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104 votes
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If gravity disappeared, would Newton's third law make everything that was pressed to the ground by gravity get pushed upwards?

As other answers explain, Newton's third law wouldn't push you upwards, because reaction disappears as soon of action (gravity) vanishes. However, we need to keep in mind that we are siting on ...
Pere's user avatar
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96 votes
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Does bottle water rise a little bit on full moon days?

Does bottle water rise a little bit on full moon days? No. Tidal forces are about the difference in gravitational pull at different points in the same body. For oceans and other very large bodies of ...
tom10's user avatar
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85 votes
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How can we see planets thousands of light years away but don't know if there are more planets in the solar system?

The problem with finding a new planet in our solar system is not that it is too faint, but knowing where to look in a big, big sky. This putative planet 9 is likely to be in the range 20-28th ...
ProfRob's user avatar
  • 129k
81 votes
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For Newton's gravitation equation, how do you account for planet size?

In general, you cannot treat arbitrary mass distributions as if they were concentrated at the center of mass for the sake of calculating the gravitational force, as your example shows. You can, ...
noah's user avatar
  • 10.3k
78 votes

If there is friction and everything stops on earth why does earth not stop due to friction?

For friction to occur there must be a medium which can exert the friction forces. For example if a raindrop falls through air there is friction against the air and it reaches some terminal velocity ...
Sebastian Riese's user avatar
78 votes
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Does a star need to be inside a galaxy?

No, stars do not need to be inside a galaxy. It is estimated that about 10% of stars do not belong to a galaxy [1]. While most intergalactic stars formed inside a galaxy and were ejected by ...
Thorondor's user avatar
  • 4,070
64 votes
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Is there a maximum distance from a planet that a moon can orbit?

The concept you're looking for is that of a planet's Hill sphere. If a planet of mass $m$ is in a roughly circular orbit of radius $a$ about a star of mass $M$, then the radius of this "sphere" is ...
Michael Seifert's user avatar
61 votes

Why does the Earth even have a magnetic field?

The crucial part is that earth's outer core is fluid, and that it's conductive. That the material happens to be iron which we know as ferromagnetic is actually rather unimportant, because the ...
leftaroundabout's user avatar
61 votes
Accepted

Does gravity get stronger the higher up you are on a mountain?

You are getting different answers from NASA and from other sources, as they are talking about slightly different things. NASA is talking about the acceleration of the GRACE satellite towards the earth,...
John Hunter's user avatar
  • 13.7k
42 votes

Simulating solar system with Newton's law

You need to use a better numerical method. Euler’s method is notoriously bad for orbital mechanics because the numerical errors always accumulate. In particular, Euler’s method does not conserve ...
Dale's user avatar
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40 votes
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Why no gravity lens around planets?

Contrary to the other answers, I will point out that gravitational lensing due to planets in the solar system is a significant and measurable effect. The measured positions of stars, as seen from a ...
ProfRob's user avatar
  • 129k
40 votes

Can we cool Earth by shooting powerful lasers into space?

Your idea to cool the Earth by shooting photons off into space is actually what already happens now! However, instead of a laser, the Earth cools itself by blackbody radiation. The Earth radiates a $...
Gary Godfrey's user avatar
  • 3,234
38 votes

Is Earth really flattened at the poles because of centrifugal force?

The description from an interial frame is that the bits of the Earth at the equator are moving faster than those near the poles, so they have more kinetic energy, and thus they are a bit farther away ...
Javier's user avatar
  • 28k
35 votes

If Earth had rings?

Yes, the rings would ultimately end up hovering above the equator (as defined by a planet's rotation). The mechanism leading to this steady state is gravitational asymmetry arising from a planet's ...
Selene Routley's user avatar
34 votes

Backyard experiments to falsify the Flat Earth theory

I live close to Lake Erie and often see scenes like in this picture. Note that the bottom of the cargo ship cannot be seen due to the curvature of the Earth.
James's user avatar
  • 1,871
33 votes

If gravity disappeared, would Newton's third law make everything that was pressed to the ground by gravity get pushed upwards?

Yes, but in almost all cases the push would be imperceptible. Reaction forces from surfaces occur when the molecules in the wall are displaced from their equilibrium position. The harder they are ...
Luke Pritchett's user avatar
33 votes

Why doesn't planet Earth expand if I accelerate upwards when standing on its surface?

Spacetime curvature makes this possible. Here's an analogy. There are two paths on opposite sides of the equator, at a constant distance from it. Someone walking east along the path north of the ...
benrg's user avatar
  • 26k
33 votes

For Newton's gravitation equation, how do you account for planet size?

So, you're right that it can matter. In this case it doesn't, because of a marvelous cancellation which may seem less marvelous after you see the “Gaussian pillbox” approaches of a classical ...
CR Drost's user avatar
  • 37.6k
30 votes

Backyard experiments to falsify the Flat Earth theory

Related to the Andrea di Biagio answer. Here is a typical flight path for the Beunos Aires-Auckland route. The distance is approximately 10,300 km by the shortest route along a sphere. Direct flights ...
ProfRob's user avatar
  • 129k
30 votes

Earth rotation: What provides the energy?

The original energy that caused the rotation of the Earth when it was formed came from the gravitational collapse of the cloud of dust and gas that formed the solar system. Maintaining rotation at ...
gandalf61's user avatar
  • 50.7k
29 votes

How fast does Haumea rotate?

From Wikipedia, the sidereal rotation period is 3.91 hrs, so the angular frequency of its rotation is $2\pi/(3.91 \, \mathrm{hrs}) = 4.48 \times 10^{-4} \, \mathrm{rad/s}$, or $7.12 \times 10^{-5} \, \...
Sebastian's user avatar
  • 1,045
28 votes

How can we see planets thousands of light years away but don't know if there are more planets in the solar system?

The reason why we can see exoplanets thousands of light years away but not a planet 200 AU away (about 30 light-hours) is because these planets are found using different techniques. The planet ...
NeutronStar's user avatar
  • 5,404
26 votes

Can the average length of the day and night of a planet be different?

It depends what you mean by day and night. The day and night are not of equal lengths now, where I live at latitude 53N. The tilt of the Earth's rotation axis with respect to the ecliptic plane means ...
ProfRob's user avatar
  • 129k
26 votes
Accepted

Why does the Earth even have a magnetic field?

The core of the Earth isn't a giant bar magnet in the sense that the underlying principles are different. A bar magnet gets its magnetic field from ferromagnetism while Earth's magnetic field is due ...
potential-at-infinity's user avatar
26 votes

Can we cool Earth by shooting powerful lasers into space?

Ambient heat is low grade energy. You can't effectively harness it to do work (which is a transfer of energy. i.e. powering laser which itself is a machine to transfer energy) because it has nowhere ...
DKNguyen's user avatar
  • 9,129
25 votes

Can the average length of the day and night of a planet be different?

I know that the opposite could happen. There is an old book called "Night Fall" about a planet that had three stars. Because there was always a star shinning on all sides it would never get dark. ...
Bill Alsept's user avatar
  • 4,027
25 votes

What is the accuracy of our knowledge about the planets orbits?

This one is tricky unless you know the magic term: ephemeris. An ephemeris gives the position of celestial bodies over time. Once you know that one, finding out information about their uncertainties ...
Cort Ammon's user avatar
25 votes

If another planet was opposite Earth, would we be able to observe it?

We could certainly be able to infer its existence through tracking the motions of the planets we can see and then simulating the gravitational motions of those planets. Adding a similar sized planet ...
Kyle Kanos's user avatar
  • 28.1k
24 votes

Is a westward flying plane heavier than an eastward one?

What you describe is called the Eötvös effect, named after the hungarian physicist Loránd Eötvös Eötvös devised ways to perform high accuracy measurements of gravity. For charting gravity over larger ...
Cleonis's user avatar
  • 20.5k

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