MegaFinalExamsKEY

# MegaFinalExamsKEY - ,2001,2002, 1.. vertical.,.

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Key for questions from the 2000, 2001, 2002, 2002 and 2003 final exams. 1. A person is hanging statically by one arm from a chin-up bar. Their upper arm is parallel to the ground and their forearm is vertical. The weight of the upper arm is 20N, and its center of gravity is 0.15 meters from the center of rotation of the elbow joint. The rest of the body (not including the arm from which the person is hanging) has a mass of 60 kg and its center of gravity has a moment arm of 0.25 meters about the elbow joint. A) What is the magnitude of the net muscle moment at the elbow? sum of moments about elbow = 0 = + (60 • 9.81• 0.25m) + (20•0.15m) - (flexor muscle moment) flexor muscle moment = (147.2) + (3) = 150.2 Nm or you could say the muscle moment is –150.2Nm B) An elbow flexor muscle and an elbow extensor muscle are co-contracting. The elbow extensor muscle is generating a force of 100N with a moment arm of 3 cm about the elbow joint. What is the elbow flexor muscle force if it has a moment arm of 4 cm about the elbow joint? the net muscle moment is what you just solved for, -150.2 Nm = +(100•0.03) – (Fm • 0.04m) (30 +150.2)/0.04 = 4504N 2. A) Explain the difference between a viscoelastic material (e.g., tendon) and a purely elastic material (e.g., a mechanical spring). if a purely elastic material is strained (i.e. stretched or compressed), the force that it resists with is proportional to the amount of strain (delta L). for a visco elastic material, the force depends on both the amount of strain and the rate of strain. an elastic material returns all the energy stored in it when it is strained. a viscoelastic material dissipates some of the energy. viscoelastic materials their force and length change over time when they are loaded. B) Give three examples (including graphs) of viscoelastic properties of tendon. Be sure to explain the examples in terms of the conditions under which the data were obtained. pick 3 of 4 graphs: hysteresis, creep, load relaxation and strain rate dependent stiffness. see Nordin & Frankel book chapter. hysteresis curve: vertical axis is force, horizontal axis is deformation (amount of stretch). curve is concave up on the stretch and on the relax portion. but relax curve is below the stretch curve. data could be obtained with a mechanical testing machine that stretches material while measuring the force and deformation simultaneously. creep: material is loaded with a constant force and the ensuing changes in length are measured. after the intial stretch, over time, the length increases slowly and further. load relaxation. a material is stretched by a certain amount and then held isometric. the force rises initially butthen over a longer time period, the tension declines as the material relaxes. strain rate dependednt stiffness: a material is stretched slowly, then relaxed. then it is stretched more rapidly. length and tension are measured simultaneously. when it is stretched more rapidly, it is stiffer, i.e. the force-length curve is steeper; for a given stretch amount, the tnesile force is greater.

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