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Re: Conservation of ME and nonconservative forces

In terms of rolling friction:

The dissipating effects of rolling friction can often be quite small with
respect to the other option of sliding friction.

For example, I have a bare wooden piece of channel molding that I use as a
marble track. One end is raised with a lab stand, and then the track
flattens out and leaves the table.* The object is to get the marble to hit a
target placed on the floor. Most students pick up on the fact that they need
to know the speed of the marble as it leaves the table, and I allow them to
use a stopwatch or a timer system to get that velocity. Some try to figure
it out before the track is level -- that doesn't work well. Others don't
ensure that the track is horizontal when the ball leaves it -- another
problem. To make a long story short, they sort through many problems until
they are confident they can figure out where the ball will land before it
leaves the table. I then change the height of the starting part of the
track, and they must place the target. They can roll the ball, but it may
not leave the track.

Students love the lab, and they often want me to reprise it when
conservation of energy comes up. After all, they figure that if they know
the initial height, they should be able to calculate the bottom velocity.

But it doesn't work. The problem is that the ball doesn't slide without
slipping at the beginning. It slips a lot, but the angle is changing, and
calculating the energy lost to friction is awful. Once it starts rolling,
calculating the energy bound up in the rotation is a pain as well, since we
usually haven't taught that yet (but we will next year).

The students want to make the track smoother, thinking that will solve the
problem. It actually makes it worse in this case. I have a roller coaster
track made by Cambridge Physics Outlet. Many people are surprised that the
wooden track is has a rubber, rather than bare wood, channel. The students
understand why this helps by the time we use it.

Unfortunately, the rubber is getting harder, and it doesn't make the balls
roll so easily. There is more sliding before it starts rolling. Ideas on how
to recondition the rubber?

*Home Depot sells these wooden channel molding. It is flexible plastic with
a thin veneer of wood. A lot of them aren't sufficiently convex to keep a
ball in it, so bring one along to check. They are really handy.

Marc Kossover