Seemingly simple system of equations

  • Thread starter ElTaco
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  • #1
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Homework Statement



There are two forces P and Q that are applied to a crate, with their respective magnitudes 100 and 200 N. One is applied at an angle upward with angle theta, and one is applied at an angle downward with angle phi. Both have positive horizontal components and the sum of the two forces has a magnitude of 250 N directly horizontal to the right. I need to find the angles theta and phi.

Homework Equations



None explicitly given.


The Attempt at a Solution



100sin(theta) = 200sin(phi)
100cos(theta) + 200cos(phi) = 250
theta = arcsin(2sin(phi))
Then I plug it in and cannot solve.

Another route I tried was keeping P and Q in instead of substituting in their magnitudes.
Psin(theta) = Qsin(phi)
Pcos(theta) + Qcos(phi) = 250
P = Qsin(phi)/sin(theta)
Qsin(phi)/tan(theta) + Qcos(phi) = 250
Again I cannot finish solving.

If something was badly worded or confusing please let me know. This problem is from a book so rather than writing the problem as exactly stated, I had to add some more information because there was a picture as well. Thanks for your time, I'm really stuck on this.
 
Last edited:

Answers and Replies

  • #2
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I'm assuming you at least have a graphing calculator. In function mode just do y1=250 and y2=100cos(arcsin(2sin(phi))) + 200cos(phi) = 250, which is what you had. Find the right range and domain and the intersection points will be your answer. you could also set it equal to 0 and fin the zeros.
 
  • #3
gneill
Mentor
20,925
2,866
Your two equations look like:

[tex]P cos(\theta) + Q cos(\phi) = F_h[/tex]
[tex]P sin(\theta) + Q sin(\phi) = 0[/tex]

Isolate cos(Θ) and sin(Θ), or cos(Φ) and sin(Φ) on one side of each equation. Square both sides of each. Add the equations. sin2 + cos2 = 1. Solve for the remaining angle. Note that sign information may be lost in the squaring process, so check the result for signs.
 
  • #4
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His solution is more intelligent. I recommend that.
 

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