# RPM Homework Problem

1. Sep 27, 2004

### Doug Desatnik

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 7.80 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 8.00 cm from the axis of rotation?
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Suppose the centripetal acceleration of the sample is 7.80 x 10^3 times as large as the acceleration due to gravity.

Question 1
Does this simply mean that the centripetal force is 7.8x10^3 x 9.80? IF so then 7.8x10^3 x 9.80 = 76400.

Question 2
I worked through the solution but the answer I get doesn't seem right to me. Can somebody maybe please point out where it is that I went wrong?

1. First I found the velocity

[tex]Ac=V^2/r[\tex] --> [tex]76400=V^2/.08[\tex] --> [tex]v=78.20 m/s[\tex]

2. Then I used the following equation

[tex]V=2\Pir/T[\tex] --> [tex]78.20=(2)(3.14)(.08)/T[\tex] --> [tex]T=.006s[\tex] --> [tex].0001 RPM[\tex]

.0001 Revolutions per Minute just seems incorrect for some reason :)

Sorry for the screwy latex code, I must be doing something wrong here to .....

Thanks in advance for the help!!

- Doug

Last edited: Sep 27, 2004
2. Sep 27, 2004

### krab

LaTeX: your [\tex] should be forward slash.

You've interpreted a time T directly as a circular rate. Wrong. T is the time of one cycle. So How many cycles per second?

Last edited: Sep 27, 2004