# Bending moment formula?

1. Jun 18, 2008

### Amy54

1. The problem statement, all variables and given/known data
what is the bending moment formula? is it related to the pure bending formula? what's that?

2. Relevant equations

3. The attempt at a solution
is the formula 1/p=M/EI?

2. Jun 18, 2008

### Defennder

There's a particular physical setup associated with the bending moment of a small section dx of a beam under gravity. The formula would depend on how you setup and balance the forces and torque on that section. Perhaps if you describe it, the homework helpers here could help you figure it out.

3. Jun 18, 2008

### Amy54

i am finding the bending moment of a rectangular beam of plywood. the dimensions change due to altering the variables throughout the experiment.

4. Jun 18, 2008

### Defennder

Yes, but how do they change? You have to be more explicit here, otherwise I don't think anyone can help. Try posting the question from the textbook or notes.

5. Jun 18, 2008

### Amy54

its not a question it is a part of a physics assignment of beam deflection.. i am trying to show for the formula (force x length^3)/(4 x young’s modulus x breadth x thickness^3) was derived. i know it was done through integration and the combining of the bending moment formula and the Torque formula (T=fr)...

6. Jun 18, 2008

### dirk_mec1

Suppose we have the situation in the picture.

We get:

$$EIv_{xx} = -FL+Fx$$

or after two integrations:

$$EIv(x) =F \frac{(x-L)^3}{6} +C_1x +C_2$$

From this point on I leave it to you....

Last edited: Jun 18, 2008
7. Jun 18, 2008

### stewartcs

http://en.wikipedia.org/wiki/Euler-Bernoulli_beam_equation

CS

8. Jun 18, 2008

### dirk_mec1

9. Jul 17, 2008

### nvn

Amy54 wrote: "What is the bending moment formula?"

d(theta)/dx = M(x)/(E*I), where theta = beam rotation, and M(x) = moment as a function of x.

Amy54 wrote: "Is it related to the pure bending formula? What's that? Is the formula 1/rho = M/(E*I)?"

Nice work, Amy54. Yes, that is the pure bending formula. For small theta, d(theta)/dx = 1/rho.