Let's say I have the following diagram:
Since it's a fixed beam, I know that there will be three components acting on point A:
1) Force acting in x direction
2) Force acting in y direction
3) Moment
However, I don't understand why If I take moment about point "A", I would need to consider the moment Ma but not the x and y forces acting on that point.
I always thought that taking moment about specific point cancels all the forces acting on that point.
I would really appreciate if someone could clarify that for me.
Best Answer
We can apply the concept of torque only about a fixed axis(I.e. an inertial frame) and in this specific question calculating torque about the pivot will also help you as torque due to hinge forces will be zero(as their arm length is zero)! But in the case of non-fixed systems we calculate torque about COM which is attributed to the fact that torque of pseudo forces about COM in COM from will be zero(hence behaving as a fixed axis).