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Physics 1028 Lecture and Textbook Notes.docx

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Physics 1029A/B
June Matthews

Chapter 3 ForcesMay 5 2014IntroductionForcesthe interaction bw separate objectsContact forces include friction tension normal or muscle forcesoAlso known as convenience forces or nonfundamental forcesContactfree forces include the four fundamental forcesoDont require direct contact bw objects and are called field forcesIn the human body muscles act as forces skeleton acts as frame on which the forces actOrganisms have receptors that can detect external forces directly or by measuring resulting accelerationthese receptors are called mechanoreceptorsFor an organism to be considered alive it must be able to recognize external stimuli and be able to respond to itLocomotion motion from one place to another can be a primary responseExerting a force can be a direct response to external stimuli Organisms more often exert forces to prevent motion31 Muscles as an Origin of ForcesSkeletal muscleattached to the bonesSmooth musclesurround abdominal organs and blood vesselsCardiac muscleoperate the heartSkeletal MuscleoResponsible for skeletal movementattached to bones by tendonsoContraction of skeletal muscle is under voluntary controlBiceps contracts bendsflexes the jointcalled a flexorTriceps is pulled making it longerTriceps contracts arm is extendedstraightenedcalled extensorBiceps is pulled making it longeroAll muscles work in pairs bc can contract but cant stretch themselvesoAntagonistic actionwhile the biceps contracts the triceps relaxesvice versaoConsists of fibres running length of muscleEach fibre is a cellsubdivided into smaller units called myofibrils each cell contains 100 myofibrilsMyofibril divided into sarcomeressmallest structural unit that can contractSarcomeres are made of two protein filaments actin and myosinThey run parallel to each other but are not joinedActin filaments are anchored to Zdiscs and extend on both sidesMyosin filaments bridge the gap bw actin filaments of 2 adjacent ZdiscsActin contains binding sites for myosin where the myosin head attachesActin allows cell to bear pulling forcesMyosin acts as a motor molecule by walking along actin rodsThe microscopic mechanism of muscle contraction is called the sliding filament modelDuring muscle contraction each sarcomere gets shorter therefore the muscle gets shorterHowever filaments dont contract or get shorter but slide on each otherThis process is caused by activationdeactivation of stimuliIn the end causes muscle to product a forceoIn vertebrates muscles extend as connective tissues called tendonsattached to bonesforce of the muscle is transferred to bone via tendonoTendons act as strong strings that are flexible but dont stretch made of collagen32 What is a Force321 ForceA force is a push or pull exerted on an object resulting from interaction bw two objectsConcept Question 31Throwing a ball toward a wall is a contactbased interactionPush ball away from self while ball is still in hand once released no interaction bw you or ball ball contacts wall resulting in interaction Concept Question 32If throw ball up or down falls towards you or groundWhile in air interaction bw ball and Earth gravitationalEarth exerts force on ball ball exerts force on EarthKnown as contactfree interaction or field interactionscan act over a distanceForce only exists if there is an interactionForces always act in pairsInteraction pair forcesforces of the pair are acting on two different objects man is exerting force on box box is exerting force on man33 Properties of a ForceForce has to be applied by a material objectForce acts in different directions picking up book holding up to shoulder moving left rightForce is a vector quantity because has magnitude and directionTwo forces act on same object equal to addition of forces resultant force called net force34 Action of a ForceExample of bumping a volleyballoForce changes the state of motion of an objectoChanges the velocity of the object Concept Question 3334a Force can change the magnitude of velocity Ex Kicking a ball 10 m
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