【Daily Question】Theoretical Foundation 2020.01.22

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Multiple-choice question: The horizontal beam AB is supported by hinge A and rod BD (see figure). Install a pulley at point O on the beam using a small shaft. Cross the soft rope over the wheel. One end of the rope is tied horizontally to the wall, while the other end holds a mass with a weight of W. The components do not have their own weight taken into account. The magnitude of the restraining force at hinge A is ( ). Answer: A. Take the AB rod together with the pulley and the object as the system under study, and draw its force diagram as shown in Figure 4-23. Write the equation of force balance: T = W, where T is the tension in the rope and r is the radius of the pulley. It follows that FBcos45°=W/4. Hint: An additional reward will be given to those who can explain the solution process; the answer can be found in the response!