical forces F1 and F2, one at each end of the bar. Determine F1 and F2 in terms of W and L.

This is easily solved by imposing translational and rotational equilibrium:
F1 + F2 = W
W*L/2 = F2*L => F2 = W/2 and F1 = W/2 (independent of L)

One can even add other given loads at given positions on the bar, and the problem is still easily solved.

*****************
But a curious student might uncover the following conundrum:
If one adds a third man exerting a third upward force F3 at a given location (say L/4 from one end), The two equilibrium equations are insufficient to solve for the values of the three unknons, F1 F2 and F3.

The three man experiment can be performed and the forces measured (even with added loads on the beam). They ARE physically determined.
How does one analytically predict this result?

Please discuss. Is it the perfect rigidity which must be relaxed? Why?
How explain this to the curious student? (and to me)

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
http://mysite.verizon.net/res12merh/



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