Two people are carrying a uniform wooden board that is 3.00 m long and weighs 160 N. If one person applies an upward force equal to 60 N at one end, at what point does the other person lift? Begin with a free-body diagram of the board.
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Equilibrium with Multiple Objects
Problem 86
Textbook Question
In Fig. 12–103, consider the right-hand (northernmost) section of the Golden Gate Bridge, which has a length d1 = 343 m. Assume the cg of this span is halfway between the tower and anchor. Determine FT1 and FT2 (which act on the northernmost cable) in terms of mg, the weight of the northernmost span, and calculate the tower height h needed for equilibrium. Assume the roadway is supported only by the suspension cables, and neglect the mass of the cables and vertical wires. [Hint: FT3 does not act on this section.]


1
Step 1: Analyze the forces acting on the northernmost section of the bridge. The forces include FT1 (tension in the cable at the anchor point), FT2 (tension in the cable at the tower), and the weight of the span, mg, acting vertically downward. FT3 does not act on this section, as stated in the problem.
Step 2: Break down the forces into components. FT1 has both horizontal and vertical components due to the angle of 19° with the horizontal. FT2 also has horizontal and vertical components, with an angle of 60° relative to the vertical. Use trigonometric functions to express these components: FT1x = FT1 * cos(19°), FT1y = FT1 * sin(19°), FT2x = FT2 * sin(60°), FT2y = FT2 * cos(60°).
Step 3: Apply equilibrium conditions. For horizontal equilibrium, the sum of horizontal forces must be zero: FT1x = FT2x. For vertical equilibrium, the sum of vertical forces must equal the weight of the span: FT1y + FT2y = mg.
Step 4: Determine the height h of the tower. The vertical component of FT2 (FT2y) is related to the geometry of the suspension cable. Use trigonometry and the given angles to relate h to the span length d1 and the angle of 60°: h = d1 * tan(60°).
Step 5: Solve the system of equations derived from the equilibrium conditions to express FT1 and FT2 in terms of mg. Substitute the trigonometric relationships and simplify to find the expressions for FT1 and FT2.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Equilibrium of Forces
In physics, an object is in equilibrium when the net force acting on it is zero. This means that all the forces acting on the object must balance each other out. In the context of the Golden Gate Bridge, the forces exerted by the suspension cables (F_T1, F_T2, and F_T3) must counteract the weight of the bridge span (mg) to maintain stability and prevent movement.
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Equilibrium
Tension in Cables
Tension refers to the pulling force transmitted through a cable or rope when it is subjected to a load. In the case of the bridge, the tension forces (F_T1 and F_T2) in the suspension cables support the weight of the bridge span. The angles at which these forces act (60° and 66°) are crucial for calculating the vertical and horizontal components of the tension, which are necessary for determining equilibrium.
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Geometry of the Bridge
The geometry of the bridge, including the lengths of the spans (d1, d2) and the height of the tower (h), plays a significant role in analyzing the forces acting on the structure. The position of the center of gravity (cg) and the angles of the cables affect how the weight is distributed and how the tension forces are resolved. Understanding these geometric relationships is essential for calculating the required forces and ensuring the bridge remains stable.
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