PHYSICS OF ANGULAR MOVEMENT I The Twist I The Tilt Twist

The principle of angular momentum is defined as the following:

angular momentum=moment of inertia x angular velocity or,AM=Iw

The relationship is as follows:


1)Angular momentum can be viewed as the quantity of rotation a body

has about some given axis as a result of its speed of rotation and 

the distribution of mass about the axis.


2) Moment of inertia (I) is the measure of how the mass of body is 

distributed about the axis of rotation.  The further the mass is 

away from the object, the larger the "I" and vice versa.  In fact

"I" increases as the square of the distance of the mass and therefore

small increase in the distance can result in relatively large increases 

in "I."


3) Angular velocity (w) is simply the stated speed of rotation about 

the axis of rotation.


4) Since AM must be conserved (unless an external force or torque

is applied) the product of "I" and "w" must stay constant.  However

the human body can change position in the air which has the effect 

of changing "I" and thus "w."  If a gymnast or diver tucks up in 

the air, "I" will decrease and then so that AM will be conserved,

"w" will increase accordingly and vice versa.


5) There can be no discussion of rotation unless axis of rotation
 
is specified.

There are two types of twisting mechanisms.  The first is the torque 

twist which is the most effective.  This mechanism is achieved by 

applying large torque relative to the longitudinal axis during 

take off.  The gymnast or diver will then have a considerable 

twisting AM in the air and then if the arms have been held wide

(large "I"), "w" can be increased quite significantly simply

by pulling the arms into the body (small "I").  
  
  The non torque twist can occur two ways.  In the first the gymnast 

or diver can begin with total body AM equal to zero about all axes. 

The second way is for the gymnast or diver to begin with some quantity

of AM about one of the non twisting axes (i.e. somersaulting or 

cartwheeling).    

The most effective way to initiate a non torque twist,if the 

diver or gymnast has angular momentum around a somersaulting axis,

is through a tilt twist "This took many years to dicover because 

it had been neglected that "AM" is a vedtor quantity, that is , 

it has a magnitude component and a directional component"(Technique, ).

To utalize this tilting mechanism, the athlete must shorten one side 

of his or her body. This can be achieved by raising one arm or

bending to one side.  "This has the effect of tilting the somersaulting

axis away from  the "AM" vector the direction of which(in order 

to be conserved) requires the body to undertake a compensating sustained

twist about the longitudinal axis as long as the tilt exists"(Technique, ).

This page was designed and researched by: Jerretta Stirman, Erin Watkins, Nichole Bunker, Lucy Evans, Gena Suh. This page was designed for a class project. The diving pictures were gathered from a web page of pictures of the Women's Springboard Diving-Masters National Champions (http://www.n2net.diving/ out97ws.html). The gymnastics pictures were gathered from the web page of INTERNATIONAL GYMNAST MAGAZINE ONLINE published by Paul Ziert and Associates, Inc.(http://www.intlgymnast.com).

Mail comments to: jerretta@mail.utexas.edu