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Re: [Phys-l] A Mechanics Question

I like Michael's answer.

He hit the main point, which is that it's a frisbee, not a point projectile. A frisbee flies. One needs to know rotation rate, launch angle relative to two axes, air density for the day, shape of frisbee, etc. - and after all that, one still cannot solve the problem :-(

Bob at PC


From: on behalf of Brian Whatcott
Sent: Sun 6/1/2008 11:47 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] A Mechanics Question

At 09:13 AM 6/1/2008, Michael E., you wrote:
Actually, since the roof is shingled, as usual, once the frisbee hits the
roof and begins to slide upward, it hits the edge of a shingle and it flips
over. Now it slides down the roof with its curved side facing downward, and
its friction is quite low, and as it picks up downward speed it almost lifts
off the roof and skims primarily with air friction. When it sails off the
roof, Billy catches it. We are unable to answer the question of how far it
landed from Billy's feet because we are not told how tall Billy is.
However, if he catches it at roughly the same height as he threw it, then it
is one meter from his feet.

On the other hand, if the frisbee did not flip, then it becomes stuck on the
roof because the sharp edges of the bottom edge of the frisbee stick on the
granules of the shingles. I know this from childhood experience, having had
to get the ladder out to retrieve any frisbee that did not flip on its top
when it hit the roof. I have a lot of experience climbing the ladder.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817

Michael is pointing to a certain lack of plausibility here, which
might be described as "mu is variable and at times reaches
high integer values(!)". However, the instructor has specified,
"..the frisbee [slides] up the roof before continuing past
the house" so we can easily suppose a corrugated aluminum roof
or something of that kind.

I can add that the instructor adds finally,
"Assume the frisbee hits the roof with the initial velocity."

[This was apparently a junior college level assignment.]

Brian Whatcott Altus OK Eureka!

Forum for Physics Educators