Physics, Astronomy & Cosmology / Re: Can gravity be said to act in two opposite directions at once ?« on: Yesterday at 21:46:55 »
Quote from: Halc
The sun exerts a considerably higher force than does the moon
Quote from: BoredChemist
the Sun's attraction is about 600 fold bigger (than the Moon)We need to clarify something that is tripping up some of the discussion and calculations above (and tripped me up, too, a few days ago).
Newton's gravity is described as an "Inverse Square Law", which means the force is proportional to 1/r2. By this measure, the Sun's gravitational influence on Earth is much bigger than the Moon's gravitational influence on the Earth.
But you can only directly measure Newton's gravitational force with giant bathroom scales if the two objects are at rest (stationary) relative to each other.
In reality, any planets, asteroids, comets or moons that we see today were not at rest relative to each other in the past, or they would have crashed together and be no more. All these remaining Solar System objects are in elliptical orbits, in "free fall". So the effects of Newton's gravity are cancelled out, when measured at the center of gravity of the object (eg at the center of the ISS).
Now we don't live at the center of gravity of the Earth (it's rather hot and pressured for my tastes), so we get a small gravitational deviation from free fall, called a tidal effect. As the difference between two points on Newton's gravity, it is an "Inverse Cube Law", proportional to 1/r3. By this measure, the Sun's tidal influence on Earth is slightly smaller than the Moon's tidal on the Earth, due to the Sun's greater distance.
So some of the contradictory conclusions above were both right - but some were talking about gravitational attraction (1/r2) which we don't directly feel on the surface of the Earth because most of it is cancelled by Earth's orbit.
While others were talking about tidal effects (1/r3) which we do feel on the surface of the Earth, even though the effect is slight.
In future posts, please clarify whether you are talking about gravitational forces or tidal forces, and why you think they are more important in this context.
Quote from: OP
Can gravity be said to act in two opposite directions at once ?Both gravitational forces and tidal forces can be handled as a vector addition at a single point.
And tidal forces can be calculated as a vector subtraction of gravitational forces between the center and surface of the Earth (two different points).