# Statics - Resultant Force

by kehler
Tags: force, resultant, statics
 P: 106 1. The problem statement, all variables and given/known data If engine 3 suddenly fails, determine the resultant of the three remaining engine thrust vectors, each of which has a magnitude of 90kN. Specify the y and z-coordinates of the point through which the line of action of the resultant passes. 3. The attempt at a solution I did it but I have no idea whether my working is right or not. The resultant force is 90kN x 3 = 270kN in the x direction. To find the y and z coordinates, I looked for the resultant moment about point 0 which i chose to be at (0,0,0). This is where I'm not sure. Why do the 2m and 3m arrows pointing out from the engines in the x direction matter? (Actually, they probably don't matter too much as they cancel out. But I have to show my working so it'll be good if someone can explain their significance) I assumed they must have some effect so I multiplied 90kN by (2/3) for the two outer engines. So, the moment due to the first force on the left is -21 x 90kN x (2/3) = -2520kN (in the z direction, ie pointing upwards) The moment due to the inner engine on the right is 12 x 90kN = 1080kN (pointing downwards) And the moment due to the rightmost engine is 21 x 90kN x (2/3) = 2520kN (pointing downwards) The resultant moment is 1080kN (pointing downwards) I know that the sum of moments about O is equal to the moment of the resultant force about O (Varignon's Theorem). So to find the y-coordinate of the point which the line of action of the resultant passes, I equated 1080kN = r x 270kN r = 4 Therefore, the resultant force is 4m to the right of point 0. The z coordinate should be zero. Can someone tell me if I did it right? Any help would be much appreciated :)
 P: 1 Yes this is correct!!
 P: 137 The 2m and 3m designations are the distance along the z-axis that the engines are away from the x-axis. These distances, coupled with the forces, will cause each engine to create moments, also. edit: Oh, geez - I just noticed that this thread originated two years ago. Sorry...

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