# lab 9 - Physics 1AL EQUILIBRIUM CENTER OF GRAVITY...

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Physics 1AL Summer Session II 2010 1 mg hand mg COG forearm pivot and support point T biceps Introduction In this lab you will use a model of the lower arm to investigate the force that the bicep muscle needs to exert for you to be able to lift and hold your lower arm and hand at various angles, and for you to be able to support masses in your hand when the lower arm is horizontal. You should review sections 8.1 to 8.4 of your text before coming to the lab. You will need to use the concepts of center of gravity, torque, and rotational equilibrium. The figure shows a diagram of an arm bent at about 90°. The upper arm is vertical and there is a mass supported by the hand. We will assume that the bicep muscle pulls vertically upwards on the upper bone (the radius ) of the lower arm. For this experiment we will work with conditions in which the tricep muscle is relaxed and not exerting a force. The lower arm rotates about a pivot point close to the base of the humerus (the bone in the upper arm). Our model of this system is simple but contains all the important features. The lower arm and hand are represented by a length of wood supported at the elbow end by a rod passing through a hole. The force of the rod on the wood is the same as the force of the humerus on the ulna and radius . A mass can be attached to the end of the wood at the hand end. The bicep is represented by a string pulling upwards with the tension

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## This note was uploaded on 09/25/2010 for the course PHYSICS 1A 1al taught by Professor Rafaelski during the Summer '10 term at UCSD.

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lab 9 - Physics 1AL EQUILIBRIUM CENTER OF GRAVITY...

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