Question

15. What is the resultant moment about point A of the three forces acting on the beam in the Examination

Figure?
A. 70 ft-lb clockwise
B. 50 ft-lb counterclockwise
C. 50 ft-lb clockwise
D. 70 ft-lb counterclockwise

Answer 1

Answer: option B. 50 ft-lb counterclockwise

Step-by-step explanation:


1) Formula: angular momentum = force × distance

2) Force 70 lb on the left side


angular momentum = 70 lb × 5' counterclokwise


3) Force 100 lb on the left side


angular momentum = 100 lb × 3' counterclockwise


4) Force 100 lb on the rigth side


angular momentum = 100 lb × 6' clockwise


5) Net angular momentum:

70 lb × 5' counterclokwise + 100 lb × 3' counterclockwise - 100 lb × 6' clockwise =

= 350 lf-ft + 300 lb-ft - 600 lf-ft = 50 lb-ft counterclockwise