Hello, Of the different rotational quantities of motion--angular velocity, angular acceleration, torque, angular momentum, etc.--, which of them has their direction perpendicular to the plane of rotation (not sure if I worded that correctly)? I am pretty certain that torque has its direction perpendicular to the plane of rotation. I've tried to research why that is, but most people say that it is simply mathematical convention, and that there is no physical reason why, or something to that effect. But I feel like if angular velocity, a quantity that is well defined, has its direction perpendicular to the plane of rotation, then we can use this fundamental quantity to reason about the others, which may give us more insight as to why they have their direction perpendicular to the plane of rotation. I'm sorry if this isn't exactly coherent; this is just a stream of thoughts, and is the only way I can seem to articulate it.