Comments made by Brian McInnes were very important.
Perhaps the physics of the centrifuge cannot be made
meaningful in the first physics course, especially before
the chapter on liquids . The merry-go-round problem could
be used as a substitute in Chapter 7. Think about a coin
positioned at a distances r from the center; the maximum
coefficient of static friction is given. Starting from rest the
platform is rotated at a given angular acceleration. I saw
this problem in many textbooks. Here are some possible
questions:
a) At what angular velocity (or after how much time, for
those who want to add another step) will the coin start
sliding?
b) Why doesn't it slide when the angular velocity is too
small?
c) Suppose the coin is in a groove and can only slide
along the radius. The coefficient of kinetic friction is
given. Show how the centrifugal acceleration (with
respect to a scale along the groove) depends on the
distance from the center.
I already told students why I do not like the centrifuge
problem; I plan to discuss the above sliding-coin problem
next week. Yes, I know that it is also a plug-and-chug
problem. But it is more appropriate for my class. Once
they understand this problem they will have some very
vague idea about what is going on in a centrifuge.
Ludwik Kowalski