Transcript Biomechanics Summary

Biomechanics
Chapter overview
What is biomechanics?
Motion
Balance and stability
Force
Projectile motion
Fluid mechanics
Now that you’ve finished … answers
page 73
page 74
page 78
page 85
page 103
page 109
What is biomechanics?
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An overview
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Biomechanics is the study of the body as a machine.
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Biomechanics can be used to:
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Refine technique
Prevent injury
Develop equipment
Correct errors
Whilst many biomechanical principles are inherent in each sport
skill, it is important to discern the most influential principles in
your sport in order to look to optimising technique.
Motion
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Motion
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To initiate motion, one must overcome inertia.
Once in motion, factors such as velocity and acceleration
become influential.
Motion can be curvilinear or rectilinear
Curvilinear motion
Rectilinear motion
Motion
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Factors such as velocity and acceleration are variable amongst
athletes and can determine the effectiveness of one’s
performance.
Newton’s laws of motion
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Newton’s first law of motion states that a still object continues to
be still, and a moving object continues to move at its current
velocity, unless an external force acts on the object.
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His second law states that the sum of the force that moves an
object is equal to the object’s mass multiplied by the
acceleration. This law can be expressed as the following
equation:
Force = mass × acceleration
(F = ma)
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Newton’s law of action reaction states that for every action there
is an equal and opposite reaction.
Balance and stability
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Balance and stability

Every athlete must be able to maintain stability and balance.
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Failure to do so can result in loss of force and or accuracy.
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Balance can be
dynamic or static.
Personal reflection
Do you find balance easier
when you are moving or
stationary? How does this
affect your performance in
your current physical
activity?

Factors affecting balance include base of support, centre of
gravity and height of gravity.
Personal reflection
Have you ever done
the stork stand
balance test? Did you
find it challenging?
Centre of gravity
Personal reflection
Think of a time when you
were performing a dynamic
movement. When your
centre of gravity falls
outside of your body, what
is your natural movement
to try to correct it?
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The height of the centre of gravity can have a significant affect
on your stability.
Personal reflection
Do you think your
height has an effect
on your stability?
Force
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Force
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Force is anything that causes or has the potential to cause the
movement, diversion or slowing of the object on which it acts.
All forces have four common properties. They have:
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magnitude (an amount, or how much is
applied)
direction (the angle at which the force is
applied)
a point of application (the specific point at
which the force is applied)
a line of action (represented by a straight
line through the point of application in the
direction that the force is acting).
Integration
Consider the sport you are
currently studying.
Describe the forces
required in terms of the
four properties listed on
the previous slide.
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Force production: Most sports require an athlete to be able to
both generate and control forces.
Integration
In your sport, can you
differentiate between the
skills that require maximum
force and those that require
control and accuracy?
Summation of forces
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To obtain maximum force, it is necessary to combine or add up
the forces applied by different body parts. This concept is known
as the summation of forces.
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The summation of force is influenced by the:
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number of body parts used in the movement
order and timing of their involvement
force and velocity generated
way in which the body and body parts are stabilised and
balanced.
Summation of forces
The long jumper combines the
forces in his legs, abdominals and
arms to propel himself further into
the pit.
Personal reflection
Choose one physical skill
from your current sport.
Which body parts do you
use to produce maximum
force? In what order?
Momentum
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Whether it is a sprinter running along a track or a bowling ball
rolling down an alley, any object or person has a certain mass
and a certain velocity. The product of these is known as the
momentum.
Personal reflection
When you sprint, how many
steps does it take you to come
to a complete stop? How does
this compare with your
classmates or competitors?
Why?
In most sports, mass is
constant, so velocity
becomes the main factor
influencing momentum. So, to
increase momentum, simply
increase velocity.
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In many sports, it is necessary for momentum to be transferred
to another object or body part. The greater the momentum an
object has, the greater its effect on other objects it collides with.
Personal reflection
Have you ever played pool?
Why is that when the white
ball strikes another ball
straight on, the white ball
stops moving while the ball
that was struck moves
forward?
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Stabilisation must occur for effective transfer of gained
momentum
For example, in tennis it is common for athletes to brace (tense
up) their muscles just before the impact of a forehand. Stopping
the rotation of the body by bracing causes a whip-like effect on
the arm
Accuracy
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It is important in many sports to be accurate in the production of
sport in both direction and magnitude.
Application
In the following sporting
situations, consider ways in which
the performer can increase their
directional accuracy.
1. Batting into space in softball
2. Digging to a setter in volleyball
3. Kicking a penalty goal in soccer
Rotary forces
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Not all forces are produced in straight lines.
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Concentric forces are applied through the centre of an object and cause
motion in a straight line
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Eccentric forces are applied
off-centre and cause rotation
in the object to which the force
is applied.
Practical example
Place your textbook on the
desk in front of you. Push the
book away from you with two
fingers, applying the force
through the centre of the
book. Now repeat this,
applying the force to one side
of centre. What do you
notice?
Rotation about an axis (torque)
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More torque, and a greater rotation or twisting movement, will be
generated when more force is applied further from the centre of
an object.
Rotational momentum
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A spinning object will have momentum that will allow it to
continue to spin until an external force is applied.
Practical example
Sit on a swivel chair and clear a
space around the chair. Ask a
friend to gently spin the chair in
any direction. You can control
the speed at which you spin by
tucking your legs into your body
to speed up and extending them
out to slow down.
The velocity of the
spin is affected by
the distribution of
the weight from the
axis of the object.
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When spinning on the ice, skaters can control the rate of spin by
moving their limbs closer to the axis of rotation (to rotate faster)
or extending their arms and legs (to rotate more slowly).
Centrifugal and centripetal forces
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Centripetal force is the force that
causes rotating objects to move
towards the centre, or axis, of
rotation.
Centrifugal force is the force that
causes rotating objects to move
away from the centre, or axis, of
rotation.
Levers
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Three types:
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First class - fulcrum between the force and
the resistance e.g. oar on row boat.
Second class - resistance between the force
and the fulcrum
Third class - force between the resistance
and the fulcrum e.g. baseball batter.
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Third class levers are the most common in sport.
Sports using bats or racquets all use third class levers.
The velocity of a lever is fastest at the point furthest from the
fulcrum.
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By increasing the length of
the bat, the velocity is
increased.
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It is important to consider strength and accuracy when looking at
levers.
It is not ideal to use a 4m long bat/racquet even if a greater
velocity could to produced.
Maximising leverage
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Due to inertia, a shorter lever is easier to move than a longer
one.
To maximise the efficiency of a lever, it is often better to begin a
lever’s motion by shortening the lever.
This can be achieved by bending the relevant joint.
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E.g. a soccer playing kicking a ball
starts with a bent knee, a javelin
thrower bends the elbow to bring the
javelin through and a tennis server
bends their elbow before extending to
hit the ball.
The ‘sweet spot’
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The sweet spot is the point that, when struck, causes no
backwards nor forwards rotation of the bat.
Personal reflection
Have you ever felt the
effects of hitting a ball
outside the sweet
spot?
Projectile motion
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Projectile motion
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Factors that influence flight
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Height of release
Angle of release
Speed of release
Gravity and air resistance
Angle of release
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The optimal trajectory of a projectile is just less than 45 degrees.
Height of release
A projectile thrown from a height will travel further.
Personal reflection
Do you think you have a
height advantage when
throwing?
The effects of spinning
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The Magnus effect explains how a spinning projectile, such as a
ball, curves in flight.
Fluid mechanics
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Fluid mechanics
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Fluid mechanics refers to forces that operate in both the water
and in air.
Buoyancy
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An object’s buoyancy determines whether or not it will float.
Buoyancy is influenced by density.
Density is a measure of mass in relation to volume.
The high density muscle
and bone in our legs
cause them to sink when
in the water whilst the low
density air and fat in our
chests keep them afloat.
Personal Reflection
Have you ever tried
floating on your back in
water? What happens
when you allow your
body to relax?
Centre of buoyancy
The centre of buoyancy of a floating object is different to its
centre of gravity.
 In swimmers, the centre of
buoyancy can change depending
on the position and movement of a
swimmer’s body, particularly the
legs, and how much of the body is
submerged.
 A swimmer’s centre of buoyancy is
closer to their heads due to the
lower density of the chest and core
compared to the legs.
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Propulsion
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Swimmers are required to both push and pull themselves
through the water.
In this pushing and pulling action the forces of lift and drag are at
work.
Lift
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Lift force occurs due to the factors involved in Bernoulli’s
Principle.
The difference in velocity of the water travelling over the surface
of the hands creates a pressure difference that, when applied
correctly, creates lift.
Drag
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Drag force opposes the forward movement of a swimmer,
providing resistance.
Swimmers can minimise this effect of drag by streamlining their
bodies and pitching the entry of their hands to ensure the water
surface is cut cleanly.
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Drag can also assist a swimmer. Lift
and drag can work together to create
propulsion.
Water resistance
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Skin resistance is the resistance created by water ‘gripping’ the
skin as is passes through.
Turbulence resistance is created due to the high pressure
created at the front of the swimmer interacting with the low
pressure formed at the rear of the swimmer, creating a suction
effect.
To minimise turbulence and
resistance, good swimmers
streamline their bodies in the
water.
Wave resistance is caused
by the wall of water that
builds up at the front of a
swimmer, blocking their
path.
Now that you have finished …
Answers
1. Define biomechanics.
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Biomechanics is the study of the body as a machine.
2. How might the science of biomechanics have influenced the
sport you are currently studying?
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Depending on the sport being studied, look for specific
techniques that utilise biomechanical principles such as the
sequential summing of forces used in a discus thrower to
maximise force produced. There may also be specific design
elements in the clothing or equipment used in the sport; e.g., the
LZR Racer suit in swimming.
3. Describe the difference between linear (rectilinear) and
curvilinear motion.
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Linear motion involves a body or object moving in a straight line
whilst curvilinear motion involves a body or object moving along
a curved path.
4. Explain Newton’s three laws of motion. How might they be used to explain the
action of a long jumper from the start of the run-up to landing in the pit?
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Newton’s first law (inertia) states that a body will remain at rest or in motion in
a straight line unless acted upon by another force. The second law
(acceleration) states that the force required to accelerate a body at a given
rate is equal to the product of that acceleration and the mass of the body. The
third law (action and reaction) states that for every action there is an equal and
opposite reaction.
In long jump the athlete uses action and reaction (third law) to begins the runup by pushing down and backwards on the ground to make the athlete’s foot
react by moving forwards and upwards. The rate of acceleration will be
determined by the force applied and the mass of the body (second law). Inertia
(first law) will keep the body moving forward in a straight line to the pit and will
ensure the body does not stop unless the athlete’s muscles to brake the run or
until the jumper hits the sand in the pit which stop forward movement through
friction. Action and reaction (third law) is also important at the point of take off
where the athlete will hit the board and attempt to apply as much downwards
and backwards force as possible to gain the maximum of forward and upwards
motion.
5. Give three examples in athletics of action and reaction forces.
1.
2.
3.
A runner in the starting blocks will apply as much force
backwards into the blocks when the gun goes to gain as much
forward motion as possible.
In high jump, the jumper will apply action and reaction as they
take off to clear the bar. The more downwards force they apply
with their foot to the ground, the harder the ground will push
back on them to propel them in the air.
In discus, the greater the forward force applied by the hand to
the discus, the more force the discus will apply back onto the
hand when released.
6. Using Newton’s second law, answer the following question. If a 15 Newton
force is applied to a 1.5 kilogram discus, what is the acceleration of the discus?
Force = Mass x Acceleration
15N = 1.5kg x Acceleration
Ac = 15/1.5
Ac = 10m/s/s
7. Discuss how a gymnast’s centre of gravity may change
throughout a routine.
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When a gymnast is upright their centre of gravity is situated
somewhere towards the centre of the midsection. When they
rotate part of their body forwards, their centre of gravity shifts
away from the midsection in the direction the body has moved. At
some points, the centre of gravity will shift entirely outside of the
gymnast’s body.
8. Outline three ways an athlete can maximise force production.
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An athlete can maximise force production by ensuring they use
summation of force to obtain the greatest amount of force
sequentially from all muscles. They can also increase their
momentum by increasing the speed or power in their run up and
finally by applying the force for a longer period of time where
possible to maximise impulse gained.
9. Outline three ways the body can absorb force.
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When landing from a jump, the ankles, knees and hips are
designed to flex to absorbed the force over a greater length of
time and thus reducing impact.
When catching a ball, moving the hands in the direction of the
incoming ball as it makes contact, will allow the ball to absorb
better and reduce the rebound effect.
Use of larger body parts will also maximise force absorption; e.g.,
catching with two whole hands rather than just fingers on one
hand.
10. Explain the difference between concentric and eccentric force.
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Concentric force is force applied through the centre of an object
resulting in movement in a straight path. Eccentric force on the
other hand, is applied off-centre and causes the object to rotate
and move in a curved path.
11. List the two factors that determine rotational momentum.
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The two factors that determine rotational momentum are mass
and velocity.
12. Identify the principle that causes a rugby union goal-kicker’s left arm to rotate
forward, across the body when their right foot kicks the ball through the uprights.
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The principle is rotational action and reaction.
This response occurs in order for the body to maintain its
equilibrium. When the rugby player’s leg swings through (action
force), the arm’s response (reaction force) prevents the body
from toppling over.
13. Explain the term ‘sweet spot’.
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The sweet spot is referred to as the point on the equipment
where it is ideal to strike the ball to maximum force and accuracy.
In this spot, the bat will not rotate forwards or backwards and
there will be no vibrating or jarring sensation felt by the striker.
14. Identify five factors that affect the trajectory of a thrown ball.
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Factors affecting the trajectory of a thrown ball include the angle
of release, height of release, speed of release, spin applied and
wind resistance.
15. Explain how you would find a person’s centre of buoyancy.
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The centre of buoyancy is found at the geometric centre of the
displaced volume.
16. Describe the types of resistance that exist in fluid
environments.
The types of resistance that exist in fluid environments include
skin resistance, turbulence resistance and wave resistance.
 Skin resistance is caused by the fluid gripping the skin as it
passes through the water.
 Turbulence resistance is caused by creating areas of high
pressure in front of the moving swimmer and low pressure
behind. As water flows from the high pressure area to the low, it
sucks the swimmer back.
 Wave resistance is caused by the swimmer making a wave or
wall in front of them, blocking their path when they swim.