# Bending moment of beam

1. Jun 15, 2009

### jon1984

Hi,

I have the equation:

f/y=M/I

And y = d/2; I = bd^3/13

However i have been trying for ages to try and get this equation in terms of d. Can anyone help me/ give advice as to how?

Thanks

2. Jun 15, 2009

### jon1984

i've got to:

2f/12M = d/bd^3

How do i collect the d's now?

3. Jun 15, 2009

### jon1984

is this correct?

d = cube.root(12M/2fb)

4. Jun 15, 2009

### PhanthomJay

The I of a rectangle is bd^3/12, you may have a typo. The rest is algebraic substitution. Is that where your problem lies...in the algebra?

5. Jun 15, 2009

### jon1984

yeah, i did use bd^3/12 when working out to find d.

I think my above solution is correct, just need someone to check

6. Jun 15, 2009

### jon1984

M = 45Nm
f = 240x10^6
b = 16mm

Im trying to find d.

I used the forumla:
f/y = M/I

re-arranged to find d:
d = cube.root(12M/2fb)

Put my values in and got an answer of:
d = 0.041m

Is this correct?

7. Jun 15, 2009

### PhanthomJay

No. You have a 'd' in the numerator and a 'd^3' in the denominator. 2f/12M = d/bd^3. That's the same as 2f/12M = 1/bd^2