Calculating revolutions per minute

  • Thread starter Sosa
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  • #1

Homework Statement



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 4.60 x 10^3 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 4.59 cm from the axis of rotation?

Homework Equations


ac=(4pi^2r) / T^2

The Attempt at a Solution


rearranged the formula to get
T = sqrt (4pi^2r) / a
T = sqrt (4pi^2*0.0459) / 4.60 x 10^3 *9.8
T = 0.009358 *60 seconds
T = 0.561 rpm but it's wrong
 
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Answers and Replies

  • #2
Asymptotic
782
528
4.60 x 103 times as large
Or do you mean 103?
Where are your equations, and solution?
 
  • #3
Or do you mean 103?
Where are your equations, and solution?
updated it!
 
  • #4
Asymptotic
782
528
Is T supposed to be the time in seconds for one revolution?
Re-evaluate the applicability of the formula you are using.
 
  • #5
mjc123
Science Advisor
Homework Helper
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688
First, check your calculation. I have done your calculation several times, in different ways, in case you, or your calculator, misinterpreted your badly-written (in terms of brackets) equation, and in no case did I get 0.009358 seconds. The correct (I think) answer is of the same order of magnitude, but not the same number.
Second, you don't multiply the period by 60 to get the rpm. Dimensional analysis should tell you that that's nonsense.
 

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