I had a student with an interesting misconception: He was stuck on
the idea that angular momentum was associated _only_ with things that
went around and around and around.
The left half of the diagram shows a particle going around and around
in a circle. It has constant angular momentum relative to the center
of the circle.
The right half of the diagram shows a particle moving in a straight line.
It has constant angular momentum relative to any fixed pivot-point
(including the pivot-point in the picture).
The angular momentum in all *six* cases is the same:
He looked at the picture for a couple of minutes, and then said "Got it."
He had been confused about that and about six other things, which made
it hard to diagnose what the problem was ... but the six other problems
evaporated as soon as the straight-line angular momentum issue was
understood.
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I'm not sure how common this misconception is, and in general I don't
like to spend time discussing misconceptions ... but somehow I found
this one particularly interesting. I think it's because the student
was really trying to do the right thing by _visualizing_ the _physics_
rather than just plugging into some formula.
The mental picture he came up with was not quite right, but you gotta
respect him for trying.