1. Not finding help here? Sign up for a free 30min 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!

Rotational movement

  1. Jun 14, 2017 #1
    A flywheel with a inertia moment of ##245 kg \cdot m^2## rotates making 20 round per second. The wheel stops 20 minutes after a braking moment action. Calculate the intensity of the braking moment

    $$ \omega = 20 \frac{round}{s} = 126 \frac{rad}{s} $$
    $$ t = 20 min = 1200 s $$

    The braking moment intensity is equal to the speed with which the angular moment changes. Because the wheel is stopped at the time ## t = 1200 s##

    $$ t = 1200 s $$
    $$ \Delta L = I \cdot \omega $$
    $$ \tau = \frac{I \cdot \omega}{t} = \frac{245 kg \cdot m^2 \cdot 126 \frac{rad}{s}}{1200 s} = 25.725 N \cdot m $$

    The text provides a result of ## 513 N \cdot m ##. What was wrong with reasoning?

     
  2. jcsd
  3. Jun 14, 2017 #2

    gneill

    User Avatar

    Staff: Mentor

    Your reasoning looks good to me. I suspect that the problem's given values have been altered at some point in order to make it a "new" question, but the answer was not updated to reflect the change.

    Note that the given answer is almost exactly 20 times what you've calculated. If I were to guess I' d say that the original question had a deceleration time of 1 minute rather than 20 minutes.
     
  4. Jun 14, 2017 #3

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It looks as if the book answer is a factor 20 off. Perhaps an earlier version of the exercise let it come to a stop in 1 minute ? And they forgot to update the answer in the back ?

    PS don't give a 5 decimal answer if all you are given is one or two decimals in the problem statement.
     
  5. Jun 14, 2017 #4

    scottdave

    User Avatar
    Homework Helper
    Gold Member

    I am stumped by your answer guide, as well. It's been awhile since I have solved one of these, so I went back and looked up to make sure I am figuring correctly, but I came up with the same answer that you did. In fact, I calculated that a 513Nm torque would stop it in 60 seconds.
     
  6. Jun 14, 2017 #5

    BvU

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Warm feeling that three of us are on the same line of tought ... :rolleyes:
     
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: Rotational movement
Loading...