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WE 37 ----------------1-------1____ __--#~/8-,_'d~ 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 _ 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 """ "' A Approachsp~d " Why canjumper Bjump farther? \ ~ 5 mph --.......;...-- --------------" B ,/ ,/ Approach speed 10 mph /' / ( " .., - 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 .", "... -....;1-,..-/~ 12 inches Jumping force -------ifJumper A is lighter than Jumper B. what is the reason that B canJump higher? Jumping force / /' ~ 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? • , 'I ...... • ,~I .. l'( i ;.. :.,~1 ,..! i- f; ~ ; < i'M' , 1'::" " -~J_ •• t' , J., ro~c.e 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 -'p I 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 , ~ k- I •r ~ 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? ; -=~ 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. ". ..... I tC:"1 ~QC/ / -- , I , , " , , I _---t----_ -I - ..... .stance II I \ \ I I Vl ." ". ". ,~_------S~~d I - . \ $/ I \ -----,-----, " -T-........ I ' ,\ I , I I o , ., \\ \ \ \ 'I 0 \ \ \ \I 0 Movement force directed at varying angles produce different results. Projectiles The principles of movement that relate to projectiles {an object that moves through space in a curvilinear path) depend upon the reason for the object in flight. If a ball is to reach its des tination in the shortest possible time (linear path), the angle at which it is thrown will be zero to ten degrees. If distance rather than time of flight is necessary, then the angle of re lease between 35-45 degrees may be advantageous. Projectiles follow a curved or curvilinear path through space, and the specific path is determined by velocity, angle of pro jection. and height of release. A projection angle which equals 90 degrees provides height but no distance. This object is thrown or hit straight up into the air. Conversely. a projection 73 angle which equals 0 degrees from ground level. provides for short distance but for no height. A throw or hit parallel to the ground moves fast for a short distance. To achieve the greatest distance for a projectile. the angl~ of projection should be about 45 degrees. Longer flight times - - - - - - - - - r nnlUst-be-accompa:n1err-by hIgher projection angles, whereas shorter flight times require that the projection angle must be decreased. Application of Force Effect oj left and right spin around a verfical axis on path . oj ball (looking down on ball). When applying force to an object, the effort shoUId be applied in the direction of the desired movement. A typical explana tion is in hitting a golf ball with a club. If the contact with the club head is correct. maximum distance and accuracy will result. In addition. the nearer the force is applied to the center of the object, the more likely the object will travel in a straight line and in the desired direction. When force is applied away from the object's center gravity. the path will be altered. Cl) ~$ ~ 1;) ...~1: - - . Wrr . ;( Pressure built up I . j'f......j < ~essure . . - Pressure built up reduced RIGHTSPlN LEFT SPIN Pressure built up Effect oJJorward and back spin on path oj ball (looking at side oj ball). //- Pressure reduced J Air resistance ~ ,...-.- ~e--··, ' / -L. . l ! //-. Air resistance 8 .." ,i ~ ~)-_ • 1 resistance Pressure reduced Air FORWARD (TOP) SPIN 1-- .... 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.