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!

Can I use this solution? Angular motion

  1. Feb 11, 2017 #1
    1. The problem statement, all variables and given/known data
    Starting from rest, a wheel has constant α = 3.0 rad/s2. During a certain 4.0 s interval, it turns through 120 rad. How much time did it take to reach that 4.0 s interval?

    ω0 = 0
    α = 3.0 rad/s2
    θf = 120 rad

    2. Relevant equations
    Δθ = ω0⋅t + ½αt2

    3. The attempt at a solution
    120 rad = (1/2)(3.0 rad/s2)(t2)
    Solving for t:
    t = [2(120 rad) / (3.0 rad/s^2)]½
    t = 8.49 s
    Subtracting 4.0 s to the said time leads to my final answer, 4.9 s

    The book says that the answer should be 8.0 s.
     
  2. jcsd
  3. Feb 11, 2017 #2
    It says "during a certain 4.0s interval " it could be between any time interval.Your equation describes us what is the time when it makes 120 rad we are not looking for that,we are looking for a time interval which object makes 120 rad.
    Can you see the differance ?
     
  4. Feb 11, 2017 #3

    Doc Al

    User Avatar

    Staff: Mentor

    During the given interval, Δθ = 120 rad. But that's not measured from the starting point.

    You found the time it takes to go from the starting point to θf = 120 rad, which is a different problem.

    Set up two equations.

    Edit: Oops, didn't see that Arman777 just said the same thing. :-)
     
  5. Feb 11, 2017 #4
    It's still pretty vague to me from how you said it. So I'll try to explain as far as I can understand.

    At the beginnig it starts to accelerate up to a certain velocity. From that point up to 4 seconds, the said theta is measured until 210 radians.
    Is that how I should interpret the problem?
     
  6. Feb 11, 2017 #5
    It made some rad ##θ_1## between ##t=0## and ##t_1##,
    After 4 sec, which lets call is ##t_2## (or ##t_1+4##) it makes ##θ_2## rad (between ##t_2## and ##t=0##)
    In between those time intervals (##t_2## and ##t_1##) it makes ##120## rad.
     
  7. Feb 11, 2017 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, but I feel it could be expressed yet more clearly.
    It accelerates at 3 rad s-2 from rest for some time t, turning through some angle in the process. Continuing with the same acceleration for another 4 seconds it turns through a further 120 radians. Find t.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Can I use this solution? Angular motion
Loading...