Also on my,

some of my forces in the X and the

Y direction, it has to be two orthogonal directions.

And so if I had a, a body on a slope, for instance.

And let's say that the X, the X axis was in my

parallel to the slope and my y axis was perpendicular to the slope,

I could also use some of the forces in the para, perpendicular

direction equal to zero and some of the moments equal to zero to

enforce static equilibrium.

Now when I go into 3D, I've also got to

include some of the forces in the X direction equaling zero,

so I don't get any acceleration, excuse me, in the Z

direction, so I don't get any acceleration in the Z direction.

In addition, now, to the sum of the moments, about point p in just the

plane around the Z axis, now I also have to have some of the moments about the,

[COUGH]

excuse me, about the X axis equal to zero and the sum of

the moments about the Y axis equal to zero to enforce static equilibrium.

So we have three independent equations for static equilibrium in

2D and six independent equations for static equilibrium in 3D.