Re: Atwood's machine problem

[Steve Clark <sclarkphd@MINDSPRING.COM>]

I recognize the problem. I have been giving it on my fall final exam
for the past several years (the students don't seem to be passing the
solution on to the peers from year to year, so I keep giving it). I got
it (and many others) from a book entitled "Mad About Physics" by
Christopher Jargodzki and Franklin Potter which I bought a number of
years ago. I haven't checked amazon.com to see if it's still around,
but I bet it it is - the copyright date is 2001 and it's published by
John Wiley & Sons. The cover price is $17.00. I thought a very
worthwhile investment.

The book is one of those where they ask questions in the front part and
answer them in the second. As best I can tell, the answers are correct.
And they give a very thorough solution to this problem.

By the way, I think these kinds of problems are great for students to
work out. However, I have to give quite a bit of guidance for this
problem. I ask the students to first draw free-body diagrams on each
object and to remember that the tension in the rope is the same
everywhere (we're assuming a massless, frictionless pulley and a
massless rope). Then I ask them to write Newton's 2nd law for each
object pictures - the monkey and then the bananas. Even with the help,
only about 1/4 of the students ever get this problem on their own. Some
of that may be because there is some pressure since it's a final exam
and some is undoubtedly because, even though they can sum forces and
set them = to ma, they don't quite believe the rope is what pulls up
the monkey. A typical third law misconception. But we keep trying.

Steve Clark, Ph.D.
Physics Instructor
Starr's Mill High School

On Friday, October 1, 2004, at 03:27  PM, Peter Schoch wrote:

> Hello all,
>
> I just got a question I can't answer, and could use some help.
>
> A student got this problem out of a book (didn't tell which one) and
> asked
> me, and I 'clutched':
>
> An Atwood's machine is in perfect balance, with equal weights on both
> sides.  One of the weights is a monkey, and the other is a stack of
> bananas.  The monkey begins to climb at a constant speed.  What
> happens to
> the bananas; ie, do they rise, fall, or stay stationary.
>
> My first inclination was to use CM and say that if the monkey climbs
> the
> bananas must fall to keep the CM stationary.  However, the book he
> Xeroxed
> this from, said the bananas rise with the monkey.
>
> Maybe it's just because it's Friday, but I don't see the 'mechanism'
> for
> that explanation.  Can somebody help?
>
> Thanks,
> Peter Schoch
> (sleep deprived with a 15 month at home)