Download 2.2 Biomechanics - Force - NCEA-Physical

Biomechanics
Principles of
Force
Newton’s first law - Inertia
• An object at rest tends to remain at rest unless acted upon
by some external force
• An object in motion tends to remain in motion and to travel
in a straight line with constant velocity unless acted upon by
an external force
• Definitions:
Object – this may be anything from an item such as a
volleyball to a human body
Force – a push or a pull (measured in newtons)
Mass – the amount of substance in a body (measured in kgs)
External – from outside the object
Forces which:
• Speed up objects
implements for hitting, such
as racquets and body parts
that apply force
• Slow objects down
Ground and air friction,
gravity and contact with
other players
Overcoming inertia
• For movement to happen, the force applied must be enough
to overcome inertia.
• The bigger an object’s mass and the bigger any resisting
force (eg. friction) the bigger the force needed to move it.
Applying force to a moving object
• When a new force is applied to an object that is moving, the
direction in which the object moves may be changed.
• For the direction of the object to change completely to the
direction of the new force applied, the new force must be
much greater than the original force. Otherwise the
movement becomes a deflection.
Inertia and the Volleyball Serve
The ball is tossed up into the air…
Direction of Force
In order for the ball to
be tossed up we need to
move the arm (overcome
the inertia of the arm)
and apply a force to the
volleyball (overcome the
inertia of the volleyball)
so that the ball is
tossed up into the air.
The force is produced
by contraction of the
arm muscles: Biceps and
Deltoids
The ball is struck to go over the net
Point of
Application
Direction of Force
The ball is currently going straight up.
In order for it to go over the net we
need to change the direction of the
ball. To do this a second force is
applied (using the hand). The greater
the force applied the greater the
change in direction.
Newton’s third law –
action / reaction
• For every action there is an equal an opposite reaction
• By applying a force backwards on the ground, the ground pushes
the runner forwards
DIRECTION OF MOVEMENT
(REACTION
FORCE
(ACTION)
• The swimmer applies a force backwards on
the water and the water pushes the
swimmer forwards
DIRECTION OF MOVEMENT
(REACTION)
FORCE (ACTION)
MOVEMENT
(REACTION)
FORCE (ACTION)
Projectile Motion
• What is a projectile?
Any body released into the air
is a projectile.
• Once it is in the air a
projectile can gain no extra
propulsion.
• In the air it comes under the
influence of gravity and air
resistance.
• A body can be released into the
air by either –
Throwing – such as a Discus
Striking – such as a tennis ball
Projection of the body itself –
such as in High jump
Vertical and Horizontal Components of
Projectile Motion
• Vertical
If you throw a ball straight
up in the air its motion is only
vertical. The force of gravity
acts on the ball to stop its
upward movement and pull it
back to earth.
• Horizontal
A throw from the boundary to
the wicket keeper in cricket
has a horizontal as well as
vertical component ie. it
moves along as well as up. Air
resistance slows the ball down
and gravity pulls it back to
earth.
Factors affecting flight:
• Speed of release - The
greater the speed of
release, the greater the
horizontal distance
• Height of release – the
greater the height of
release, the greater the
horizontal distance
• Angle of release – There is
an optimum angle of release
for each object, but that
will depend on a number of
factors including
aerodynamics of the object
and height of release
Trajectory – flight path of
projectiles.
All projectiles have a
parabolic flight path
Angle of Release – requirements for different activities
•
•
•
•
•
•
Activity
Tennis Serve
Volleyball float serve
Long jump
Discus
High jump
Standing back
somersault
•
•
•
•
•
Angle
-3 to
13 to
17 to
35 to
40 to
• 75
(degrees)
15
20
22
39
50
What are the different objectives for our projectiles?
And, what do we need to do to maximise our chances of
success?
• Height such as high jump, pole vault
Maximum possible speed of release
Maximum possible angle of release
• Distance such as discus, long jump
Maximum possible speed of release
Correct angle of release for activity
Application of spin in some cases
• Speed such as rugby pass
Maximum possible speed of release
Lowest possible angle of release
• Accuracy such as archery, netball
FORCE SUMMATION
• Any desired movement is a combination of a number of
forces. A sequence of movement is used to produce optimal
velocity (best speed).
• Force Summation = the sum of all forces.
• We know that when we want to throw a ball a long distance
we cannot just rely on the strength of our arm alone. We
include our torso (upper body) and legs, which have the
strongest muscles in the body, as well.
• The greater the force required the more body parts must be
used.
• Timing of the movement is
essential and follows a unique
pattern. Each segment should
be moved at the instant the
previous segment begins to slow
down. Correct sequencing is
especially important when
maximal force is desired.
• In humans, the slowest,
strongest and heaviest parts
(with the most inertia) are
moved first, followed by the
next strongest/heaviest ie. the
torso and thighs, followed by
the shoulder, arm and then the
fingers which are the fastest,
weakest and lightest part.
• Use the Javelin as an example. Can you
identify the first, second, etc…body
parts to start moving?
• Speed of release is important as the faster the speed of
release the greater the force imparted (given) to the
object/body. That is why in most throwing events you see the
performer building up faster and faster until the desired
speed is hit for the release.
• In addition to this the longer you are in contact with the
object the longer the forces have to impart to that object.
That is why in Javelin we take a run up, large step and then
pull the arm through from an outstretched position.
• To gain maximal force
summation you need
stabilisation of body parts.
For example: a stable lead
leg in the volley ball serveopposite to the arm
executing the serve.
Stabilised Leg
Levers
• A lever is a length of solid material (eg a bone)
that is used to apply force to another object.
• Within the lever system there are 3 parts:
Fulcrum – a pivot point
Load (resistance) – weight that needs to be moved
Effort – the force applied to move the load
First Class Lever System
• This is perhaps the most easily understood lever
system. The load and effort are applied either
side of a central fulcrum. The best example of
common use in training is probably the ‘tricep
curl’
L
F
E
The Second Class Lever System
• The resistance as in the loaded barrow is
between the fulcrum of the wheel and the
gardener’s effort at the handles.
• There are few obvious such levers in the
body. The closest accepted example is a
heel raise.
• In a heel raise the principle effort is from
the muscles of the lower leg and one would
expect the centre of mass to be in front of
this. The fulcrum near the ends of the toe
joint would be outside the centre of mass.
The Third Class Lever System
• The effort is applied between the resistance and the
fulcrum
• Most of the lever systems in the body are third class,
they are not particularly good at producing force but
are rather designed to operate at speed.
Force Arm & Lever Length
• The longer the lever, the more effective
it is in producing velocity (Force/Time)
• A Longer lever travels a greater
distance, when moving the same amount
of degrees
• “Merry-Go-Round/Roundabout example
Force Arm & Lever Length
• Tennis players can
hit a tennis ball
harder with a
straight arm drive
rather than with a
bent elbow because
the lever (including
the racket) is
longer and moves at
a faster speed
Force Arm & Lever Length
• In baseball, hockey, golf, field hockey,
and other sports, long levers produce
more shear force and thus better
performance
• For quickness of movement, it is
sometimes desirable to have a short
lever arm,
– Catcher shortens the windup,
by only bringing the ball next to the
ear
to make a quick throw to second

What class of
lever is this?
LOAD
EFFORT
FULCRUM