negative mass, was Re: g

[John Gastineau <johng@pcep-physics.physics.ncsu.edu>]

Exactly. The negative mass still falls. Now what do you get if you tie a 1kg
mass to a -1kg mass with a string? What's the net force on it? Now cut the
string.

All this isn't original with me. There was an article a while back (in TPT?
I have no idea) titled something like "negative mass can be positively
amusing." 

To respond to Paul Camp's comment--I meant that for the sake of this thought
experiment that we were to _take_ both the inertial and gravitational mass
as negative, not that they were necessarily both negative. You're right--if
you separate the two the experiment gets curiouser and curiouser... 

Here is another fun one. Check out 

http://www2.ncsu.edu/ncsu/pams/physics/PCEP/impec/slowfall.html

This page holds a scan of a real letter that my department got some years
ago. (Actually, it arrived days before I was to teach F=ma to my first
college class. Prime material.) The author poses a serious question that
gets at the relation between gravitational force and acceleration very
nicely. I like to give the letter to my students and ask them to criticize
the author's physics. The process gets them working at a surprisingly high
level. 

BTW, the web page is part of a class site that's really designed only for
internal use. Poke around, but it is not pretty. 

JEG

At 03:46 PM 3/31/96 -0800, you wrote:
> > Fun thought experiment: what would happen if you release a -1kg ball? Both 
> > the 
> > gravitational mass and inertial mass are negative. Your immediate response
> > probably isn't correct.)
> >
> > JEG
>
> Since F=mg, and g is downward and m is neg., then the force would be 
> upward.  Then, since a =F/m, F is upward, and m is negative, the 
> acceleration is downward.  So it would behave exactly the same as when 
> grav. and inertial mass are both positive (but only for this special 
> case, we can't have friction, for example, adding to an object's 
> acceleration, can we?).  
>
> Am I right?  :)
>
> Having fun,
> Dave
>
> David J. Hamilton, Ed.D.                    "And gladly wolde he lerne,
> Franklin HS, Portland, OR                    and gladly teche."
> djhamil@teleport.com                          Geoffrey Chaucer


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John E. Gastineau                                PV34PV3P
North Carolina State University Physics
Physics Courseware Evaluation Project
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