Equipment:
Theory: For an object stuck to the wall of a rotating cylinder, there are three forces to worry about. The force of gravity is balanced by the frictional force, and there is also a normal force directed out from the wall, pointing towards the center of the circular path. The normal force, in other words, accounts for the centripetal acceleration of the object.
The faster the rotation rate, the larger the normal force. The maximum value of the static force of friction is proportional to this normal force, so it also increases as the rotation rate increases. As the cylinder slows the normal force is reduced, as is the maximum value of the static force of friction. When the magnitude of the maximum force of friction drops below the force of gravity, the object will slide down the wall.
Description: This is a miniature version of the Barrel of Fun carnival ride in which people stand against the wall of a rotating circular platform. When the platform is spinning fast enough the floor drops away, but the large frictional force prevents people from sliding down the wall.
Assemble the support for the cylinder by bolting the three support screws to the platform and sliding a wooden block over each screw. The cylinder will slide down between the supports onto the platform. Spin the cylinder by applying a force to the inner wall of the cylinder; when it is spinning rapidly inject an object into the cylinder near the top. Try to match the object's speed to the rotation speed of the cylinder. A piece of sponge, or a rubber stopper, or something equivalent, will stick to the wall quite nicely. This will suddenly fall down the wall at some point as the cylinder slows.
Note that a short film of the carnival ride is available on laserdisk.
Setup time: 15 minutes
References: (available in the demo room)
Other demonstrations in this category : Ball on a string 1D50.10 -- Whirligig conical pendulum 1D50.20 -- Pail of water 1D50.40 -- Penny on a coathanger 1D50.45 -- Rolling chain 1D50.70
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