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Unformatted text preview: 911 Chapter 12 Static Equilibrium and Elasticity Conceptual Problems 1 • ( a ) False. The conditions ∑ = i i F r and ∑ = i i τ r must be satisfied. ( b ) True. The necessary and sufficient conditions for static equilibrium are ∑ = i i F r and ∑ = i i τ r . ( c ) True. The conditions ∑ = i i F r and ∑ = i i τ r must be satisfied. ( d ) False. An object is in equilibrium provided the conditions ∑ = i i F r and ∑ = i i τ r are satisfied. 2 • False. The location of the center of gravity depends on the mass distribution. 3 • No. The definition of the center of gravity does not require that there be any material at its location. 4 • Determine the Concept When the acceleration of gravity is not constant over an object, the center of gravity is the pivot point for balance. 5 •• Determine the Concept This technique works because the center of mass must be directly under the balance point. Thus, a line drawn straight downward will pass through the center of mass, and another line drawn straight downward when the figure is hanging from another point will also pass through the center of mass. The center of mass is where the lines cross. *6 • Determine the Concept No. Because the floor can exert no horizontal force, neither can the wall. Consequently, the friction force between the wall and the ladder is zero regardless of the coefficient of friction between the wall and the ladder. Chapter 12 912 7 • Determine the Concept We know that equal lengths of aluminum and steel wire of the same diameter will stretch different amounts when subjected to the same tension. Also, because we are neglecting the mass of the wires, the tension in them is independent of which one is closer to the roof and depends only on W. correct. is ) ( b 8 • Determine the Concept Yes; if it were otherwise, angular momentum conservation would depend on the choice of coordinates. *9 • Determine the Concept The condition that the bar is in rotational equilibrium is that the net torque acting on it be zero; i.e., R 1 M 1 = R 2 M 2 . This condition is satisfied provided R 1 = R 2 and M 1 = M 2 . correct. is ) ( c 10 •• Determine the Concept You cannot stand up because your body’s center of gravity must be above your feet. *11 •• Determine the Concept The tensile strengths of stone and concrete are at least an order of magnitude lower than their compressive strengths, so you want to build compressive structures to match their properties. Estimation and Approximation 12 •• Picture the Problem The diagram to the right shows the forces acting on the crate as it is being lifted at its left end. Note that when the crowbar lifts the crate, only half the weight of the crate is supported by the bar. Choose the coordinate system shown and let the subscript ″ pb ″ refer to the pry bar. The diagram below shows the forces acting on the pry bar as it is being used to lift the end of the crate. Static Equilibrium and Elasticity 913 Assume that the maximum force...
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This note was uploaded on 08/18/2010 for the course IMT 12253 taught by Professor Mikael during the Spring '10 term at École Normale Supérieure.
 Spring '10
 Mikael

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