Mechanics Homework Help?

Question

My problem is similar to this problem http://www.chegg.com/homework-help/questions-and-answers/fb-600-n-fc-700-n-determine-magnitude-coordinate-direction-angles-resultant-force-acting-f-q2823588

(I don't have a chegg account) but the problem is the same sort of problem I have to do. In my problem, the strings attached to the flag pole are in the negative x axis. I just have to find the magnitude of the resultant force. I think I get how to do it, but does the fact that its in the negative x axis make a difference? because apparently the answer that I got isn't right. I thought the negative part wouldn't matter since everything is squared when finding the magnitude? Thanks for any help!

Answer 1

The negative x axis will make no difference. Use absolute values for the force vectors so the resultant is positive.

Answer 2

This sort of problem isn't absolutely difficult, but there is quite a lot of tedious detail calculation (for example, of direction cosines) to be done. I suspect that you've made a slip somewhere. If you think it worth your time to show your working here, I dare say we could find it! (You're right about the negative axis, by the way).

Answer 3

Well, one way of dealing with that is to define the vectors in an orthonormal basis (ex, ey, ez)

We'll have something like that :

Fc = +/- 700 ex

Fb = +/- 600*cos(Fb^Fc)ex +/- 600*sin(Fb^Fc)ey

(adapt the signs to your example)



Indeed, it's easy to get the magnitude of a vector (here the sum of the two) once it is expressed in an orthonormal basis

if Fb + Fc = x ex + y ey.

|| Fb + Fc || = ( x^2 + y^2)^(1/2) ("cf Pythagor")