Re: [Phys-l] A question about the Earth's gravity

[John Mallinckrodt <ajm@csupomona.edu>]

> But the question is about the Earth's gravity.  Properly that is  
> the force
> that the Earth exerts on the body.

That's "proper" in Newtonian physics, but we know better now and have  
for nearly a hundred years.

I'm not saying that we shouldn't teach the antiquated, but still  
useful concept that bodies attract each other by virtue of their mass  
and separation, but we shouldn't actively work against eventual  
better understanding by pretending that centrifugal effects are  
merely unfortunate "corrections" to "real" gravitational forces.   
Indeed, the modern perspective is simple, practical, and easily  
explained: The "gravitational force" in *any* reference frame is m  
times the local value of g and the way you find the local value of g  
is by dropping an object and measuring its acceleration.

John Mallinckrodt
Cal Poly Pomona