PHYSICS OF SPORT MOVIES (Updated June 2011)
The following
QuickTime movies are concerned with the physics of bouncing balls, collisions,
ball spin,
vibrations and other aspects of physics related to sport. The physics is described
briefly
in the relevant
sections of my web site and in more detail in journal publications.
PC users can
download QuickTime for free at http://www.apple.com/quicktime/download
You can save a
movie (with Firefox) by clicking on the down
triangle & can then change time in hh:mm:ss to frame
number (F) by clicking in the time/F box.
The down triangle doesnÕt work in recent versions of Firefox. Use: Tools
– Page Info – Media – Save As
FALLING
If a slinky is held vertically at the
top end and then released, does the bottom end fall faster or slower than the top end? Hitting a ball with a bat or
racquet involves a similar process in that the handle starts
vibrating well after the ball is struck. Slinky explanation.
See also www.youtube.com/watch?v=eCMmmEEyOO0
The bottom end of a pencil (or a tree) can
slide in either direction when it falls. Or first one way then the other, as in
this slow motion video. On a sports surface, a person can twist an ankle if the foot
or shoe grips instead of sliding.
Click here
for a few explanations of these spinning effects
SPINNING
Video film taken at 600 fps showing the
ball spin in a kick serve. The spin is mostly sidespin, with a small amount of
topspin. The racquet head needs to rise upwards to generate topspin. It does, but only
just, so topspin is relatively small. Here is
another kick serve showing a bit more topspin
since the head rises faster on impact.
Polyester
tennis strings are now preferred by professional players since they
generate slightly more spin (but are
stiffer on the arm). They behave a bit like spaghetti strings (which were banned in the 1980s and which were
unwoven) in that they move sideways easily since polyester is slippery.
When the strings snap back,
they increase the spin and reduce the ball speed parallel to the strings. The effect is
shown here and here and here . Note that bottom of the ball moves backwards,
with tangential COR ~ 0.5
If a ball is
projected forward while spinning about a vertical axis, does it roll
forward or what? Here
are three film clips, spin1 spin2
(both 30 fps) and spin3 (300 fps) showing that the ball
curves, as in tenpin bowling. The ball rolls along a small circular track
at the bottom of the
ball. At low spin rates, the spin axis rotates into a horizontal position.
A steel ball on a steel plate rolls to
a stop and then keeps spinning for a much longer
time.
This clip (at 300 fps) shows a golf
ball struck with a billiard cue. The ball spins backward as it slides forward on
the carpet, then spins forward as it continues to slide forward, and then
starts rolling when v = Rw.
WOBBLE BOARDS & MUSICAL SAWS
The vibration frequency of a wobble
board varies with the amount it bends. How come? The
vibration frequency of a tennis racquet drops when the strings
are installed.
The vibration frequency of a metal saw
also varies with the amount it bends. How come? The standing wave frequency
depends on wave speed which depends on saw stiffness which depends
on the amount the saw bends. Theme music from ÒOne flew
over the cuckooÕs nestÓ uses a musical saw.
BOUNCE MOVIES TENNIS MOVIES GOLF MOVIES
BASEBALL & SOFTBALL
MOVIES BILLIARD
BALL MOVIES
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