Re: correct answer

[Maurice Barnhill <mvb@UDel.Edu>]

Steven T. Ratliff wrote:
> On Oct. 9, Mark Shapiro wrote:
> >       In preparing for class today I was reviewing some problems in
> > Halliday, Resnick and Walker (5th ed) - Chapter 7.  I believe the
> answer
> > to problem 15E on page 150 is incorrect.
> >
> >       The problem states that a 12 m long firehose is uncoiled by
> > pulling the nozzle end horizontally along a frictionless surface at
> the
> > steady speed of 2.3 m/s.  The mass/length of the hose is 0.25kg/m.
> How
> > much work has been done on the hose by the applied force when the
> entire
> > hose is moving?
>
> An interesting comment on this.  In the fourth edition, the problem
> is on p. 184 (problem 27E).  Everything is much the same except the
> statement of the problem to be solved:  "How much kinetic energy is
> imparted in uncoiling 12 m of hose?"
>
> This makes the problem easier, and the answer is certainly correct in
> this case (with the obvious reasonable assumptions).

No, the answer is still not correct.  The answer would be correct only
if no rotation were imparted to the rope, which is by no means
guaranteed.  In particular, the diagram in the text does not have any
kind of restraint to prevent the normal tendancy of the firehose to whip
around.
-- 
Maurice Barnhill, mvb@udel.edu                
http://www.physics.udel.edu/~barnhill/          
Physics Dept., University of Delaware, Newark, DE 19716