W9.1 Angular momentum Banana Skins
By far the most common misconception about angular momentum is ......
WRONG!
![]() | Only a particle moving in a circle has angular momentum |
WRONG!
![]() |
Not true - review Key Point 5.16; there is no mention of rotation here!
With that in mind, let’s try a little problem…
The figure shows a particle moving in a straight line at constant speed, past some point O. Describe the behaviour of the particle’s angular momentum about O.
Solution
Reveal

Solution
It is the same at all points along the line : 12 kgm2s-1, in a direction into the page. It is tempting, but incorrect, to think it is a maximum when the radius and velocity vectors are perpendicular.
Let’s consider a general case:
At the point P, the distance away from the origin is given by
The (vector) equation for the total angular momentum,
, at any point is given by

This vector is directed into the page and has magnitude given by L, where

So, in the case where and
are perpendicular,
their cross product is a maximum (sin~φ=1)
but this is balanced by
being its minimum value.
You may like to think about this when you are tackling the roundabout question (Q6.15)