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How many revolutions per minute is the sample making

  1. Oct 22, 2005 #1
    A centrifuge is a device in which a small container of material is rotated at a high speed on a circular path. Such a device is used in medical laboratories, for instance, to cause the more dense red blood cells to settle through the less dense blood serum and collect at the bottom of the container. Suppose the centripetal acceleration of the sample is 8.75 x 103 times as large as the acceleration due to gravity. How many revolutions per minute is the sample making, if it is located at a radius of 2.80 cm from the axis of rotation?

    1st - figured out the accelration = 8.75E3 * 9.8 = 155 m/s
    2nd converted it to m/hour 155 m/s * 60s/h = 9300 m/h
    3rd calculated the total distance of 1 rotation = 2*.28*pi = 1.76 m
    4th divided velocity / distance = 5284 rpm

    But it says my answer is incorrect. What am I doing wrong (beside working on physics homework at 3am)?


  2. jcsd
  3. Oct 22, 2005 #2
    8.75 x 10^3?
    why change to m/hour?
    a=w^2*r, find w and then find the answer.
  4. Oct 22, 2005 #3


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    Homework Helper

    She meant m/min - there are only 60 seconds in a minute, and the problem asks for revolutions per minute.

    Serena, I've not worked through the whole thing, but I do see a problem in your step 1. You have 8.75 x 10^3 times about 10, but you end up with 155. It should be on the order of 10000. Also, the units are m/s^2, not m/s, but I assume that was a typo.

    Your methodology looks fine, though.
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