1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Block pull, Problems with trig to isolate for theta.

  1. Jul 28, 2013 #1
    1. The problem statement, all variables and given/known data
    What value(s) of θ will result in the block being accelerated 9.0m/s2 to the right?

    2. Relevant equations
    ƩFx = max
    N = mg - P sin(θ)

    3. The attempt at a solution
    P cos(θ) - μkN = max
    P cos(θ) - μk(mg - P sin(θ)) = max
    P cos(θ) + μk P sin(θ) = max + μkmg
    cos(θ) + μk sin(θ) = (1/P)(max + μkmg)

    From here I can't find a trig identity that will help me isolate for theta.
  2. jcsd
  3. Jul 28, 2013 #2
    ## a \cos \theta + b \sin \theta = r (\cos \alpha \cos \theta + \sin \alpha \sin \theta) ##, where ## r = \sqrt {a^2 + b^2} ## and ## \cos \alpha = a/r , \ \sin \alpha = b/r ##.
    Last edited: Jul 28, 2013
  4. Jul 28, 2013 #3


    User Avatar

    Staff: Mentor

    Another approach is to replace one of the trig functions (say cos) using the identity sin2 + cos2 = 1. Rearrange the resulting expression so that squaring both sides eliminates the square root.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted