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Re: conservation of momentum



On Thu, 15 Nov 2001, Justin Parke wrote:

If you will bear with me, I have another question about a
homework problem where the book and I disagree. This one is
from Halliday, Resnick, and Walker Fundamentals of Physics,
5th edition. p. 211 number 47.

"You are on an iceboat on frictionless, flat ice; you and the
boat have a combined mass M. Along with you are two stones of
masses m1 and m2 such that M=6m1=12m2. To get the boat
moving, you throw the stones rearward, either in succession or
together, but in each case with a cetain speed v(rel) relative
to the boat. What is the resulting speed of the boat if you
throw the stones
a) simultaneously
b) m1 then m2
c) m2 then m1?

book:
a) .2 v(rel) (I agree)
b) .210 v(rel)
c) .209 v(rel)

I don't get b) and c).

I don't either, to three sig figs I get .210 v(rel) for both b and
c. To four sig figs I find

b) (2/15 + 1/13) v(rel) = 0.2103 v(rel)
c) (1/15 + 1/7) v(rel) = 0.2095 v(rel)

But perhaps you are getting

b) (2/13 + 1/12) v(rel) = 0.237 v(rel)
c) (1/14 + 1/6) v(rel) = 0.238 v(rel)

If so, I suspect you aren't properly interpreting v(rel) as I did
as well at first. The speed v(rel) should be taken wrt the boat
after the throw, not before.

John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm