1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    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!

Homework Help: Phase constant sign (quick question)?

  1. Sep 1, 2016 #1
    1. The problem statement, all variables and given/known data
    A = 20 cm
    Given the graph, what is the phase constant(in degrees)?
    (I have already solved for amplitude and frequency)
    2. Relevant equations
    x(t) = A cos (wt + ϕ)
    3. The attempt at a solution
    x(t) = A cos (wt + ϕ)
    Taking from t = 0, we find:
    10 = 20cos(w*0 + ϕ)
    10 = 20cos(ϕ)
    1/2 = cos(ϕ)
    cos-1(1/2) = ϕ
    ϕ = 60 degrees.

    I tried submitting the answer and it said check signs, so I entered -60 degrees.
    Why is this answer negative?
  2. jcsd
  3. Sep 1, 2016 #2


    User Avatar

    Staff: Mentor

    Hi tyrostoken, Welcome to Physics Forums!

    The cosine trig function is many-to-one: Note that cos(Φ) = cos(-Φ). So when you solved for Φ the answer might have been either +60° or -60°. It's up to you to check which one fulfills the given conditions.
  4. Sep 1, 2016 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    To add to gneill's answer...
    Imagine shifting the curve left until the value at t=0 is once again 0.5. This solves your equation, but the graph is visibly different. So you need another fact about the graph to fix the phase. What obvious difference is there in the graphs?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted