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negative mass, was Re: g

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

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

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

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

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.


At 03:46 PM 3/31/96 -0800, you wrote:
Fun thought experiment: what would happen if you release a -1kg ball? Both
gravitational mass and inertial mass are negative. Your immediate response
probably isn't correct.)


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,

David J. Hamilton, Ed.D. "And gladly wolde he lerne,
Franklin HS, Portland, OR and gladly teche." Geoffrey Chaucer

PLEASE do not reply to the email address in the header of this message; use
the address instead.

John E. Gastineau PV34PV3P
North Carolina State University Physics
Physics Courseware Evaluation Project
Box 8202
Raleigh, NC 27695-8202

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