What exactly is your difficulty? It is difficult to know what suggestions would help if you don't show any work of your own.
O(0,0,0), A(4,0,0), B(1,3,0) and C(1,1,2) are vertices of a parallelepiped having OA,OB and OC as concurrent edges. Forces of magnitudes 7, 2 and 4 act on a particle at O in the directions pointing towards A,B and C respectively. Determine the resultant force on the particle. What is the component of the resultant, along the diagonal of the parallelepiped.
Well I know how to calculate the resultant by taking the unit vectors along the directions on which the forces are acting , multiplying them by the magnitude for the forces so that I get the 3 forces as 3 vectors and the resultant is just the addition of the 3 vectors.
What I'm having the difficulty is getting the component of the resultant along the diagonal of the parallelepiped.
I think I can get the component by taking the dot product of the resultant with the unit vector along the diagonal of the parallelpiped but I just cannot find a way to get the unit vector. Or is there any other easy way of doing it?