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COMMENTS
Tuesday 21st March 2006
Bob Sollish wrote:
In reference to your article on what makes a ball wobble I would
have to say that your explanation is lacking - moving air (stray
breezes) doesn't cause it. The answer lies in an understanding of
fluid mechanics - in this case the fluid in question being air.
(my Mechanical Engineering degree comes in handy here)
When a ball is spinning the flow of air around the ball is laminar.
When the balls is moving through the air without spinning - or very
little spinning - the flow is called turbulent. (picture the smooth
flow over an airplane wing in a wind tunnel - that's laminar)
The ball transitions from one to the other unevenly - one side or
part of the ball will lose its laminar flow and start to become
turbulent - this is called "boundary layer separation."
This separation on one side of the ball can create the effect of
the "bottom dropping out" where the ball drops down suddenly
towards the table - shorter than expected. In effect the spin on
the ball has "stalled". A normally struck ball never spins
down enough to do this within the length of the table.
The second effect in play here is that a ball with no spin at all
will "buffet" in the air - oscillating from side to side
as well as up and down, as it moves - you can see this effect clearly
with knuckle ball pitchers in baseball. (Tim Wakefield being the
best modern day example) In a slow motion replay the ball can be
seen to "dance" left and right as it moves towards the
plate. The turbulent motion with no boundary layer - will cause
alternating low pressure areas on the sides of the ball (perpedicular
to its motion) causing the ball to be drawn quickly to the side
as it flies. The effect is identical to that of table tennis "wobble",
and a baseball is clearly too heavy to be blown around by stray
breezes.
Sorry about the explanation being pretty technical - but the real
answers for things like this usually are.
