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!

Angular motion using Newton's Laws

  1. Jun 19, 2007 #1
    1. The problem statement, all variables and given/known data

    A flywheel takes 5 seconds to rotate through 5 revolutions starting from rest. If it is accelerating at a constant rate during that time, what is the angular acceleration?

    2. Relevant equations

    ∅=ω t+at^2

    3. The attempt at a solution

    having done my calculations i have concluded that the angular acceleration is zero, please can someone confirm this?

    Cheers
     
  2. jcsd
  3. Jun 19, 2007 #2

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    how did you perform your calculations?
     
  4. Jun 19, 2007 #3
    i inserted the numbers into the equation above and in this resulted in:

    0 = 12.5 a

    0/12.5 = a

    therefore a = 0
     
  5. Jun 19, 2007 #4

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    If the weel starts from rest, the angular velocity = 0. yes, but 5 revolutions is how many radians?..

    If it during constant acceleration makes 5 revolutions in 5 seconds, how can the acceleration be zero then? There should ring a bell..
     
  6. Jun 19, 2007 #5

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    [tex] \theta = \theta _{0} + \omega_{0} t + \frac{1}{2} \alpha t^{2} [/tex]
     
  7. Jun 19, 2007 #6
    thats what i did think (hence the post), however does it make difference that its asking for angular acceleration?(sorry if this is a daft question, but due to me working on these numbers all day my head is in a bad state currently)
     
  8. Jun 19, 2007 #7

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    But after 5 relevolutions, how many radians have you then covered?..

    not 0...

    The units for angular acceleration is [rad/(s^2)], and angular velocity [rad/s] , and of course angle [rad]
     
  9. Jun 19, 2007 #8

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    malawi_glenn asked two key questions:
    1. If the flywheel makes 5 revolutions in 5 seconds starting from rest, how can the acceleration be zero?
    2. How many radians is 5 revolutions. Hint: It is not zero.
     
  10. Jun 19, 2007 #9
    ok after acquainting my head with the wall to gather my thoughts i now have a new answer.. brace yourself

    a= -6.28
     
  11. Jun 19, 2007 #10

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    well it should not be negataive because the weel is increasing its velocity..

    and your choice for angle covered in 5 revolutions was?
     
    Last edited: Jun 19, 2007
  12. Jun 19, 2007 #11
    ∅=ω t+at^2

    ∅=not sure, i thought it was 78.55, but since this gives a negative result then i guess its wrong.

    ω t = 157.10

    t^2 = 25

    can you point me in the right direction to working out ∅ as my chain of thought is wrong
     
  13. Jun 19, 2007 #12

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    well do you know how many radians ONE revolution is?

    Your negative result does not come from that. And I only see a "
    Square = not sure"
    Can you write in latex?

    Just plug in the total radians covered in 5 revs, in the forumla:

    theta = 0.5*alpha*t^2
     
  14. Jun 19, 2007 #13
    [ tex ] theta = ω t+at^2 [ /tex ]

    testing the latex
     
  15. Jun 19, 2007 #14

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

  16. Jun 19, 2007 #15

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    But do you know how many radians ONE revolution is?
     
  17. Jun 19, 2007 #16
    1 rev = 2 pi rad
     
  18. Jun 19, 2007 #17

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    good.

    5 rev = 10pi

    10 pi = (1/2) * alpha * 5^2

    20 pi / 25 = a = ?
     
  19. Jun 19, 2007 #18
    [tex] \theta = \omega t + {1/2} \alpha t^{2} [/tex]
     
  20. Jun 19, 2007 #19
    i cant relate this to me resolving theta?
     
  21. Jun 19, 2007 #20

    malawi_glenn

    User Avatar
    Science Advisor
    Homework Helper

    You were asked to calculate the angular acceleration, given that the weel started from rest and accelerated constant so it fulfilled 5 revs in 5 sec...

    Sorry I cant help you more that this.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Angular motion using Newton's Laws
Loading...