- #1
rsq_a
- 107
- 1
I can't seem to wrap my head around the signs of shears and moments when applied to beams. Consider a cantilever beam that goes from x = 0 to x = L (with positive deflection, or y(x), corresponding to a deflection upwards).
The standard equations tell us that
[tex]
\text{Moment} = EI \frac{d^2 y}{dx^2}
[/tex]
[tex]
\text{Shear} = EI \frac{d^3 y}{dx^3}
[/tex]
[tex]
\text{Load} = EI \frac{d^4 y}{dx^4}
[/tex]
Now consider what happens when we change [tex]x = -x[/tex] (that is, we put our coordinate system so that the beam begins at x = 0 and goes to x = -L).
Why does that change the shear to negative, but keep the sign of the moments and loads the same?
The standard equations tell us that
[tex]
\text{Moment} = EI \frac{d^2 y}{dx^2}
[/tex]
[tex]
\text{Shear} = EI \frac{d^3 y}{dx^3}
[/tex]
[tex]
\text{Load} = EI \frac{d^4 y}{dx^4}
[/tex]
Now consider what happens when we change [tex]x = -x[/tex] (that is, we put our coordinate system so that the beam begins at x = 0 and goes to x = -L).
Why does that change the shear to negative, but keep the sign of the moments and loads the same?