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Chapter 3

Chapter 3 - Forces.docx

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Western University
Physics 1301A/B
Martin Zinke- Allmang

Chapter 3: Forces  Force: defined by the interaction between separate objects  Mainly characterized by its two quantitative properties: magnitude and direction  Categorized as either contact forces or contact-free forces  4 fundamental forces: gravity, electromagnetics, strong nuclear force, weak nuclear force  Contact-free forces; don’t require any direct contact between objects, also called field forces  Field forces can be long-range forces  Forces dealt with daily are non-fundamental except for gravity; also called convenience forces  Contact forces such as tension, normal, or muscle forces  Contact forces act only when physical contact between the objects is established  Laws of mechanics (kinematics & dynamics) allow for the understanding of the anatomical design and physiological function of muscles as a source of forces, and the skeleton as the frame on which these forces act  Organisms have a range of receptors called mechano-receptors that detect external forces directly or by measuring the resulting acceleration  Necessary conditions of the widely-accepted definition of life:  Metabolism & growth  Recognition of external stimuli, combined with the ability to respond  Reproduction  Locomotion isn’t a direct response of the organism to external stimuli, but it is one possible consequence of the primary response, which is to exert a force  Organisms tend to exert forces to prevent motion (e.g. when we hold objects or keep our body in a particular posture) 3.1: Muscles as an Origin of Forces  Muscle Tissues: distinguished in anatomy on the basis of their structural differences, and in physiology on the basis of their functional purposes  3 categories:  Skeletal muscles that are attached to the bones  Smooth muscles that surround abdominal organs & blood vessels  Cardiac muscles that operate the heart  Skeletal muscles’ actions relate most directly to the everyday experience with our bodies  Skeletal muscle: attached to the skeleton, mostly by tendons; responsible for skeletal movement  When skeletal muscles contract, the bone doesn’t always move; contraction of skeletal muscle is under voluntary control  When biceps contract, it flexes the joint; thus a biceps is called a flexor  Simultaneously, the triceps contracts & the arm is extended at the elbow joint, thus it is called an extensor  All muscles, regardless of their type, work in pairs because they can contract but cannot stretch themselves; Muscle can pull but cannot push  The need for two complementary sets of muscles = rooted in the physical mechanism of muscle action: a muscle can only contract actively, but then has to be stretch passively as another muscle contracts; this is called antagonistic action  When the biceps contracts, the triceps relaxes, and vice versa; always opposite -> antagonistic pair  Although muscle action always exerts a force on a component of the skeleton, it doesn’t necessarily cause locomotion  Skeletal muscles consist of bundles of fibres running the length of the muscle  Each fibre is a cell subdivided into smaller repetitive units called myofibrils  Muscle cells contain about 100 myofibrils  Myofibril divided in the elongated direction into sarcomeres, the basic contractile units of the muscle  Sarcomere = smallest structural unit in the muscle that can contract  A sarcomeric unit is confined by both ends by stiff Z-discs  Sarcomere made up of 2 types of protein filaments: Actin and Myosin  Run alternatively parallel to each other, but are not joined  Actin Filaments: anchored in the Z-discs & extend on both sides  Myosin Filaments: Bridge the gap between the actin filaments of two adjacent Z-discs  Myosin consists of a tail & a specialized binding head called the myosin head  Actin contains binding sites for myosin  Each myosin filament is surrounded by 6 actin filaments  Actin proteins allow the cell to bear tensile (pulling) forces  Myosin protein acts as a motor molecule by walking along the actin rods  Microscopic mechanism of muscle contraction is called the sliding filament model  As a muscle contracts, the myosin filaments bind via the myosin heads onto the actin filaments, forming chemical bonds called cross-bridges  After myosin heads hook onto the actins, the myosin filaments pull the actin filaments toward the centre of the sarcomere  The binding of myosin to actin by the myosin heads causes a release of energy, resulting in the myosin head swiveling, pulling the myosin filament over the actin filament  As the actin filaments slide over the myosin filaments, the length of the entire sarcomere is shortened  The collective shortening of the sarcomeres in many myofibrils causes muscle contraction  The sliding motions causes the muscle to contract, producing force  When the myosin filament hits the Z-discs on both sides, a further shortening of the muscle would require filament crumbling, which doesn’t occur  Each sarcomere, and thus each muscle, can shorten by slightly more than 20%  When a sarcomere is elongated by more than 35%, overstretching occurs, but there are mechanisms that protect muscles from this  In vertebrates, muscles aren’t directly connected to bones, but extend as connective tissues called tendons, which are attached to the bones  The force of the muscle is transferred to the bone via a tendon  Tendons act like extremely strong strings that are flexible, but don’t’ stretch  Made of large strands of white, fibrous proteins called collagen  Different from muscle tissue, bur originates within the muscle to provide maximum strength  Achilles tendon: the thickets & strongest tendon in the body, extending from the calf muscle to the heel bone 3.2: What is a Force?  Muscles, tendons and bones are three separate tissues that interact as a particular functional group 3.2.1: Force  A force is a push or a pull exerted on an object, resulting from the interaction between two objects  When a ball is in the air, neither you or the wall have an effect on the wall  In the broadest definition:  A force represents the interaction of two distinguishable objects.  The calf muscle cannot exert a contact force on the heel bone because the muscle and the bone are not in contact with each other  The tendon creates this contact; thus, the calf muscle exerts a force on the Achilles tendon, and in turn, the Achilles tendon exerts a force on the heel bone  There is no single isolated force, all forces act in pairs  When two forces of interaction are applied on two different objects that are interacting, the forces of the pair are acting on two different objects  They are called interaction pair forces 3.3: Properties of a Force  A force must be exerted on a material object  Forces are not applied spontaneously; something is pushing or pulling these objects  Therefore, force must also be applied by a material object  For any force, you must be able to identify both the object that exerts the force and the object upon which the force is exerted  Force not only has a magnitude, but it also has direction; it is a vector quantity General Properti
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