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Rolling Cans
Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993
Question:
Rolling Cans, why do they roll different?
I was rolling cans of soup with my students the other day and we
noted that they rolled down a ramp in very different ways. Some rolled slowly
and gained speed while others seemed to leap away after a slow start. Some
stopped quickly once they were off the ramp and others continued on for a
considerable distance. Could someone please help me with the physics of this
phenomenon?
Replies:
Sounds like a really fun experiment! Of course, it all depends
on what is in the can. If the can contains very viscous material, or
essentially solid material, like a can of molasses, it will roll as a solid
body, with the inside having exactly the same angular speed as the outside.
If the can contains a liquid, then the inside can behave quite differently,
and if there are several different materials inside, you could have quite
complex interactions, with heavy solid materials trying to stay on the bottom
of the can all the time, and therefore preventing its rotation to some extent,
for example. The basic physics is that of rotational angular momentum
coupling to the gravitational force pulling the can down the plane, and the
rotation caused by the fact that the surface of the can rolls, rather than
sliding. There should be some treatments of slightly less complex systems
(wheels rolling down inclined planes, for example) in a mechanics text that
discusses moments of inertia. Then, on the inside of your can, you have a
fluid mechanics problem, which should be treated in books that discuss
viscosity, since fluid motion in a rotating cylinder is a standard measure of
viscosity. And finally you have possible solid objects that are sliding
against the can on the inside, causing a frictional force. Hopefully, with
all these elements, you can at least model most of the different behaviors you
saw!
Arthur Smith
How about doing some experiments in which you control some of the
variables Arthur Smith mentioned? Roll a can containing pure water, another
containing molasses, another containing water and sand...do some experiments
at constant volume (full cans) and others at constant weight. Vary the size
of the can, keeping the contents fixed, and do this for several different
contents. See if you can justify the results against the physical principles
Arthur Smith described.
Topper
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