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Name: Robert
Status: Student
Grade: 9-12
Location: SC
Country: United States
Date: October 2006


Question:
Centripetal pressure. I am interested in the pressure developed while spinning water in a container. From what I read objects spinning at thousands of revolutions per minute yield very high psi levels. This is what I am after. If I take about 8 ounces of water and spin it in a 6 inch diameter container at 10,000 rpm, what psi would I get?

Let us say a pipe was attached to the edge of the container. How much pressure would be in the pipe? What would it take to get 100 psi? More rpms, diameter, water?



Replies:
Hi Robert,

I think you are confused with regards to the difference between force and pressure. Force is the result of an acceleration (such as the acceleration due to gravity, or the acceleration of a rocket) acting on a mass. Pressure, on the other hand, is the result of a force acting on a unit area, thus the units of PSI, or pounds (of force) per square inch (of area). The same amount of force will produce different amounts of pressure, depending on how much area the force is acting on.

There is no such thing as centripetal pressure. Objects spinning at high RPM do NOT develop "high psi levels", or any pressure at all. They experience high force. In other words, the object being spun is forced outward as if it weighed much more than its normal weight. The formula for the force developed when an object is spun, is....

Centripetal Force = Mass x Velocity (squared) x Radius.

MASS is the mass of the object being spun VELOCITY is the speed of the object as it is spinning RADIUS is the radial distance from the axis center to the object.

Pressure is force per unit area, and in the above formula, you will notice that there is no mention at all of area. Therefore, trying to determine "pressure" is meaningless.

In your example, the 8 ounces of water, when spun at high RPM will be forced outward and appear to "weigh" much more than it does at rest. This effect is used when training astronauts. They are placed in a seat that is hinged at the top, and connected to the end of a long arm that is spun at high speed. As the rotational speed increases, the astronaut feels heavier and heavier, and experiences the same effect as the increase in weight felt when a rocket accelerates. The astronaut does not feel pressure. He simply feels that he weighs much more then normal; that is, his body is pressed against his seat with increasing force. Regards,

Bob Wilson.



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