If the centre of mass M is to the right of the pivot point
S, the arm tilts to the right.
If M is to the left of S, the arm tilts
to the left.
Adding a ball or other mass to the left of M moves M
slightly to the left.
Adding a ball or other mass to the right of M moves M
slightly to the right.
In this kind of system M should normally be above S.
Tilting the arm to the left will cause M to move slightly
to the left. Imagine M high above S to see
this. If tilting moves M to the other side of S,
the arm will be stable in both positions.
With 0 to 5 balls on the arm, and the arm tilted to the right, M
should be to the right of S.
Adding the 6th ball should move M to the left side of S
and start tilting.
With the arm tilted to the right and empty except for the first ball arriving
on the left end, M should still be to the right of S
to avoid tilting. If this cannot be achieved, the arm may still operate
correctly if the ball rolls quickly to the right end.
When the arm is tilted to the left, and the balls roll of, M
should stay to the left of S even when only one ball is left
on the left end of the arm. Otherwise the arm may tilt back to the right
before the last ball has left.