Question

Determine the magnitude of the couple force f so that the resultant couple moment on the crank is zero?

Answer 1

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Suppose positive torque is one that tries turning CCW and negative turns CW.

Then 150lb on the left side of the image is positive because if we only look at that force, it would try to turn the object CCW (Counter Clock Wise).

going from left to right:

F1=150 × cos(45-30)

L1=5+5=10inch

F2= -F × cos((45-30)

L2=5 inch

F3=-F × cos(45+30)

L3=4inch

F4= -150 × cos(45-30)

L4=4+4=8inch

T = F1×L1 + F2×L2 + F3×L3 + F4×L4 = 0