Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Forces w/o third-law partners???????



At 07:40 PM 10/20/99 -0700, Leigh Palmer wrote in part:

the force that acts on a body in an
Earth based lab, that I have been calling "weight", doesn't have a
third law partner. The gravitational force the body exerts on the
Earth isn't quite the same magnitude and it doesn't act in exactly in
the opposite direction.

I should have added that the centrifugal force accounts for this
discrepancy. It is not magic, and its not GR, either. It's simple
classical physics.

That appears to be tantamount to a statement that momentum is not conserved.

Are we really going to claim that gravitation doesn't conserve momentum?
Yikes!

Forces without third-law partners? Somebody please tell me this doesn't
say what it seems to say.

Are you under the impression that vertical components of momentum are
conserved in the laboratory? If they were, we wouldn't need tables or
chairs. If you shift your frame of reference outside and include the
Earth in your system you might notice that the system is rotating on
its axis, and that the table has to exert an extra force over and
above the gravitational force on any object placed on it to accelerate
the object. That's why the object doesn't move with constant velocity;
it moves in a circle that it completes in one day.

The laboratory on the Earth's surface is not an inertial reference
frame. The vertical component of translational momentum is never
conserved in the laboratory; conservation of momentum only holds for
systems on which no external forces act.

Leigh