1. Apr 18, 2009

Freyster98

1. The problem statement, all variables and given/known data

Derive an expression geometrically for the radius of curvature of the following beam. This is part of a lab assignment for the bending of a simply supported beam with overhangs.

** I did this crappy diagram with AutoCAD, so I couldn't ( or didn't know how to ) include greek letters. Let's let r= $$\rho$$, and d= $$\delta$$ for my derivation.

2. Relevant equations

a2+b2=c2

3. The attempt at a solution

I just used the pythagorean theorem to solve for $$\rho$$.

Starting with: $$\rho$$2= ($$\rho$$-$$\delta$$)2+(L/2)2.

Factoring out ($$\rho$$-$$\delta$$)2 , solving for $$\rho$$ and simplifying , I end up with the following expression:

$$\rho$$=($$\delta$$/2)+(L2/8$$\delta$$)

I guess I have this question...is this the proper way to derive the radius of curvature geometrically? Is it ok to do it this way?

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Last edited: Apr 18, 2009
2. Apr 19, 2009

tiny-tim

Hi Freyster98!

(have a rho: ρ and a delta: δ )

Yes, Pythagoras is fine (though you seem to have lost a factor of 2 somewhere ).

But there is quicker method (with less likelihood of a mistake):

Hint: similar triangles

3. Apr 19, 2009

Freyster98

I ran through it a few times...I don't see where I'm losing a factor of 2.

4. Apr 20, 2009

tiny-tim

sorry … my similar triangles method (have you tried that yet?) gave me the diameter, not the radius

so i got an extra 2

5. Apr 20, 2009

Freyster98

Ok, thanks. No, I haven't tried the similar triangles because, well, I don't get it :uhh:

6. Apr 20, 2009

tiny-tim

ok … the triangle with sides d and L/2 is similar to the triangle with sides L/2 and … ?