Question
Q2
When designing a sign like the one shown in the figure below it is necessary to take into account any forces from wind acting across the surface of the sign. For example, if the sign is not designed properly, the support may twist or the bolts securing the post to the base may shear and fail. It is reasonable to assume that the distributed force from the wind can be treated as a single resultant point force acting along the y axis at the center of the sign. If the distance from the base of the sign to the center is 6' in the -X direction and 20' in the +Z direction, using a vector cross product determine the moment about the base of the sign if Find = 500lbs. Mbase = -1 0000i + 0j - 3000k ft lb Mbase = -10000i + 0j + 3000k ft lb
Explanation / Answer
Vector R =6i +0 j + 20 k ft
F = (0 i + 500 j + 0 k )ib
Moment = R X F
= (6i + 0 j + 20 k) X ((0 i + 500 j + 0 k) { i X j = k ; jX k = i ; k X i = j ; }
= 3000 k - 10000 i + 0 j ftib
