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: Particle in Positive Direction

  1. Apr 10, 2012 #1
    1. The problem statement, all variables and given/known data
    d) When is the particle moving in the positive direction?
    [itex]f(t) = cos(πt/4), t ≤ 10[/itex]

    2. Relevant equations
    [itex]f '(t) = -(π/4)sin(π(t)/4)[/itex]

    3. The attempt at a solution
    [itex]0 < -(π/4)sin(π(t)/4)[/itex]


    [itex]t>4n 0<=n<=2[/itex]

    [itex]t>4 t>8[/itex]

    The answer says 8>t>4

    im probably skipping a simple step. wut do?
  2. jcsd
  3. Apr 11, 2012 #2


    Staff: Mentor

    So after multiplying both sides by -4/π, you get sin(πt/4) < 0.
    Sketch a graph of y = sin(πt/4) and you'll see that 4 < t < 8 is indeed the interval.

    I don't get what you did to arrive at the inequality below.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook