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Angular kinematics

  1. Jan 14, 2014 #1
    Question

    The wheel of a car has radius 0.2m and initially rotates at 120 rpm. In the next minute it makes 90 revolutions
    What is the cars angular acceleration?


    Attempt

    So I tried to get the final angular velocity and couldn't and have tried multiple times with the angular kinematics equations but I'm not getting the correct answer

    Which is -0.105 rads^-1 b
     
  2. jcsd
  3. Jan 14, 2014 #2
    How many radians are there in 120 revolutions?
    How many seconds are there in a minute?
    What is the angular velocity of the wheel initially?
     
  4. Jan 14, 2014 #3
    I got the angular velocity as 4pi rad/s

    And I know θ= 180pi rad in 60 seconds
     
  5. Jan 14, 2014 #4
    Also I calculated the final angular velocity to be 3pi rad/s
     
  6. Jan 14, 2014 #5

    tiny-tim

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    Science Advisor
    Homework Helper

    Hi KiNGGeexD! :smile:

    This is essentially the same type of question as A car initially moves at 120 mph. In the first hour it moves 90 miles.
    What is the cars acceleration? …

    solve it the same way. :wink:
     
  7. Jan 14, 2014 #6
    Where did you get the 180 from? I get 240.

    Did you know that the equations for angular displacement, angular velocity and angular acceleration are of the same mathematical form as those for linear displacement, linear velocity, and linear acceleration? Well they are. If θ is the angular displacement, ω is the angular velocity, and α is the angular acceleration, what is the equation for θ as a function of time t in terms of the initial angular displacement θ0, the initial angular velocity ω0, and the angular acceleration α?

    Chet
     
  8. Jan 14, 2014 #7
    I did it that way exactly but still got that question wrong
     
  9. Jan 14, 2014 #8
    Θ= 90 rev * 2pi
     
  10. Jan 14, 2014 #9
    I got the same answer you got. Maybe they want it as -60 revs/min^2.

    Chet
     
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