Re: Forces on Free Falling Objects

[brian whatcott <inet@INTELLISYS.NET>]

At 09:29 11/2/99 -0500, Scott Beattie wrote:
> ... we're studying Newton's
> Third Law.  When reading one of the examples of acting and reacting
> forces in my text, I was baffled.  They gave an example of a boulder
> falling towards the earth with the earth's gravity pulling down on the
> boulder and the boulder pulling up on the earth.  I was thinking that if
> every falling object had a force within them that pulled up on the
> earth, than eventually a force could be found to overcome earth's
> gravity. ... Will we ever find a force great enough to halt a falling
> object to a rest? ...
> Scott Beattie

This phrasing, 'the force in a mass' is very similar to the way
 Newton described inertia in his great work "Principles" which
you can easily read in an English translation at your local
library (1)

But that was then.  Teachers prefer to describe the force of one
 object on another in a slightly different way these days.
Massive objects are always attracted to each other.
That's the force you have in mind. But you don't feel any force
if you are dropping freely. Until the wind blows hard.

Henry Cavendish who loved to experiment with physical effects created
an interesting experiment that could actually show up this force,
201 years ago.

 This is not so simple! Nobody can easily move massive objects around,
 the size of the Earth. That's what you'd need to do to change the
weight of things, very much.

Instead, he looked for a force acting sidewards,
where a much smaller force could just about be detected.

 He hung a strong fiber inside a long vertical tube.
 At the bottom of the tube was a wider can, where he hung a
cross bar carrying two lead bobs, one each end, each about
2 inches in diameter.

The fiber was hanging in the tube and the
tube was mounted on the can. The crossbar and bobs
were able to swing inside the can.
It protected them from the draft in his laboratory.

Both bobs were equally attracted to the Earth, but
at the side of each bob he set up a fixed lead ball about 12
inches in diameter (and heavy!), and he looked for the small
swing of the cross frame as the movable bobs
 were attracted to the fixed balls.

This effect is very very small. The sidewards force is much less
than a millionth as big as the weight (that's the downwards force)
 on the bob.

 He couldn't touch the bar or it would swing wildly.
 So he fixed a mirror to the bar and shone a candle light ray onto
 it as a kind of very light pointer.

People later called it "weighing the Earth" because you could work
out the mass of the Earth using his experimental results and a
Newtonian formula.

Ref (1)   Its in the reference section:
Newton: "Great Books of the Western World"
Encyclopedia Britannica Press

Sincerely,

brian whatcott <inet@intellisys.net>
Altus OK