Transcript biomechanics - CastleSchoolPE

BIOMECHANICS
Angular Motion
Angular Motion
The same quantities used to explain
linear motion are applied to angular
motion. In rotating bodies they take
on there angular form : • ANGULAR DISPLACEMENT
• ANGULAR VELOCITY
• ANGULAR ACCELERATION
• ANGULAR DISPLACEMENT –
Distance of a body rotating around an axis, measured in
degrees (1 complete rotation is 360o)
• ANGULAR VELOCITY –
The angle through which the body rotates about an axis
in 1 second. E.G A trampolinist performing a tucked back
somersault turns through 360o in 2 seconds. What is the
resulting angular velocity?
• ANGULAR ACCELERATION –
The rate of change of angular velocity.
Moment of Inertia
An objects resistance to rotational
change.
The moment of inertia (MI) is determined by its
mass and the distribution of its mass around the
axis and rotation.
PRACTICAL : Whilst sitting on the hall floor see how many times
you can spin round 360o with one push!
You should have found that when you have a wider shape
you spin a lot slower and therefore less times. When you
tuck in you should have found that you spin a lot faster and
do more rotations. (Why is this?)
Where the objects mass is concentrated about the axis, the
lower the moment of inertia and the greater the angular
velocity e.g. when tucking in whilst spinning.
Can you think of any other sporting examples?
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Gymnastics – Somersaults
Trampolining
Diving
Figure Skating
The further the objects mass is away from the axis the
greater its moment of inertia and the slower the rate of
rotation, the more force required to make it rotate and stop
rotating.
(See page 204 in
Sport & PE for
differences in MI’s)
ANGULAR MOMENTUM: This is the product of AV x MI
This relates to Newtons 1st Law i.e. an object will continue to
rotate with a constant angular momentum unless acted on by
a net force.
If MI increases, AV decreases and vice versa. Therefore
Angular momentum is conserved and remains the same.
TASK!!
Copy the graph from the next slide and add onto it the lines
representing: Angular Momentum
Angular Velocity
Moment of Inertia
Explain the shape of the 3 curves you have drawn and the
reasons behind them.
Angular Momentum
Moment of Inertia
Angular Velocity
Time (s)
ANSWERS!
• Angular momentum remains constant during the flight.
• There are no external forces acting
• The moment of Inertia decreases (in tuck phase)
• Because of reduction in distribution/spread of mass.
• Angular velocity increases (during tuck phase)
•Angular momentum = angular velocity x moment of inertia
Another example question?
1. Explain the mechanical principles that allow spinning ice
skaters to adjust their rate of spin.
(6 Marks)
ANSWER!
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Ice may be regarded as a friction free surface/friction is
negligible.
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During spins angular momentum remains constant;
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Angular momentum is the quantity of rotation;
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Angular momentum = AV X MI;
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Angular velocity = rate of spin/how faster skater spins;
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Moment of inertia = distribution/spread of mass around axis;
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Changing/reducing moment of inertia affects/ increases
angular velocity;
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Skater brings arms into body allowing rate of spin to
increase.