Moment of a system of forces

Q. Moment of a system of forces

Question 
Two forces F1 = 800N and F2 = 2000N act on the inverted L shaped frame as shown in the figure. Determine the moment about point A.

Answer 

F1 = 800 j and

F2 = 2000 (0.6i - 0.8j) = 1200i - 1600j

=> F

= F1 + F2

= 800 j + (1200 i - 1600 j)

= 1200 i - 800 j

r = 5i + 2k

Moment

= r x F

= (5, 0, 2) x (1200, - 800, 0)

= (1600, 2400, - 4000).