Transcript Lect 3 Force Production

Distance and displacement
Distance and displacement are quantities used to describe the extent of a
body's motion. Distance is the length of the path a body follows and
displacement is the length of a straight line joining the start and finish
points e.g. in a 400m race on a track the length of the path the athlete
follows (distance) is 400m but their displacement will be zero metres
(they finish where they start).
Speed and velocity
Speed and velocity describe the rate at which a body moves from one location to
another. These two terms are often thought, incorrectly, to be the same. Average
speed of a body is obtained by dividing the distance by the time taken where as
the average Velocity is obtained by dividing the displacement by the time taken
e.g. consider a swimmer in a 50m race in a 25m length pool who completes the
race in 60 seconds - distance is 50m and displacement is 0m (swimmer is back
where they started) so speed is 50/60= 0.83m/s and velocity is 0/60=0 m/s
Speed and Velocity = distance traveled ÷ time taken
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Acceleration
Acceleration is defined as the rate at which velocity changes with respect
to time.
average acceleration = (final velocity - initial velocity) ÷ elapsed time
From Newton's 2nd law:
Force = Mass x Acceleration
Acceleration = Force ÷ Mass
If the mass of a sprinter is 70kg and the force exerted on the starting
blocks is 700N then acceleration = 700 ÷ 70 = 10 msec²
Acceleration due to gravity
Whilst a body is in the air it is subject to a
downward acceleration, due to gravity, of
approximately 9.81m/s²
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Uniformly accelerated motion
When a body experiences the same acceleration throughout some interval
of time, its acceleration is said to be constant or uniform. In these
circumstances, the following equations apply:
Final velocity = initial velocity + (acceleration x time)
Distance = (initial velocity x time) + (½ x acceleration x time²)
Moment of force (torque)
The moment of force or torque is defined as the application of a force at a
perpendicular distance to a joint or point of rotation.
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Linear Kinetics
Kinetics is concerned with what causes a body to move the way it does.
Momentum, inertia, mass, weight and force
Momentum: mass x velocity
Inertia: the resistance to acceleration - reluctance of a body to change
whatever it is doing
Mass: the quantity of matter of which a body is composed of - not affected by
gravity - measured in kilograms (kg)
Weight: force due to gravity - is mass x gravity (9.81m/s²)
Force: a pushing a pulling action that causes a change of state (rest/motion)
of a body - is proportional to mass x acceleration - is measured in Newtons (N)
where 1N is the force that will produce an acceleration of 1 m/s² in a body of
1kg mass
The classification of forces, external or internal, depends on the definition of
the 'system'. In biomechanics, the body is seen as the 'system' so any force
exerted by one part of the system on another is known as an internal force all
other forces are external
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So weight is the force of gravity between two bodies, usually some
small object in contact with the earth.
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A cricket ball that has a mass of 0.2 kg has the same mass wherever it is ?
Look at 3 different scenarios:
1) In Bendigo ?
2) On top of Mount Everest ?
3) On outer space ?
Explain.
The weight of a cricket ball is the same wherever it is ?
Look at 3 different scenarios:
1) In Bendigo ?
2) On top of Mount Everest ?
3) On outerspace ?
Explain.
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Do a force acceleration example.
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Refer to the following information for the next six questions.
Static Equilibrium
Little Nellie Newton wishes to be a gymnast and hangs from a variety of
positions as shown. Since she is not accelerating, the net force on her is
zero. This means the upward pull of the rope(s) equals the downward pull
of gravity. She weighs 300 N.
Enter the scale reading, in newtons, for each case.
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In the example below,
the action-reaction pair
is shown by the arrows
(vectors), and the
action-reaction
described in words.
For the remaining situations, discuss
with your neighbor the direction of the
"action" vectors.
In the first form provided, verbally state
the "reaction" to the given "action".
In the second form provided, state the
direction of the "reaction" vector.
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Force Production
Mass, Weight
Types of Force
Force Summation
Action Reaction
Inertia
COG
Force, Gravity
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Forces
• Internal forces – produced by the muscles
• External forces – gravity, air resistance
etc.
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Forces
“something that causes an object to be
deformed or moved.”
(Roberts & Falkenburg, 1992)
Force can:
• Get objects moving
• Stop objects moving
• Change the direction of a moving object
• Change the speed of a moving object
• Balance another force to keep an object still
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External Forces Worksheet
• Get objects moving
Gravity
Friction
Air resistance
Ground reaction force
Point of application
Direction
Resultant action
• Stop objects moving
• Change the direction of a
moving object
• Change the speed of a moving
object
• Balance another force to keep
an object still
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Summation of Joint Forces
In whole body sports skills:
• Using (recruiting) joints in the order, big to
small, will make objects move faster.
(acceleration).
• Using (recruiting) joints in the order, small to
big, will usually result in deceleration.
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Implications for the Coach
There are two practical principles that apply specifically to running,
jumping and throwing where the athlete is concerned with creating
optimal force and speed:
● Use all the joints that can be used
● Use every joint in order
Use All the Joints That Can be Used
The forces from each joint must be combined to produce the
maximum effect. This is best done when all joints that can be used are
used. This will help to get the most speed or acceleration out of a
movement.
In the shot put, for example, the knee, hip, shoulder, elbow, wrist
and finger joints should all be used to exert the greatest force on the
shot. Beginners frequently miss out early joint movements such as the
knee or hip action, or fail to complete a movement fully by not using
the wrist or fingers.
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Use Every Joint in Order
When several joints are used in a skill, their sequence and timing are
important. This principle tells us when the joints should be used. Movement
should begin with the big muscle groups and move out through the
progressively smaller muscles, from big to small. This pattern produces
optimal forces and flowing, continuous movement.
The continuous, flowing movement produces a summation of forces,
forces adding together. The force generated by one part of the body is built
on by the force of subsequent joints. In the well timed shot put, the hip
action commences just as the leg extension decelerates. The shoulder
action commences as the hip rotation decelerates and so on.
The release velocity of an implement depends on the speed of the last
part of the body at release. The correct sequence and timing allow the
athlete to attain maximal release velocity.
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Force Summation
•
•
Combination of forces to produce a maximal force
2 types
– Simultaneously: explosive action of all body parts occurs at the same
time
• Eg: high jump take-off, vertical jump for rebound in basketball
– Sequentially: body parts are moved in sequence to generate a great
force
• Eg: throwing, striking, kicking
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• What are the 4 parts to a sequential
force summation technique (page
54)
• Write down (or draw) the steps in
force summation for a throw from
the outfield in softball
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Homework Task
•
Answer the following questions
– 1. Pick one benefit of biomechanics for a sport of your choice
and discuss in detail
– 2. For the same sport analyse and discuss in detail one form of
equipment that has used biomechanics
– 3. Describe the sequence of body parts in the force summation
for a tennis serve
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Locating Centre of Gravity
Single segmental objects have their Centre of
Gravity directly above the base of support.
Sometimes the Centre of Gravity can be
found outside the body. (Arc)
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Gravity: A Constant Force
• The Earth’s rotation produces a force
called gravity.
• Gravity provides a constant force on
matter, which is commonly understood as
weight.
• Weight is mass multiplied by gravity.
• Gravity causes objects to travel toward
the earth at a constant rate of
acceleration. 10m/sec2
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Human Body Centre of Gravity
• Standing still – centre of gravity is located
in the abdominal cavity, about 6 inches
above the pubis symphysis.
• As your position changes – so does your
centre of gravity.
• The position of the centre of gravity will
determine whether the body is in balance.
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Definition
The centre of gravity is defined as the point
around which a body’s weight is equally
balanced in all directions.
(Hall, 1999)
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Centre of Gravity, Stability &
Balance
Stability and balance will be easier if:
• The mass is large
• The base is large
• The centre of gravity is low
• The centre of gravity is located over the base
Headstand
Handstand
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Centre of Gravity
• When the force of gravity acts on a body,
it acts through the centre of gravity and
always moves towards the centre of the
earth.
• Symmetrical objects like balls and cubes
have their CoG in the exact centre of the
object.
• Objects are 3 dimensional, so the CoG
will be at the point where the axes of all 3
planes meet.
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Inertia - Newton’s First Law
Newton’s first law of motion states that:
“a body will continue in a state of rest or in a
straight line of uniform motion unless acted
on by an external force.”
(Roberts & Falkenburg, 1992)
• Inertia is the resistance of an object to
change.
• An object at rest will remain at rest unless
acted upon by a outside force.
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Center of Gravity
Gravity is a force which is always present and is a pulling force in the direction of the centre of the earth. This
force acts on every body through an imaginary point called the centre of gravity (GG). A solid object like a shot or
discus has its CG in the centre and this is a fixed point.
The human body is a complex and constantly changing shape. The centre of gravity now moves according to
the positioning of the body and limbs. The CG may be inside the body, for example, when standing or it may be
outside the body as in the pole vault and flop high jump bar clearances.
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Types of Forces
Force without motion – isometric force
Force with motion – isotonic force
Sub-maximal force
Maximal force
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