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WE 37
----------------1-------1­____
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OBJECTIVES
1. To identify a.nd locate the
major parts of the skeleton.
and the major bones of the
body.
2. To describe the function and
significance of the type of
joints of the body.
3. To describe the function and
identify the location of the
major skeletal muscles of the
body used in movement.
4. To explain the significance of
the center of gravity in
relation to human
movement.
5. To explain how stability is
effected and controlled by
moving the center of graVity.
6. To explain how the body
uses its system of levers for
mechanical advantage.
7. To explain how Newton's
Laws of Motion effect human
movement.
8. To explain how the laws of
projectiles effect human
movement and activity.
KEY WORDS AND TERMS
•
Axial
a Appendicular
a Scapula
!! Clavicle
Humerus
JlI Ulna
Radius
iJl Carpals
:i Metacarpal
:I Phalanges
Pelvis
Femur
Tibia
Fibula
Tarsals
Meun:arsaIs" .'
Joint
Pivot Joint
•
•
•
•
•
•
•
•
•
•
BaIl and Socket Joint
• Hinge
• SaddleJoint
• EllipsoidJoint
Joint
• Condylar Joint
• Center
GraVity
• Base ofofSupport
•
•• Levers
Bar
• Rigid
Fulcrum
• Force
• Resistance
• Force Ann
•
Ann
• Resistance
Isl-2nd-3rd Class Levers
• Projectiles
• Projection Angle
•
·
ongymnast
somersaults in the
A dancer leaps and lands
his or her feet. These movements. which we often take
A
for granted. require strong bones and muscles. as well as a
air.
trained body. Actions that seem simple. such as walking.
bending and jumping. all depend on the coordination of the
---sk-eletal-afld-mtlseular-syst~ms.
The principles that govern movement apply to the human
body. Learning how to apply these principles to movement
skills contributes to the successful participation in all phys­
ical aCtivifte$.l~oWle(ilgeofthese principles allows us to in­
telligently !elect oPr Ihovement patterns and skills and to
efficiently use oui 'body. Understanding the Principles of
Movement lays a foundation for peIfecting the functions of
our human body.
Like the framework of a building. your skeleton supports
your body. However. more than just bones make up your skel­
eton~ It also consists of cartilage and ligaments. that bind one
bone to another as in the knee joint. The skeleton also con­
sists of tendons. which join muscle to bone like the shoulder
joint.
Structure of the Body
Skeletal System
The human skeleton is made up of 206 bones and is divided
into two major parts. the axial skeleton and the appendicular
skeleton. The axial skeleton consists of the skull. spinal
column. sternum. and ribs. The appendicular skeleton con':
sists of the bones of the upper and lower extremities. The
scapula, clavicle, humerus, ulna. radius. carpal bones. meta­
carpals and phalanges are bones found in the upper extrem­
ities. The pelvis. femur. tibia. fibula, tarsal bones, metatarsals
and phalanges are bones located in the lower extremities.
The longest bone in the human body is the femur (thigh
bone) and it is' normally 27% of our total height. The femur is
the strongest bone in the human body. Measured ounce for
ounce. it can withstand greater pressure and support a greater
weight than a steel rod of the same size.
When two or more bones form a connection. the resulting
articulation is called a joint. The purpose of most joints is to
provide movement of the bones of the skeleton. The structure
and function of the joints of the body are determined by the
relationship between the shape of the joint and the move­
ments that it permits. Joints of the body operate in much the
same way that railroad tracks determine the route available
to the train by allowing movement as long as it follows the
route ofthe track. Your body has many types ofjoints and most
kinds occur in more than one part of the body.
Skull - - - - - !
--~---"----~~-
Humerus-....
Ribs
Carpal bones
Ulna _ _...:1
/
".
-i
Metacarpals
~--Femur
-Phalanges
Skeletal System
A pivotjOint allows your head toturn from side to side. This
movement allows you to take in broad scenes without turning
your entire body. This allows you to keep your eyes on a ball
when your body is turned away.
A ball and socketjoint in your shoulder and hip areas pro­
vides movement in nearly all directions. No other kind ofjoint
lets you move in so many different ways. This is the primary
joint in throwing and kicking.
A hinge joint allows your arms to flex and to extend. This
motion makes it possible to reach out to an object, to pull it
inward and to even lift it using your elbows.
A saddle joint lets you lean forward and backward at the
ankles. In your thumbs. the saddle JOint gives you a tight grtp
on small objects such as a bat. racquet. or rope.
An ellipsoid joint lets your WIists pivot up and down and
sideways. They aid your hands in perlonmng a vartety of skills
. w.
---­
----------------~-__JHk_e_hit_ting-a-nd-throwing.---
A condylar joint lets your knees bend. extend and rotate
slightly. This action allows you to sit down. to stand up and
to walk smoothly as in the squatting position of the catcher.
Try to moye the pages of this book without bending your
fingers or try walking without bending your knees:Hard. isn't
it? Joints solve such movement problems for you.
,
....
,
I
"
1.
\
.
--Ball and
Socket
. Hinge
Ellipsoid
Pivot
Joint
Joints found in human body
_~ .....
-
Saddle
Condylar
Muscular System
Muscles are the engines of your body. They are heavy and
make up about 42% of a man's body weight and about 36%
of a woman's body weight. Leg muscles such as the quadri­
-----ceps,_hams.1I:ings ancLgastrocnemJus help you run. Chest
muscles (pectoralis major and minor) help you breathe. Your
heart. the busiest of all the muscles. beats 40-70 million times
a year to keep you alive by pumping blood through your body.
Skeletal muscles are responsible for the movement and po­
sitioning of the bony segments of the body. Skeletal muscles
consist of bundles of muscle fibers resembling strands of wire
within a cable. Muscle fibers vary greatly in length. Some ~
be over 1112 inches long. while others are less than .04 inches
in length.
Skeletal muscles pull on the bones. thus causing move­
ment. They never work by pushing. Movement occurs when
a muscle shgrtens as in the pullup. This is known as a con­
centric contraction. It takes between two and four thou­
sandths of a second for an impulse (electrical. mechanical,
chemical, thennal) to stimulate the muscle fiber to contract.
----
-----~--------- ----- ------~
Bicep
Latisimus dorsi
Quadriceps
Gastrocnemius
Soleus
Muscles of the Human Body
{.o I
Concentric contraction
Muscle shortens
Eccentric contraction
Muscle lengthens
Movement can occur by the lengthening of the muscle as in
loweling yourself from a pullup. This is known as an eccen~
tric contraction. "Voluntary" muscles contract when the brain
sends appropriate signals via the nervous system in all con­
scious movement. "Involuntary" muscles contract automat­
ically to assist in bodily functions such as circulation and
digestion, and the heart beat.
Skeletal muscles help a person swing a tennis racket and
a baseball bat, to run a mile and swim across a swimming
pool and to shoot baskets and bump a volleyball. There are
over 3 times as many muscles in the adult body as there are
bones; approximately 656 muscles total. Most often. muscles
work in pairs. As an example, when the biceps muscle in your
upper ann contract, it pulls your forearm upward. The triceps
which are on the opposite side of biceps relax, thus allowing
for the movement to occur. To lower your ann, the triceps con­
tract and the biceps relax.
Motion and Factors
Affecting Motion
Motion is a movement produced by a force that is either a push
or a pull. The amount of motion depends upon the amount of
force in relationship to the amount of resistance. In order to
lift a box, you must be strong enough to lift the weight (resis­
tance) of the box. In the human body. muscular contractions
produce the movements that put the body into motion. Gravity
is a natural force that often resists motion.
Effects of Gravity
The natural force in the universe that pulls everything to­
wards the center of the earth is known as graVity. It acts on
all bodies by pulling through the weight center of the object.
The weight center or balance point is known as the center of
gravity. The location of this point in any object remains fixed
as long as the body shape does not change. In bodies of uni­
The center oj gravtty is
form size and shape. the center of gravity is at the geometric
constantly changing in the
center. When· the size and shape vary. the center of gravity
body.
shifts toward the more weighted section. Finally. when the
shape or position of an object changes. the location of the
_.- -eenter-of---gmvity-a1so-moves.--As...y-OllLpOsitiOnmDYeSJJ:.QID_a_
standing position to a squatting position. the center of gravity
adjUSts in proportion.
, Since the human body is capable of many positions and
generally uses the arms and legs. the location of the center of
gravity is constantly changing. The location of the center of
gravity depends on the arrangement of the body segments at
any given time. When an arm or leg is moved. when the trunk
is flexed. or when a weight is added to the body, the location
of the center ofgravity changes according to the direction, dis­
~ ..
tance, and amount of weight that is shifted or added.
The location of the center of gravity in a human being varies
according to-body build, age, sex, and the posture of the body.
Simply by standing or sitting, by raising your arms over your
head, or by flexing your stomach changes the posture position
and causes the center of gravity to shift for each position. As
a reference point, for the average person in the standing po­
sition, the location of the center of gravity is at about the area
of the navel.
_.
Sta bility and Balance
The ability of a person to maintain balance in movement is
recognized as a fundamental skill. Because we generally hold
ourselves in an upright position and because the law of gravity.
constantly pulls us toward earth, problems with stability are
ever present. A body is only stable when the center of gravity
is directly over its base of support. A base of support is formed
by the connection of body parts that are in contact with the
ground including all the area within the supporting parts. For
example, if you are standing with your feet shoulder width
apart. the base of support is that area that extends from toe
to toe and heel to heel. Either consciously but mostly uncon­ SCiously, we spend most of the time we are awake adjusting
the center of gravity to maintain balance.
Several "principles" are present when examining stability:
1) The larger the base oj support. the more stable the object
or person. For example, standing on two feet is more stable
than standing on one foot. 2) The closer that the line of gravity
falls at the center of the base of support, the more stable the
body. The line of gravity is that line that falls perpendicular
to the earth's surface and goes from the center of gravity to
the base of support. If that line falls within the base of sup­ port, balance is maintained. 3) When an external weight such
as a book bag, grocery bag or suitcase is added anywhere to
the body, the line of gravity shifts toward the weight. This re­
qUires a body alignment change so that the center of gravity
stays within the base of support. 4) The base of support should
A body is only stable when
the center oj graVity is
directly over its base oj
support.
.
Base of support
be widened in the direction that force is being applied or ab­
sorbed in order to maintain stability. If you are caught in a
heavy wind. you will face the wind and lean forward with your
back foot extended to widen your base of support. 5) The lower
the center of gravity. the more stable the body; the higher the
-------_-~cent_et:__of-l!r_avit__v_._____tbe-Ie_ss__stable__t_he-_ooav.F_or__exam_ole_;_--the--
Center
of
gravity
Base
of
~..&...4,,)support
1. The larger the base of support the more
stable the object or person.
..................
Base
of
sup~ort
2. The closer the line of gravity falls at the center of
the base of support. the more stable the body.
Center
of
gravity
Center
of
gravity
3. When an external weight is added anywhere to a
body. the line of gravity shifts toward the weight.
4. Widen base of support in the direction
of force.
Center
of
gravity
5. The lower the center oLgravity. the more stable the body.
I
A wide base of support adds to the stability of an object. In
addition to the height of the center of gravity, a narrow base
of support makes it difficult to maintain stability. For in­ stance, walking on a balance beam or ice skating is difficult
because of the narrow base of support. The problem is keeping
-·_·-the-cent~r-ef-gFavJty-o¥er-the-hase-Of.sUPPOl:tTh.~e"".r=efi=o~r.e=''-"th=e
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wider the base of support, the more stable and easier to ac­
complish the task.
Newton's Laws of Motion
In the seventeenth century. Sir Isaac Newton discovered three
laws that explain why objects move as they do. Many of the
movements encountered in physical education and dance are
directly related to Newton's Laws of Motion.
Newton's first law of motion says that an object or body w1ll
remain ina state of rest or continue in uniform motion until
the body is ~acted upon by a force that is large enough to alter
the object. In other words, a body at rest w1ll remain at rest
or a body in motion will continue at a constant speed and in
a'straight line unless it is acted upon by another force. As an
example, imagine the frightening situation of sitting in a ve­
hicle Without a seat belt and the vehicle suddenly stops. The
movement of the person in the car as he or she moves toward
the Windshield is Newton's first law in action.
Newton's second law of motion states that the acceleration
(speed) of an object is directly proportional to the force causing
it and is inversely proportional to the mass of the object. As
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A
Approachsp~d
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Why canjumper Bjump
farther?
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5 mph
--.......;...-- --------------"­
B
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Approach speed
10 mph
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Newton's Second Law of Motion
50 Ibs.
force
Applied to a light object
__ ...
,. .,
-
0
50 Ibs.
force
Applied to a heavy object
Accelerates
faster
Accelerates
slower
Applying Newton"s Second Law of Motion
\
B
A
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"... -....;1-,..-/~
12 inches
Jumping force
-------ifJumper A
is lighter than
Jumper B. what is the reason
that B canJump higher?
Jumping force
/
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24 inches
/
an example, the application of the same amount of force to a
golf ball and to a 12 pound shot put (metal ball) will cause the
golf ball to accelerate more than the shot put because of the
golf ball's. smaller mass. In addition. if these two objects are
equally accelerated. the shot put will make a larger dent in the
ground because of its greater mass.
When we consider the human body as the object for accel­
eration in a situation such as jumping. the force that is ap­
plied results from the strength of the contracting muscles and
the mass of the body itself. If you wish to increase the height
of your jump, you must either apply more force by strength­
ening the muscles and/or you must reduce your mass.
Newton's third law of motion states that for every action
there is an equal and opposite reaction. Whenever a body
exerts a force upon another body, the second body exerts an
equal and opPOsite force upon the first. This law explains why
one makes slow progress when walking on ice or on very soft
sand as compared to walking on a finn, non-slippery surface.
The. force of the foot pushing back against the ice or sand is
diminished and causes a weak equal and opposite rea,ction.
Experimenting with Newton's Laws of Motion should
become a common practice because of the significant way that
they contribute to efficient movement. The following move­
ment patterns are examples of the.laws that Can be ~pplied
to movement activities.
Initiating motion occurs in the anticipation and prepara­
tion for movement. This is the beginning of a movement skill.
An example would be your preparation as you wait for a tennis
ball to be served to you.
Changing direction refers to your position and the ability
to change direction or start and stop efficiently. To change di­
rection requires that you control your center of gravity Within
your base of support without falling. Changing direction
should be practiced in the specific activity so that the playing
surface and foot wear can be tested. An example would be
trying to make a sharp turn on a slippery surface like a wet
grass field.
Acceleration and deceleration require changes in the "ef­
fective" force that is applied. To accelerate you must get all
your body parts moving quickly and to decelerate you slow
down the body's movement. For example. a sprinter must
move hisjher anns quickly and forcefully to accelerate
throughout the race and must slow down the body parts in
order to slow down and stop.
Reactive force provides a strong ca~e for staying on the
ground in all cases involving power. Reactive force is the ap­
plication of force against the ground as in running. turning.
or pushing off for a throw. In order to achieve a maximum
effort. all forces of the body. legs. hips. trunk. shoulders. and
anTIS combine in a coordinated effort to produce a desired
result. This is called summation Offorces. Care must be ex­
ercised to maintain ground ·contact or potential for effort is lost.
For example. it is easier to throw a ball with one foot firmly
planted on the ground than it is when you are airborne.
Newton's Laws of Motion have valuable practical applica­
tion in our daily lives and in physical education. Whatever the
phySical activity. lifting. pushing. pulling. throwing. kicking.
jumping. or.moving. the knOWledge and understanding of the'
laws of motion are essential to the development of effective
movement and for achieving success.
How ts the use oj spikes an
application oj Newton's Third
Law oj Motion?
Initiating slight mouement
when preparing
to hit the ball
ts to your aduantage.
Moving your arms can
jorcejully and qUickly help
you to sprintjaster.
•
•
Which person is able to throw
the balljurther?
•
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Force produces motion. stops motion and prevents motion. It
may increase speed. decrease speed or cause Objects or op­
ponents to change direction. Forces may push or pull to cause
motion or balance each other so that bodies remain sta­
tionary. Force is the effect that one body has on another. How
· a force affects a l::x>dy is determined -by the size (or magnitude)
of the force, the direction in which the force is moving and the
point at which the force is applied to the object. A large person
with a large mass that can run fast is capable of producing
more force than a small person running the same speed.
-----T-herefore;-t:he--iarge-football-playeFS-ar-e-eft-en-plaeed-in-th@------line to block. The formula for Force is known as:
Force
=
~~~
Mass X Acceleration
Types of Motion
There are two basic types of motion and these are known as
Linear and Rotary. There are other forms of motion, but they
are either vartations of linear and rotary motion or a combi­
nation of both.
Linear Motion (translatory motion) occurs when the body
or object moves as a whole in a straight line with all the parts
movtngat the same speed and in the same direction. Linear
motion usually occurs when the body is transported byan­ other object, such as in skate boarding, skiing, bicycle riding,
or rlding in an automobile. The body may also move in a linear
pathway such as in walking or running, but it does so through
the rotary movements of the legs.
Rotary Motion (angular motion) is movement in an arc or
a circle around an axis or a fixed point. The arc may be small
or it may be a complete circle. Two related examples are the
softball pitch using the underhand pitch and the windmill
pitch. Most of the joint motions-in the body are angular move­
ments in which the body part moves in an arc about a fixed
point. The running action of the legs, rotation of the arms in
underarm, overhand, and sidearm actions are all examples.
A variation of linear motion is curvilinear motion. An object
begins motion in a linear pathway but due to gravity, air re­ sistance, or some other force, the motion becomes a curved
pathway. Resistance causes the linear motion to become cur­ vilinear. The curvilinear motion that results differs from rotary
motion. Although its pathway is curved, it does not neces­ sarily move in a circle or in the arc of a circle.
Throwing is an example oj
rotary motion.
Levers
A lever is a simple machine that's used as a device to transmit
energy. It is a rigid bar that can rotate around a fixed point
whenever a force is applied to overcome a resistance. A sticK
_ _ _ _ .~~used-to-pr:y-an-ObjeeuS-a--Jtw.e-rrl-he-lenger-and-stff>nger-th.p-e
---I
lever, the greater the capacity to apply force. In our daily lives,
we use levers. In the kitchen we may use hand can openers,
nut picks, and bottle openers. In our house we use a hammer,
a tack lifter, a crowbar, and a wheelbarrow. Even though each
of these have different shapes and sizes, each is a rigid bar.
Whenever a force 1s applied to one of them, it tUrns about a
fixed po1nt known as a fulcrum. In a wheelbarrow the fulcrum
1s the axle.
In the human body, 1t 1s necessary to recognize that every
bone in the skeleton can be viewed as a lever. The bone serves
as the rigid bar, thejo1nt serves as thefulcrum and the con­ tracting muscle serves as theforce. The distance between the­ fulcrum and the resistance is known as the Resistance Ann
and the distance between the fulcrum and. the contracting
muscle is known as the Force Ann. A longer force arm pro­ duces power, whereas a longer resistance ann produces speed
and movement. A crow bar typically has a short claw to grab
the head of a nail. This is the resistance ann. The crow bar
has a long handle to pry called the force ann.
The body as a lever.
Fulcrum ",.."
\---­
Lor:gerforce arm produces
more power.
•
•
Force arm
Force arm
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There are three types of levers:
1. In a First Class Lever. the fulcrum (axis of rotation) Is
located between the force application and the resistance. The
length of the force arm determines the output of the lever.
RESiSfANeE----------­
FULCRUM
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Common examples of first class levers Include a teeter­ totter. a crowbar. a pair of pliers and a hammer removtng a
na1l. First class levers are unusual In the musculo-skeletal
system because body levers are not generally arranged with
the axis of rotation between the muscle attachment and the
resistance.
Does the length of the force ann create the same
advantage in a 2nd class lever? ­
Which wheelbarrow would be
easier to lift? Why?
;
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Force arm
Force arm
2. In a Second Class Lever. the resistance is between the
force and the fulcrum. This lever favors force at the expense
of speed and range of movement because the force ann is
always longer than the resistance arm. Lengthening the force
arm makes 1t easier for you to 11ft the object.
EFFORT
RESISTANCE
~
FULCRUM
Second Class Lever
Common examples of second class lever systems include a
wheelbarrow. a rowboat, a nutcracker and a door. Examples
of second class lever systems in the body where the muscle is
the force are even more unusual than examples of first class
lever systems. The push up is an example of the total body
acting as a second class lever, where the feeract as the axis
of rotation, the center of gravity is the resistance and the
hands pushing against the ground is the movement force.
I
Third Class Lever
....---.---------------)rr~~__"\
RESISTANCE
RESISTANCE
A longer resistance arm
produces more speed. !f the
longer club is movingfaster
when it hits the ball, which
club will hit the ballfurther?
3. In a Third Class Lever, the force application is between
the fulcrum and the resistance. This lever is most effective in
producing speed and range of motion. The third class lever is
the most common lever in the body and can be found
throughout the upper and lower.eXtremities.
(Muscle)
RESISTANCE
EFFORT
FULCRUM
(Joint)
Third Class Lever
Common examples of third class levers are shovels. fishing
poles. and most implements used in sports such as rackets.
bats. fencing foils. and weights, In addition. most of the body
arrangements are third class levers when the muscle is the
movement force. because the muscle attachments are closer
to the joint axis of rotation than are the resistance that the
muscles are moving. This.results in an inefficient source of
power but produces a lever that is capable of fast. movements
capable of moving relatively light objects at high speed.
In the body's third class levers. power cannot be increased
by lengthening the force arm (the distance from the muscle
attachment to the joint). However. power can be increased by
two other methods. The first method is to strengthen the
muscle in order to increase the power of the force. The second
method is to lengthen the resistance ann, thereby increasing
the speed of the resistance arm. This will produce more force
at impact. An example of this is the increased amount of force
---generated-by-h1tting-a--tenBis--ball with a-small-r-acquet--Y..s.-a
regular tennis racquet.
Having a basic understanding of the lever system provides
an insight into our ability to change our personal performance
of activities by being aware of the mechanical advantage cre­ ated by levers and adjusting the resistance ann, force, and
force arm when appropriate in order to increase our perfor­ mance.
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Movement force directed at
varying angles produce different results.
Projectiles
The principles of movement that relate to projectiles {an object
that moves through space in a curvilinear path) depend upon
the reason for the object in flight. If a ball is to reach its des­ tination in the shortest possible time (linear path), the angle
at which it is thrown will be zero to ten degrees. If distance
rather than time of flight is necessary, then the angle of re­
lease between 35-45 degrees may be advantageous.
Projectiles follow a curved or curvilinear path through space,
and the specific path is determined by velocity, angle of pro­
jection. and height of release. A projection angle which equals
90 degrees provides height but no distance. This object is
thrown or hit straight up into the air. Conversely. a projection
73
angle which equals 0 degrees from ground level. provides for
short distance but for no height. A throw or hit parallel to the
ground moves fast for a short distance.
To achieve the greatest distance for a projectile. the angl~
of projection should be about 45 degrees. Longer flight times
- - - - - - - - - r nnlUst-be-accompa:n1err-by hIgher projection angles, whereas
shorter flight times require that the projection angle must be
decreased.
Application of Force
Effect oj left and right spin
around a verfical axis on path
. oj ball (looking down on ball).
When applying force to an object, the effort shoUId be applied
in the direction of the desired movement. A typical explana­
tion is in hitting a golf ball with a club. If the contact with the
club head is correct. maximum distance and accuracy will
result. In addition. the nearer the force is applied to the center
of the object, the more likely the object will travel in a straight
line and in the desired direction. When force is applied away
from the object's center gravity. the path will be altered.
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LEFT SPIN
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Effect oJJorward and back
spin on path oj ball (looking at
side oj ball).
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Air
FORWARD (TOP) SPIN
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Air resistance
Pressure
built up
BACK SPIN
Absorption of Force
As a body receives a force. the momentum of the force at
impact will determine if any body adjustments are needed to
assure a gradual reduction of the impact. The greater the mo­
mentum. the greater the impact will be and the greater the
necessity to gx:adually reduce the momentum.
The impact of the force can be reduced by increasing the
surlace area and the time and distance over which the force
is received. The larger the surlace area receiving the force. the
less force received by anyone area. For exarnple~ when
catching a ball. the impact of the ball should not be received
on the tips of the fingers because an injury may result. There­
fore surface area should be increased by using the palm of the
hand. a glove. or two hands instead ofjust one..
Study Questions
1. In jumping. stress is most likely put· on which major
.bones?
2. What are the bones of the arm? The chest?
3. Which muscles are pIimaIily used to run? To push with
your arms? To pull with your arms?
4. As you kick a ball. the pIimary joints that are used are
the knee and hip. What type ofjoints are these?
5. Why is the ellipsoid joint in your wrists an advantage over
a simple hinge joint?
6. Why is the center of gravity important in the body?
7. Why does the center of gravity constantly change in
humans?
8. How does a person maintain balance?
9. There are 4 pIinciples that apply to stability. Name them
and give an example of each.
10. Define a lever.
11. Name what the rtgid bar. fulcrum. and force are in the
human body.
12. A diving board is an example of a first class lever. If you
lengthen the board when you jump. what wi]] happen to
the height of your takeoff? Why?
13. How do you make the 3rd class levers of your body more
efficient in producing movement?
14. How does Newton's first law of motion explain why it is
clifficult to quickly and sharply change direction when you
are running?
.. - . . - . - ..
__ .
--------­
15. How does Newton's third law of motion explain why
starting blocks are used as an advantage in sprinting
events?
16. Using the laws of projectiles. explain how you-would thro~
a ball as far as possible?- - _ - __ . _ - - - _
.• ....•
.
For Further Application of Mechanical Principles
Application oj Mechanical Principles
Thejollowing mechanical principles have application to all physical
activity and sports. Review the list and apply the principles that will
enhance the performance oj your selected sport or activity.
Effects oj Gravity on Posture
1. The line oj gravity oj each segment oj the body is an extension oj
the line qf gravity oj the segments above and below and thus
gravity actually helps to maintain the position ojthe various seg­
ments since it pulls each downward evenly into the one below.
2. The more nearly the vertical the long axis oj each segment. the
greater the stabilizing effect oj gravity.
3. The greater the angle oj inclination oj the total body or any seg­
ment ojthe bodyjrom the vertical, the greater the muscular effort
necessary to hold it.
4. In order to havejull control oj one's body at all times relaxation
oj muscle is as important as stimulation oj musCle.
Balance or Equilibrium
1. Stabiltty is directly proportional to the area oj the base on which
the body rests.
2. Stability in a given direction is directly proportional to the hori­
zontal distance oj the center oj gravttyjrom the edge oj the base
toward the given direction oj movement.
3. Stabiltty is directly proportional to the weight oj the body.
4. For equilibrium to exist, the center oj gravity qf a body mustjall
within its base.
5. External weights added to the body become part oj the total body
weight and affect location oj the center oj graVity. dispiactng it in
the direction oj the added weight.
Levers
1. The mechanical advantage oj a lever is rep'resented by the ratio
oj the length oj thejorce arm to the resistance arm.
2. The longer thejorce arm, the greater the movement offorce.
3. The shorter thejorce arm, the smaller the movement ojjorce but
the more immediate the action.
4. To be most effective, thejorce must be directed at right angles to
the lever.
Newton's Law of Motion
1. An object which is at rest will remain at rest and an object in
motion will remain in motion unless acted upon by aforce.
2. When a body is acted upon by aforce. its resulting acceleration
(change in speed) is proportional to thejorce and inversely pro­
portional to the mass.
3. For every action force. there is an equal and opposite reaction
Jorce.
4. To turn an object.force must be exerted at a distance to its axis
and the greater the distance, the greater the rotational or spin­
ning elJect.
5. The axis of a revolving body is a straight line about which all other
----p-arts rotate or spin irra-plane--at,tght-angles.
6. Acceleration oj rotary movements depends not only on mass but
also upon its distribution about the axis (the closer the mass to the
axis. the easier it ts to turn).
7. A turning body isolatedJrom extemalJorces u·.il have a constant
angular momentum.
8. Momentum. angular or linear. can be tfansjerredJrom one object
to another and from body part to whole body.
9. The angular velocity oj two moving body parts is inversely pro­
portional to the moment oj inertia about their common axis.
Movements oj a Pendulum (Underhand Throw)
1. Movements oj a pendulum are produced by theJorce oj gravity.
2. The upward movement oj a pendulum is alJected by the mo­
mentum developed in the downward movement.
3. The amplitude oj the pendulum (range oj swing) depends upon the
heightJrom which its movement is initiated.
Force
1. Force must be ofsuffiCient magnitude to overcome inertia if move­
ment is to result.
2. The greater the mass oj the object imparting the Jorce and the
Jaster it is mOVing, the greater theJorce imparted.
3. The greater the time and the distance over which acceleration can
be developed. the greater the momentum possible.
4. The more contributing muscles used. the less energy wasted.
5. The moreJully a muscle is stretched. the greater theJorce that it
can exert.
6. An object acted upon by twoJorces. moves in the direction oj the
resultant oj the twoJorces.
7. TheJurtherJrom the center oj gravity theJorce is applied, the less
Jorce necessary to rotate the object.
Projectiles
1. A projectile carries out two independent motions. a horizontal
motion which is gradually decreased by air resistance and a ver­
tical downward motion which is accelerated by theJorce ojgraVity.
2. The distance a projectile travels depends upon the initial speed
and the angle at which it is projected.
3. The greater the speed ojprojection. the lower the angle can befor
a given distance.