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Unformatted text preview: 1 Lecture 20 Thursday, April 3 2 Which situation satisfies both the first and second conditions for equilibrium? A. A seagull gliding at a constant angle below the horizontal at a constant speed. B. An automobile crankshaft turning at an increasing angular speed in the engine of a parked car. C. A thrown baseball that does not rotate as it sails thru the air. D. None of the above. 3 The center of gravity of an object coincides with its center of mass A. if the object has a symmetric shape. B. if the object is moving at constant velocity. C. if the net torque on the object is zero. D. if the acceleration of gravity is uniform throughout the body. 4 When calculating torques to apply the second condition of equilibrium to an object A. the reference point must be chosen at the object’s center of mass. B. the reference point must be chosen at the point where the strongest force is applied. C. the reference point must be chosen at a point where an unknown force is applied. D. the reference point can be located anywhere. 5 If tensile stress on an object is increased slightly beyond the yield point A. the strain increases. B. fracture takes place. C. the strain decreases if the material is ductile. D. none of the above. 6 The only motions of a rigid body cm ! s 1 ! s 2 ! s 3 ! ! 1 ! ! 2 ! ! 3 Translation of the cm in three independent directions Rotation about the cm in three independent directions. 7 Preventing the object’s translation • If the center of mass is not moving, its acceleration is zero, this means: ! F ! = F x = ! F y = ! F z = ! " # $ % $ (3 equations) 8 Preventing the object’s rotation • If the object is not rotating about the center of mass, its angular acceleration is zero. This means: • Thus we have a total of six equations to accommodate the total of six degrees of freedom of the object. ! ! " = ! x = " ! y = " !...
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 Spring '08
 Chamberlin
 Physics, Force, Mass, Strain, rod, length 2L

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