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# Principles of biomechanics

## on Feb 19, 2012

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## Principles of biomechanicsDocument Transcript

• 9 External linksOverviewThe moment of inertia of an object about a given axis describes how difficult it is to change its angular motion about thataxis. Therefore, it encompasses not just how much mass the object has overall, but how far each bit of mass is from theaxis. The further out the objects mass is, the more rotational inertia the object has, and the more torque (force* distancefrom axis of rotation) is required to change its rotation rate. For example, consider two hoops, A and B, made of the samematerial and of equal mass. Hoop A is larger in diameter but thinner than B. It requires more effort to accelerate hoop A(change its angular velocity) because its mass is distributed farther from its axis of rotation: mass that is farther out from thataxis must, for a given angular velocity, move more quickly than mass closer in. So in this case, hoop A has a larger momentof inertia than hoop B.Divers reducing their moments of inertia to increase their rates of rotationThe moment of inertia of an object can change if its shape changes. Figure skaters who begin a spin with arms outstretchedprovide a striking example. By pulling in their arms, they reduce their moment of inertia, causing them to spin faster (by theconservation of angular momentum).The moment of inertia has two forms, a scalar form, I, (used when the axis of rotation is specified) and a moregeneral tensor form that does not require the axis of rotation to be specified. The scalar moment of inertia, I, (often calledsimply the "moment of inertia") allows a succinct analysis of many simple problems in rotational dynamics, such as objectsrolling down inclines and the behavior of pulleys. For instance, while a block of any shape will slide down a frictionlessdecline at the same rate, rolling objects may descend at different rates, depending on their moments of inertia. A hoop willdescend more slowly than a solid disk of equal mass and radius because more of its mass is located far from the axis ofrotation. However, for (more complicated) problems in which the axis of rotation can change, the scalar treatment isinadequate, and the tensor treatment must be used (although shortcuts are possible in special situations). Examplesrequiring such a treatment include gyroscopes, tops, and even satellites, all objects whose alignment can change.The moment of inertia is also called the mass moment of inertia (especially by mechanical engineers) to avoid confusionwith the second moment of area, which is sometimes called the area moment of inertia (especially by structural engineers).The easiest way to differentiate these quantities is through their units (kg·m² as opposed to m4). In addition, moment ofinertia should not be confused with polar moment of inertia (more specifically, polar moment of inertia of area), which isa measure of an objects ability to resist torsion(twisting) only, although, mathematically, they are similar: if the solid forwhich the moment of inertia is being calculated has uniform thickness in the direction of the rotating axis, and also hasuniform mass density, the difference between the two types of moments of inertia is a factor of mass per unit area.
• InertiaInertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resistany change in its motion. The principle of inertia is one of the fundamental principles of classical physics which are used todescribe the motion of matter and how it is affected by applied forces. Inertia comes from the Latin word, iners, meaningidle, or lazy. Isaac Newton defined inertia as his first law in his Philosophiæ Naturalis Principia Mathematica, which states:[1]The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavours topreserve its present state, whether it be of rest or of moving uniformly forward in a straight line.In common usage the term "inertia" may refer to an objects "amount of resistance to change in velocity" (which is quantifiedby its mass), or sometimes to its momentum, depending on the context. The term "inertia" is more properly understood asshorthand for "the principle of inertia" as described by Newton in his First Law of Motion; that an object not subject to any netexternal force moves at a constant velocity. Thus an object will continue moving at its current velocityuntil some forcecauses its speed or direction to change.On the surface of the Earth inertia is often masked by the effects of friction and gravity, both of which tend to decrease thespeed of moving objects (commonly to the point of rest). This misled classical theorists such as Aristotle, who believed thatobjects would move only as long as force was applied to them.
• 2A/2B BIOMECHANICS 2nded.www.flickr.com/photos/keithallison/4062960920/1©PE STUDIES REVISION SEMINARSCONTENTIntroduction to Biomechanics•What is it?•Benefits of BiomechanicsTypes of motion in Physical Activity•Linear•Angular•GeneralCoordination of linear motion•Types of forces•Kinematic Chain•Simultaneous force summation•Sequential force summationStability and Balance•Balance•Stability•Centre of Gravity (COG)•Base of Support•Factors affecting balance and stability2CONTENTNewton’s laws of motion•Force production•Newton’s 1stLaw of motion•Inertia•Newton’s 2ndLaw of motion•Momentum
• •Conservation of momentum•Impulse•Flattening the arc•Newton’s 3rdLaw of motionProjectile motion•Trajectory of a projectile•Factors affecting flight of a projectile•Angle of release•Height of release•Velocity at take off•Gravity•Air resistance•Spin 32. SEQUENTIALLY• Where body parts are moved in sequence to produce a force.• Generally used to produce maximal force in whole body actions suchas throwing, kicking and striking• E.g. A baseball pitcher, striking in golf, kicking in rugbyCO ORDINATION CONTINUUM.FORCE SUMMATIONFOR MAXIMAL OR SUBMAXIMAL FORCEwww.flickr.com/photos/flash716/2579185404/ www.flickr.com/photos/martindo/3040647348/4©PE STUDIES REVISION SEMINARSHomeSUCCESSFUL SUMMATION OF FORCE/MOMENTUM•Body parts move in a sequence to generate the largest force oracceleration possible.•To sequentially produce maximal force effectively, the followingprinciples need to be applied:1. The stronger and larger muscles of the thighs and trunk are moved firstfollowed by the smaller and faster muscles2. Sequentially accelerate each body part so that optimum momentum passesfrom one body part to the next.3. Each body part should be stable so that the next body part acceleratesaround a stable base to transfer momentum
• 4. Use as many body parts as possible, so force can be applied over themaximum possible time5. Follow through is important to prevent deceleration of last segment andsafe dissipation of force.6. Ensure all forces are directed towards the target©PE STUDIES REVISION SEMINARS 5HomeSEQUENTIAL SUMMATION OF FORCES - THROWINGBig body parts of legs andtrunk initiate movementWide base provides stable basefor acceleration of eachsegmentMaximise number of segmentsusedFollow through towards thetarget to prevent decelerationof final segment and maximisemomentum towards the target©PE STUDIES REVISION SEMINARS 6HomeDETERMINING THE CENTRE OF GRAVITY• To determine ones COG, simply draw a box around the objectsouter extremities• Then draw diagonal lines through the box, with the point ofintersection determining the objects approximate COG.www.flickr.com/photos/dearlydeparted/3834132754/7©PE STUDIES REVISION SEMINARSHomeApproximateCOGFACTORS AFFECTING BALANCE & STABILITYLow COG =↑ stabilityHigh COG =↓ stability8
• ©PE STUDIES REVISION SEMINARSHome3. THE HEIGHT OF THE COG ABOVE THE BASE OF SUPPORT• The line of gravity or pull of gravity will always pass vertically through thecentre of an object’s mass.• The higher the centre of gravity above the base of support, the less stablethe object is. Athletes often lower their centre of gravity by bending theknees in order to increase their stabilityMore stable Less stableLow COG Higher COGWide base of support – 4 point contact Small base of support – 2 point contactLine of gravity in middle of support Similar line of gravitySTABILITY VARIES WITH BODY POSITION©PE STUDIES REVISION SEMINARS 9HomeNEWTON’S 1STLAW OF MOTIONNewton’s First Law of Motion - InertiaThe size of the force required to change the state of motion of an objectdepends on the mass of the object. The greater the mass of the object,the greater the force needed to move it.The 8kg medicine ball has agreater inertia because of itsgreater mass and thereforerequires a greater force to move itThe golf ball on the left willremain stationary on the tee untila force (applied by the club) isapplied to it10©PE STUDIES REVISION SEMINARSHome“A body continues in its state of rest or state of motion unless acted upon bya force”. Newton’s Second Law of Motion – acceleration / momentumThe greater the force applied to an object, the faster the acceleration will be.Acceleration is directly proportional to the force applied.
• A small force applied to a ball usinga putter results in slow accelerationA large force applied to a ball using adriver results in faster accelerationNEWTON’S 2NDLAW OF MOTIONwww.flickr.com/photos/mandj98/2567621739 www.flickr.com/photos/jae_yong/679089763/11©PE STUDIES REVISION SEMINARSHome“The rate of change of acceleration to a body is proportional to the forceapplied to it”.• If all other factors are constant (i.e. Speed of release, height of release,spin, air resistance);©PE STUDIES REVISION SEMINARS 121. ANGLE OF RELEASEHome•An angle of less than 45⁰ results in shorter horizontal distances,shorter vertical distances and shorter flight times•This might be useful in the following sports;•Throwing in softball, cricket etc, stab pass in AFL•An angle of greater than 45⁰ results in shorter horizontaldistances , greater vertical distances and longer flight times.•This might be useful in the following sports;•High Jump, Pole Vault, punting in American Footballwww.flickr.com/photos/stuseeger/434121246/www.flickr.com/photos/thatpicturetakr/2461107564/sizes/l/©PE STUDIES REVISION SEMINARS 13VERTICAL MOTIONHORIZONTAL MOTION1. ANGLE OF RELEASEHomeAngle of release = 45⁰•Vertical and horizontalvelocity is equal•Max horizontal distance
• attainedAngle of release > 45⁰•Vertical velocity is greaterthan horizontal•↑ height and flight time•↓horizontal distanceAngle of release < 45⁰•Horizontal velocity isgreater than vertical•↓ height and flight time•↓horizontal distance
• Q4E Case Study 10 - Cricket Batting 1 Batting is a side-on game – or at least it used t Proposed Subject usage: Sports Science A level & 1st/2nd yr Degre ECB Cricket Coaches Level 1 - 4 GCSE / A-level Sports Science 2008 - Crick National Curriculum 2008-2009 (Key Stage 3. Evaluating and improving performance. Judge how good a performance it AQA GCSE PE 2009 Specification 6.2 Analysis of performance The specification will assess a candidate‟s ability to analyse performance s Determine its strengths and weaknesses Improve its quality and effectiveness AQA A Level PE 2009 Specification At AS, candidates are required to observe, analyse and evaluate performan 21.2 Observe the chosen performer in relation to the competent performa techniques for a chosen activityCricket Batting 1:The demands for International cricket batsmen have changed considerably over the past ten ymatches and one day internationals is testament to this. There is no doubt in my mind that 20increasing the range of flamboyant strokes we see today, the reverse sweep or flick over fine lbowler. Nobody seems to play with a straight bat anymore? Batsmen are intent on working thepick up a quick single with a bat path that only coincides with the ball for a split second… effecjustify the risks taken?Given that batting is such an important part of the game, it is surprising that little biomechanicthe sport. The biomechanics of the off-drive and on-drive have been found to be very similar, wexecution. Grip force patterns of top & bottom hands along with kinematic analysis of selectedhowever, the underlying theme from the biomechanics literature is that batting has many diffecompare the flowing and rhythmical drives of Sachin Tendulkar, Brian Lara & Sunil Gavaskar, tHayden & Adam Gilchrist, or the skill and determination of an Aravinda de Silva, Ricky Pontingbecoming more apparent from the research is the degree of variation the same individual has wlook at players when they first arrive at the crease and then after scoring 50+ runs, why is this