I can imagine a pencil rotating about an axis parallel to its length and perpendicular to its length at the same time and I have also seen several self balancing mechanisms online (got no idea wt that is) where a ball spins in multiple directions
So what do they mean by an object can only have one axis of rotation?
In: Physics
Let’s talk about vectors.
If we have a vector in the x direction and the one in the y direction, we can add them together, and we get one pointing at a diagonal.
It doesn’t make sense if we could have a vector pointing in two directions at once, it’s just the sum of two other vectors.
Now, rotational velocity can also be described as a vector. Look at the way its rotating and curl the fingers on your right hand, leaving your thumb extended. Your fingers should curl the same direction as the rotation. The way your thumb points uniquely describes the direction of the vector of the rotational velocity. Note how it points along the axis on which the object is rotating.
Since we can describe the rotation around an axis, adding a rotation around another axis lets us add the two vectors together, and the new axis of rotation will point in the direction of that new vector.
For totally free motion, you only need translational motion in 3 axes and rotational motion in 3 axes. This is called 6 degrees of freedom.
Port/starboard
Fore/aft
Ventral/dorsal
Yaw
Pitch
Roll
These are the 6 directions
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