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Re: rolling

At 17:21 -0500 10/25/01, Kilmer, Skip wrote:

I encourage my students to be observant and bring interesting problems to
class. This afternoon one of them brought the following:
In the lab are some wheel and axles from old Hall's carts, two 2-cm steel
disk wheels attached to a 5-cm steel axle. He put one of them on an inclined
board, about 15 degrees incline, and it rolled down the board. Next he
turned the wheels about 20 degrees to the slope, and it rolled down 20
degrees to the slope. His question, and mine to anyone who wants to play
with it, is why does the thing roll sideways? We solved it to his
satisfaction, but not really to mine.

Here's my take on it. When you tilted the axle relative to the axis
of the slope, you put a sideways load on the wheels, but since it
apparently wasn't greater than the maximum possible static friction
force, it wasn't enough to get the wheel set to slip along the slope.
Did you try larger angles to see if you could get the downward
component of gravity to put a large enough sideways load on the
wheels to get them to slip?

My recollection is that those wheel sets are reasonably heavy and
pretty uniform, so the sideways force would be just about the same on
both wheels, meaning that, unless it actually starts sliding
sideways, it won't rotate and so it should roll "sideways" across the
incline.

Does this make any sense to you?

Try taking an unpowered toy car (one, all of whose wheels are free to
rotate) and lock the rear wheels and set it to rolling down the
incline (I usually do it by taping the axle to the body of the car
with masking tape). What happens? Then reverse ends and do the same
thing, so that the locked wheels are in front. Now what happens? Does
it make what happened with the Hall Cart Wheels make any more sense?

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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