Cuesta College, San Luis Obispo, CA
A uniform beam with a mass of 10 kg and length 2.0 m has a 3.0 kg load hanging from its end, and is suspended by a horizontal cable attached to a wall. (Calculate all torques with respect to the pivot.) The __________ force has the longest perpendicular lever arm. (A) horizontal cable tension.
(B) beam weight.
(C) load.
(D) (There is a tie.)
Correct answer (highlight to unhide): (A)
The weight of the boom acts at its center of gravity, straight downwards. The perpendicular lever arm ℓw for the weight force w extends from the pivot to perpendicularly intercept the weight force line of action, such that this will be a horizontal line of length:
ℓw = (1.0 m)·cos(70°) = 0.34 m.
Similarly the perpendicular lever arm ℓload for the load force Fload extends from the pivot to the perpendicularly intercept the load force of action, such that:
ℓload = (2.0 m)·cos(70°) = 0.68 m.
The perpendicular lever arm for the horizontal cable force extends from the pivot point to perpendicularly intercept the tension force of action, such that:
ℓcable = (2.0 m)·sin(70°) = 1.9 m.
Exam code: quiz05nWaW
(A) : 21 students
(B) : 11 students
(C) : 14 students
(D) : 3 students
Success level: 43%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53
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