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: Trig Function

  1. Sep 14, 2005 #1
    Trig Functions

    [tex]2y - 5 = \sin(144t - 45)[/tex]

    How can I find when the object is at equilibrum? I know it's when y = 0, but how do I solve from there? I tried arcsine but it gives me a domain error.

    How can I find the minimum in between [itex]0 \le t \le 10[/tex]?

    Is the period of oscillation 0.625 degrees?
     
    Last edited: Sep 14, 2005
  2. jcsd
  3. Sep 14, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What makes you think equilibrium is when y = 0? The midpoint of the motion will be when sin() = 0.
     
  4. Sep 14, 2005 #3
    Also, if you solve explicitly for y, you'll see that there is a vertical shift, meaning that the y-axis is not the midpoint of this graph. Use Doc Al's advice.

    As for the period, I didn't check your numbers, but remember in a sine graph in the form of [tex]a \sin{(bx+c)}+d[/tex] that [tex]\frac{2\pi}{|b|}[/tex] is equal to the period. That's in radians of course.
     
  5. Sep 15, 2005 #4
    Alright, I revised my answers given the replies so far. I think the period is 2.5 degrees, maximum height (the question is about a spring oscillating) is 7.5m and the first equilibrum is at t = 0.3125. Can anyone tell me if I'm correct?

    I'm still having trouble with the minimum in between [itex]0 \le t \le 10[/tex]?
     
    Last edited: Sep 15, 2005
  6. Sep 15, 2005 #5

    Doc Al

    User Avatar

    Staff: Mentor

    The period should be in seconds, not degrees. (144 is in what units?) Rewrite your expression like this:
    [tex]y = 2.5 + 0.5 \sin(144t - 45)[/tex]

    If you understand what this says, you should be able "read off" the equilibrium position, the amplitude, and the maximum and minimum values of y.
     
  7. Sep 15, 2005 #6
    Sorry, I meant at what times is the function at it's minimum between 0 <= t <= 10

    As for the period, is it correct to say 2.5s instead of 2.5 degrees? I used what Jameson gave me: 360/|b| = T

    I made the mistake of thinking the amplitude was 5 (no idea where I got that number, I've been juggling questions all night ;)... I see the max height should 3m, correct (assuming it's m vs t)?
     
  8. Sep 16, 2005 #7

    Doc Al

    User Avatar

    Staff: Mentor

    y will be a minimum wherever sin() is at its minimum, which is when sin() = -1.

    If the 144 is degrees/sec, then 2.5s is correct.

    Right. Since the sin function oscillates between -1 and +1, y will oscillate between 2 and 3.
     
  9. Sep 17, 2005 #8
    Thanks for your help!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook