MO = rOA × F = |i j k| |2 3 4| |Fx Fy Fz|

MO = √(800^2 + (-600)^2) = 1000 N·m

Calculate the position vector from O to A.

by Ferdinand P. Beer, E. Russell Johnston Jr., David F. Mazurek, and Michael E. Plesha is a widely used textbook in engineering mechanics courses. Here's a deep dive into the solutions of this book:

MO = (3(0) - 4(0))i - (2(0) - 4(200))j + (2(0) - 3(200))k = 0i + 800j - 600k