# Statics: two unknown angles and resultant force

by drnoisewater1
Tags: angles, force, resultant, statics, unknown
 P: 2 1. The problem statement, all variables and given/known data Two forces P and Q with respective magnitudes 100N and 200N are applied to the upper corner of a crate. The sum of the two forces is to the right (+x direction) with a magnitude of 250N. Find the angles that P and Q make with their sum - that is, with the horizontal line through +x axis. 2. Relevant equations R = P + Q where R is the resultant vector and P and Q are vectors given in the problem Rx = Px + Qx = 250 Ry = Py + Qy = 0 3. The attempt at a solution All the information is given except the two angles. Plugging in the given values gives me the equation: $$\Sigma$$Rx = 100cos($$\theta$$) + 200cos($$\phi$$) = 250N $$\Sigma$$Ry = 100sin($$\theta$$) - 200sin($$\phi$$) = 0 where theta is the angle between P and R and phi is the angle between Q and R. Basically this comes down to confusion on algebra for me. I attempted substitution and that got me nowhere. I know there is some kind of trick to solving this, but I cannot figure it out. There are two equations and two unknowns so there must be a way to do it. I have worked a similar problem where one of the angles is known and the other was supposed to be at a maximum and calculus could be used there. Is that a possibility in this problem or is there just a little trick that I am missing? 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution
 PF Gold P: 619 How about $$tan(\theta)$$ does that equal anything?
 P: 2 I tried that, but still had phi in the expression. Rx: sin($$\theta$$) = 2sin($$\phi$$) Ry: 2.5 - 2cos($$\phi$$) I thought that since magnitude of R is sqrt[Rx^2 + Ry^2] = 250 things might cancel out. Well all the angles canceled out so i just got an incorrect expression. I tried Ry/Rx just for fun, to get a tan expression and that was not beneficial because nothing canceled out there either. I just thought about this though: if you take tan inverse of Ry/Rx the resultant angle will be 0 since the resultant is about the x-axis. However, that means little as I don't know if you can evaluate arctan[0.8sin$$\phi$$) - tan($$\phi$$)]