Transcript Biomechanicsx

PROJECTILES
• the motion of objects in flight
– human bodies
– shot / discus / javelin / hammer
– soccer / rugby / cricket tennis / golf balls
•
is governed by the forces acting
– weight
– air resistance
– Magnus effect
– aerodynamic lift
•
and the direction of motion
height of
release
angle of
release
DISTANCE
TRAVELLED BY
PROJECTILE
speed of
release
Motion and Movement - Newton’s Laws
FORCE
FORCE
• FORCE is push or pull
• the unit is the NEWTON (10 N is approx the weight of 1 kg)
• force changes the state of motion of an object
• force causes acceleration or deceleration or change of
direction
• the more force the bigger the acceleration
• force changes the shape of an object
•
•
•
•
WEIGHT
FRICTION
REACTION FORCES
AIR RESISTANCE / FLUID FRICTION
– all these forces affect the sportsperson
Module 2562 A.2.4
Motion and Movement - Newton’s Laws
NEWTON’S FIRST LAW of MOTION
NEWTON’S FIRST LAW
• this law is used when ZERO NET FORCE is
applied to an object
• this doesn’t mean that zero force acts, but
that all forces MUST CANCEL OUT
•
with zero net force an object
– is STATIONARY or
– moves at CONSTANT SPEED in the
SAME DIRECTION
•
•
•
a sprinter in full stride has four forces acting
but they cancel out exactly
therefore he / she travels at constant speed
Module 2562 A.2.5
NEWTON’S SECOND LAW of
NEWTON’S SECOND LAW
MOTION
Motion and Movement - Newton’s Laws
•
this law is used when a NET FORCE acts on an
object
•
net force FORWARDS produces
ACCELERATION
•
net force BACKWARDS produces
DECELERATION
•
net force SIDEWAYS produces CHANGE OF
DIRECTION
•
the bigger the force the bigger the
acceleration
•
•
•
the sprinter slows down at the end of a race
there is a net force backwards
Module 2562 A.2.6
so the sprinter decelerates
Motion and Movement - Newton’s Laws
NEWTON’S THIRD LAW of MOTION
NEWTON’S THIRD LAW
•
this law is used when two bodies EXERT
FORCES ON ONE ANOTHER
•
ACTION AND REACTION ARE EQUAL and
OPPOSITE IN DIRECTION
•
action of jumper down on ground
=
reaction of ground up on jumper
the harder you push down on the ground,
the more the ground pushes up on you
•
•
this upward force on the jumper is the force
acting to cause the take off
Module 2562 A.2.7
Motion and Movement - The Effects of Force
The EFFECTS of FORCE
EFFECTS of FORCE
• force causes linear acceleration or deceleration
• including change of direction
• the point of action of a force affects what
happens
• friction acts at the feet of a sportsperson, not
enough of it and the person’s feet slip
• if a force acts through the person’s centre of mass
(CofM), then linear motion is caused
• if a force acts to one side of the CoM then
rotation is caused
• like take-off in the high jump, the reaction force
acts to one side of the CoM
Module 2562 A.2.8
CENTRE OF MASS
CENTRE of MASS (CoM)
• this is the single point in
a body which
represents all the
spread out mass of a
body
• the weight acts at the
CoM since gravity
acts on mass to
produce weight
WHERE IS THE CENTRE OF
MASS?
• position of centre of
mass depends on shape
of body
• this is how the high
jumper can have his
CoM pass under the bar
• but he could still clear
the bar
BALANCE and TOPPLING
BALANCE
• to keep on balance the
CoM must be over the
base of support
TOPPLING
• the CoM must be over the
base of support if a person is
to be on balance
• toppling would be caused by
the weight acting at the CoM
creating a moment about the
near edge of the base of
support
• this can be used by divers or
gymnasts to initiate a
controlled spinning (twisting)
fall and lead into somersaults
or twists
CENTRE OF MASS - GENERATION OF
ROTATION
FORCE ACTING AT TAKE-OFF THROUGH CoM
•
the line of action of a force on a jumper
before take-off determines whether or
not he rotates in the air after take off
•
if a force acts directly through the centre
of mass of an object, then linear
acceleration will occur (Newton's second
law), no turning or rotating
•
example :
– basketballer : force acts through CoM
therefore jumper does not rotate in
air
CENTRE OF MASS - GENERATION OF
ROTATION
FORCE ACTING AT TAKE-OFF NOT THROUGH
CoM
•
a force which acts eccentrically to the
centre of mass of a body will cause the
body to begin to rotate (will initiate
angular acceleration)
•
this is because the force will have a
moment about the CoM and will cause
turning
•
example :
– high jumper : force acts to one side
of CoM therefore jumper turns in air
STABILITY AND BASE OF
SUPPORT
Stability is defined as the ability to hold
or maintain a position in space.
3 key terms
Centre of Gravity (COG) the theoretical point
where all the body weight is concentrated or the
theoretical point about which the body weight is
evenly distributed.
Line of Gravity (LOG)– a straight line from the
COG to the floor
Base of Support (BOS) – The area in contact with
the floor.
There are four basic principles
underlying stability.
Principle #1
• “The closer the line of gravity is to the centre
of the base of support the greater the
probability of maintaining balance.”
Why is this body in a stable position?
• “The line of gravity from the centre of gravity
passes through the centre of the base of
support.”
What happens if we move the line of
gravity closer to edges of the base of
support?
• We become more unstable. The chance of
losing balance increases.
Principle #2
“The wider the base of support,
the greater the probability of
maintaining balance”
Consider wrestling. Is a wrestler more stable on all
fours or standing up? Why?
• This is because the size of the base of support is
larger so the center of gravity has further to
travel to get outside the edges of the base of
support so this becomes a stable position.
Principle #3
• “The probability of maintaining balance is increased when
the centre of gravity is lowered in relation to the base of
support.
This is because; ;.
As the centre of gravity is lowered, the distance the line of
gravity has to travel to reach the edges of the base of support is
greater than when the center of gravity is higher.
What other applications does this principle have in sports?
• In some sports we need to lower ourselves to improve
stability e,g. a rugby scrum packs in low to improve stability
Principle #4
“ The further one body part moves away from the line of gravity,
the probability of maintaining balance decreases unless another
body part moves to compensate for it.”
Consider the person doing the shot put.
What have they done to maintain balance in the throw?
As the shot putt and trunk moves back the right leg has moved out in the opposite
direction to compensate for this to keep balance.
This is because;
The line of gravity from the centre of gravity is kept above the base of support (foot on
ground is base of support).
QUESTIONS
• Explain the relationship between the 3
components – BOS, COG and LOG
• What is the effect of moving the COG outside
the body?
• What will happen if the LOG falls outside the
BOS?
class 1
effort in
muscle
pivot at
joint
E-P-L
class 2
JOINTS AS
LEVERS
load is force
applied
E-L-P
class 3
L-E-P
LEVERS
levers have an pivot (fulcrum), effort and load
•
and are a means of applying forces
at a distance from the source of
the force
CLASSIFICATION OF LEVERS
•
•
•
•
•
•
class 1 lever : pivot between effort
and load
see-saw lever found rarely in the body
example : triceps / elbow
class 2 lever : load between pivot
and effort
wheelbarrow lever, load bigger than
effort
example : calf muscle / ankle
•
•
•
class 3 lever : effort between pivot
and load
mechanical disadvantage, effort bigger
than load, most common system found
in body
example : quads / knee and biceps /
elbow
MOMENT OF INERTIA (MI)
• the equivalent of mass for rotating systems
• rotational inertia
• MI depends on the spread of mass away from the axis of spin, hence body
shape
• the more spread out the mass, the bigger the MI
• bodies with arms held out wide have large MI
• the further the mass is away from the axis of rotation increases the MI
dramatically
• sportspeople use this to control all spinning or turning movements
• pikes and tucks are good examples of use of MI, both reduce MI
CONSERVATION OF ANGULAR MOMENTUM
ANGULAR MOMENTUM (H)
angular momentum
= moment of inertia x angular velocity
= rotational inertia x rate of spin
H
=
Ix w
CONSERVATION of ANGULAR MOMENTUM
this is a law of the universe which says that angular momentum of a spinning body
remains the same (provided no external forces act)
a body which is spinning / twisting / tumbling will keep its value of H once the
movement has started
therefore if MI (I) changes by changing body shape
then w must also change to keep angular momentum (H) the same
if MI (I) increases (body spread out more) then w must decrease (rate of spin gets less)
•
•
strictly, this is only exactly true if the body has no contact with its
surroundings, as for example a high diver doing piked or tucked
somersaults in the air
but it is almost true for the spinning skater !
CONSERVATION OF ANGULAR MOMENTUM EXAMPLES
THE SPINNING SKATER
arms wide - MI large - spin slowly
arms narrow - MI small - spin quickly
THE TUMBLING GYMNAST
• body position open - MI large - spin
slowly
• body position tucked - MI small - spin
quickly
Force Summation
STUDENTS WILL HAVE AN
UNDERSTANDING OF THE 5
PRINCIPALS OF FORCE SUMMATION
FORCE SUMMATION
• When we are trying to generate as much
momentum as possible in activities such as
throwing, kicking and striking the amount of
momentum we can give to an object is
determined by the sum of all forces generated
by the different body parts. This is called force
summation.
• There are 5 basic guidelines for giving an
object as much momentum as possible.
#1 Using Body Segments
• We should look to use as many body segments
as possible when trying to give an object
maximum momentum. Why?
• Because we can maximise the muscular force
that each muscle group associated with each
segment can generate
#2 Stretch out
• Before we begin the sequence of movements,
such as the hitting, throwing or kicking action we
should stretch our muscles out to their optimal
length. Why?
• It allows the muscle to be contracted with
optimal force. It gets the blood flowing through
the muscle and helps warm the muscle and
therefore decreases the risk of injury.
#3 Sequencing of body segments
• Generally to give maximum momentum to an object in kicking,
throwing and hitting we move larger muscle groups first and the
smaller muscle groups closer to the object last.
• In effect we use the body like a giant whip.
• What are the benefits of this?
• The momentum generated by larger segments can be passed on to
smaller ones until we make contact/release etc.
• In a shot putt illustration, how do we see this principle being
applied?
• Look at the order of execution sequence
• i.e. legstrunkshoulderarmswristhandsfingers
shot
#4 Timing of Body Segments
• Generally to give maximum momentum to an
object in kicking, throwing and hitting we need to
make sure that the right body segment is adding to
the overall momentum at the right time.
• What could happen if the timing of the body
segments is “out of order”?
• Not only does it lack coordination but maximum
force generation is lost or lessened
• How does correct timing ensure maximum
momentum?
• It means we use those larger muscle groups first
and the smaller muscle groups last.
#5 Full range of motion
• Generally, to give maximum momentum to an
object in throwing, kicking or striking, we need to
move the segments through the greatest range of
motion as we possibly can.
• What are the benefits of this?
• The greater the range of motion the higher the
speed of the extremities on release/contact.
• How does correct timing ensure maximum
momentum?
• No speed or force is lost throughout the
entire movement.
• We can apply all this information to an
example of the javelin throw.
• Using your knowledge of generating
momentum, explain how the athlete
generates maximum momentum to the javelin
upon release.
ANSWER
• They use the large muscle groups of the legs and trunk to initiate
the force. This force passes onto the shoulder, arms and finally
hand at release. Forces are getting increasingly larger up to
release.
• The arm is fully extended at the shoulder prior to the throwing
action
• Timing is legstrunkshoulderarmshand
• The arm moves through its full range of movement to maximise
lever length and force summation