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.
 
        

 

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.
 

 

A high ball toss helps to increase the amount of topspin in a kick serve.

Here, a rising and a falling ball is struck by a vertical aluminum bar,

representing an upside-down serve where the racquet approaches horizontally.
 
 

 

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 thrown or struck with topspin, the spin axis could rotate

during the flight and end up pointing in a different direction.

Here is an example of how it could happen (at 300 fps). And here

is what does happen (with thin rod through ball axis to tilt ball).
 

 

 

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.
 

 

AERODYNAMICS

 

A very simple wind tunnel using a plastic sheet rolled into

a cone. It blows air at 4 m/s onto a polystyrene ball supported

by two cotton threads (to reduce sideways motion). The angle

from the vertical measures the drag force.
 

 

Measuring the drag force on a balloon.

The forces on the balloon include gravity,

the buoyant force and the drag force.
 

 

Magnus force on a spinning balloon.

Clockwise spin     Counter CW spin     At 300 fps

 

Magnus force on a paper cylinder
 

 

Magnus force on two plastic cups taped end to end and launched with

backspin using two rubber bands. The result is a typical baseball Òpop-upÓ.

Here is the method and here is a backspin Ògolf shotÓ at lower spin.
 

 

Slow motion 300 fps movie showing a 10 gram polystyrene

ball thrown with a home-made Lacrosse type launcher at

23 m/s with backspin at 2000 rpm. Defies gravity !
 

 

The same 10 gram ball launched with sidespin shows an equally dramatic

effect in the horizontal direction due to the horizontal Magnus force.

The spin axis is slightly tilted. The small vertical component of the Magnus

force counters gravity, so the ball moves in a horizontal plane.
 

 

Reverse swing of a cricket ball occurs at ball speeds around 90 mph,

provided the seam is aligned at around 20-30 degrees to the path of

the ball.  It can be easily demonstrated with a balloon at 1 mph.
 

 

BOUNCE MOVIES         TENNIS MOVIES          GOLF MOVIES

 

BASEBALL & SOFTBALL MOVIES         BILLIARD BALL MOVIES

 

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