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!

Homework Help: Please help before I go crazy

  1. Sep 15, 2009 #1
    1. The problem statement, all variables and given/known data
    A crankshaft with a diameter of 3.0 cm , rotating at 2300 rpm comes to a halt in 1.30 s . What is the tangential acceleration of a point on the surface of the crankshaft?

    2. Relevant equations

    3. The attempt at a solution
    -2300 rpm=138000r/s=13006.19m/s





    the answer is completely wrong.
    Last edited: Sep 15, 2009
  2. jcsd
  3. Sep 15, 2009 #2


    User Avatar
    Science Advisor

    What are the units on those numbers?
  4. Sep 15, 2009 #3
    sorry, edited
  5. Sep 15, 2009 #4


    User Avatar
    Gold Member

    did you convert the units, cm to m and rpm to rps?

    edit: you did, but:

    2300 rpm=2300r/m=2300r/(60 sec)=2300/60 r/s
    Last edited: Sep 15, 2009
  6. Sep 15, 2009 #5
    yes, if you see i said the radius = 1.5cm=.015m and 2300rpm=13006.19m/s
  7. Sep 15, 2009 #6


    User Avatar
    Science Advisor

    Giving names the variables so we know what is being discussed.

    W (angular velocity in radians per second)
    v (tangent velocity in meters per second)
    r (radius in meters)
    @ (angular acceleration in radians per sec per sec)
    A (tangential acceleration in meters per sec per sec)
    t (time in seconds)

    Wf is final and Wi is initial; but since it is going to zero, we can just use W = @t

    Drizzle is right. You've done the unit conversions in step 3 incorrectly.
  8. Sep 15, 2009 #7
    ahh finally!! im clumsy.. thank you
  9. Sep 15, 2009 #8


    User Avatar
    Gold Member

    no you’re not, just think logically next time after you did the conversion of units, i.e. could the number of rounds in a second be greater than the number of rounds in min of the same system or should it be less? just a bit of advice :wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook