Re: How does a ball spin on your finger?

A ball spinning on a finger is similar to a top spinning on a level 
surface, a gyroscope spinning in its housing, or even spinning satellite 
or football.  In all these examples, angular momentum from spinning 
produces stability because angular momentum will be conserved unless a 
force (or really a torque) upsets the system.  Angular momentum is a 
function of rotational speed and mass distribution (moment of inertia).  
So the angular momentum depends on the spinning speed and the shape of the 
ball.  There are many MadSci replies in the archives about angular momentum.

Unfortunately with a basketball because of the shape, it more difficult to 
generate enough angular momentum.  The ball must be spun very quickly, or 
on average the finger must stay under the center of gravity.  Consider 
balancing a hammer or a broom on the palm of your hand.  You can adjust 
your hand to keep the center of gravity to keep the hammer or broom from 
toppling.  For the case of the hammer, if the head of the hammer is in 
your hand is pretty easy to balance.  However, if the end of the handle is 
on your palm, it will be difficult to keep it balanced.

Experienced spinners make very subtle adjustments to keep the ball 
balanced and spinning.  It takes some practice to avoid over-adjusting the 
torques to keep the ball balanced.

Sincerely,
Tom "Curly Neal" Cull