Write4U - 08 November 2011 02:15 AM

I am not convinced that when a particle enters the gravity field of a massive body (which may extend for millions of miles), that massive body is affected by the gravity field of that particle (which may extend less than millimeter).

Well, I would say, get used to it. It is Newton’s third law:

The mutual forces of action and reaction between two bodies are equal, opposite and collinear.

Let’s do the calculation. You throw a weight of 1 kg with a velocity of 12 km/s away (that is you throw it away that hard that it leaves the earth’s gravitational field ).

That means you gave it a momentum of p = m.v = 1 kg x 12 km/s = 12 km.kg/s
The earth’s mass is 6×10^24 kg (a 6 with 24 zeroes).

Now the earth will get a velocity of your throw of v = p/m = 12/(6 x 10^24) = 2 x 10^-24 km/s in the opposite direction.
That is 3600 s/h x 2 x 10^-24 km/s = 7.2 x 10^-21 km/h: 0.000000000000000000006 km/h. Do you think somebody notices?

Of course, a 1 kg meteorite flying by at millions of kilometers away will have less impact, but it will cause a tiny change. A photon even less. But if you want, you can try to calculate now for yourself.

Edit: typo

OK. lets throw this rock not from the earth (which would indeed affect te momentum of the earth), but throw it into the gravitational field of the earth from outside that field, where it begins to fall toward the earth along the earth’s gravitational field. This action does not affect the earth, until the rock’s gravitational field actually is in range of the earth and begins to exert it’s own gravitational effect on the earth. Then there would be a mutual interaction. It is conceivable that the rock (with sufficient speed) may be thrown clear of that gravitational field and never have any influence on the massive body itself.
Unless everything has an infinite gravitational field, why would a particle have any effect on a massive object which is outside of the gravitational field of the particle? But I can imagine that a massive object with an extended gravitational field begins to affect the particle long before the particle’s gravitational field distorts that massive object’s field to a degree that it affects the earth itself.  IMO with objects of different mass, their gravitationally effective distance and influence must be figured into the equation.

Write4U: it makes no sense to ask if Newton’s 3rd law is true.

If a massive body changes the trajectory of a particle (by its gravitation or its electric charge or what ever), then the trajectory of this massive body is changed too, but the change will be inversely proportional to the proportions of their masses.

And a gravitational field is infinite, even when at great distance becomes smaller and smaller. The formula for calculating gravity G.M1.m2/(r^2) never becomes zero for any distance r you fill in, and M1 can be huge, and m2 tiny small, both masses feel their conjoint gravity.

This is not a question of opinion. This is established science.

Just to repeat myself: you must not mix up what is scientific fact, scientific speculation, philosophy and free wheeling ideas. Studying the first three is a good antidote against the last.

GdB,

Thanks for indulging my ignorance. But your explanation that an object (any object) has an infinite gravity field clears up a lot.

I had a visual of a car speeding along on a dusty road.  It seemed to me that the dust in the pocket of turbulence behind the car would have no influence on the car or its speed. But perhaps I am wrong there also..

Write4U - 08 November 2011 01:21 PM

I had a visual of a car speeding along on a dusty road.  It seemed to me that the dust in the pocket of turbulence behind the car would have no influence on the car or its speed. But perhaps I am wrong there also..

Well, the gravitational influence is also very, very tiny, but it is there. More important is all the work done by the car to move all the dust particles on the road (where it warms up the road due to friction) and the swirling up in the air. This energy loss you will recognise eventually by your increased fuel consumption.

GdB: ...you must not mix up what is scientific fact, scientific speculation, philosophy and free wheeling ideas…

Absolutely, though I would think that scientific speculation, and philosophy should not totally insulate itself from “free wheeling ideas”, lest it lose a source for creative devlopment. And even what is considered scientific fact is subject to change should new and reproduceable data consistently show that the underlying assumptions no longer hold.