How many revolutions per minute is the sample making

In summary, a centrifuge is a device that rotates a small container of material at a high speed to separate different components. In this conversation, the discussion centers around determining the revolutions per minute of a sample in a centrifuge, given its centripetal acceleration and radius. After calculating the acceleration and converting it to m/hour, the total distance of one rotation is determined and used to find the revolutions per minute. However, there is an error in the calculation of the acceleration and a correction is suggested.
  • #1
Serena_Greene
9
0
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)?

Thanks!

-Serena
 
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  • #2
8.75 x 10^3?
why change to m/hour?
a=w^2*r, find w and then find the answer.
 
  • #3
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.
 

1. How do you measure revolutions per minute (RPM) for a sample?

The most common way to measure RPM for a sample is by using a tachometer, which is a device that measures the rotational speed of an object. This can be done by attaching the tachometer to the rotating part of the sample and reading the RPM on the device's display.

2. What is considered a normal or average RPM for a sample?

The average RPM for a sample can vary depending on the type of sample and the experiment being conducted. However, a typical range for RPM is between 100 to 5000 RPM. It is important to consult the experimental protocol or previous studies to determine the expected RPM for a specific sample.

3. How can you calculate RPM if you do not have a tachometer?

If a tachometer is not available, RPM can still be calculated by measuring the time it takes for the sample to complete one full revolution and using the following formula: RPM = 60 / time in seconds per revolution.

4. Can the RPM of a sample change during an experiment?

Yes, the RPM of a sample can change during an experiment due to various factors such as changes in temperature, friction, or external forces. It is important to monitor and record the RPM throughout the experiment to ensure accurate data collection.

5. Is there a maximum RPM for a sample?

The maximum RPM for a sample can vary depending on the material, size, and design of the sample. However, most samples have a maximum RPM limit to prevent damage or failure. It is important to follow the manufacturer's guidelines and safety precautions when determining the maximum RPM for a sample.

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