# Tag Info

Accepted

### 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 ...
• 2,857
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### How come the Sun's gravity can hold distant planets in orbit, but cannot rip humans off Earth?

The Sun is keeping you close. After all, you are orbiting it just like the Earth. You don't fly off into space because the Earth and you experience the same acceleration due to the Sun's gravitational ...
• 28.4k
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### Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?

The Roche limit denotes how close a body held together by its own gravity can come. Since gravity tends to be the only thing holding moon-sized objects together, you won't find natural moons closer ...
• 8,863

### Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?

The Roche limit applies only to bodies which are held together purely by internal gravitational attraction. Compact objects such as artificial satellites are held together by the much stronger inter-...
• 8,510

### Why would black hole rip me apart?

You don't need GR to see this effect. It's due to tidal forces. Suppose you are 2 meters tall. Then the force of the Earth on your feet is $GMm/r^2$, and the force on your head is $GMm/(r+2)^2$. The ...
• 21.5k
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### Why do tidal waves appear so suddenly in some rivers?

What you are describing is called a tidal bore. Quoted from Wikipedia - Tidal bore - Description: Bores occur in relatively few locations worldwide, usually in areas with a large tidal range (...
• 40.1k
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### If the tidal bulge on the earth speeds the moon up, how does the moon move to a higher orbit?

To understand this, let’s start with a simpler example of orbital mechanics. Suppose we have a rocket in a circular orbit that wishes to transfer to a higher circular orbit. This proceeds in the ...
• 104k
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### Why would black hole rip me apart?

The problem is that when you are falling, all of you can't be in the same inertial frame. That is, whilst your centre of mass might be inertial, parts of your body will be feeling accelerating forces ...
• 133k

### Can we run out of gravitational (tidal) energy?

The moon does work on the Earth because the Moon orbits around the Earth at a different speed than the Earth's rotation. So, as time passes, different parts of the Earth feel the maximum strength of ...
• 24.2k

### How can a grain of sand be "spaghettified" when nearing a black hole?

Spagghetification occurs when the gravitational potential energy on the side of the grain of sand closer to the center of the black hole is much larger than the potential energy on the other side. The ...
• 51.5k

### How come the Sun's gravity can hold distant planets in orbit, but cannot rip humans off Earth?

First of all, planets are much, much more massive than you are and so the sun's gravity exerts a much larger force on the planets than it does on you. Secondly (and more importantly) you are much ...
• 893

### Why do tidal waves appear so suddenly in some rivers?

The phenomenon which you describe is known as a tidal bore. To understand it, one has to go beyond the strictly idealized linear theory for waves as taught in most courses of physics and consider ...
• 20.4k

### Why do some location on Earth have only one tidal maximum per day instead of two?

The tides are a result of the response of the Earth's oceans to the tidal forces exerted on the water by the Moon and the Sun. The responses are vastly complicated by the Earth's rotation about its ...
• 41.7k
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### If water is essentially incompressible, why are there tides?

incompressible: not able to be compressed. compressible: In thermodynamics and fluid mechanics, compressibility is a measure of the relative volume change of a fluid or solid as a response to a ...
• 234k

### Why aren't satellites disintegrated even though they orbit earth within earth's Roche Limits?

The Roche limit is a limit on objects being held together by their own gravity. Satellites are held together by much stronger forces. Different parts of the satellite are ultimately connected by ...
• 103k

### Can we run out of gravitational (tidal) energy?

Ultimately, the short answer is yes. The moon is slowly moving away from the Earth, at a rate of 4 centimeters per year. This is due to it being in a slightly higher orbit than equilibrium. At ...
• 320

### How can a grain of sand be "spaghettified" when nearing a black hole?

The difference in gravity between the "front" and the "rear" of a grain of sand sent towards a black hole should/must be negligible, considering how weak gravity as a force is, ...

### Is the tidal force the only real force-like quality of gravity?

The tidal force is electromagnetic as well. Suppose you are falling into a black hole and being spaghettified by the tidal force. If we turned off the electromagnetic force your atoms would just drift ...
• 358k
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### Relative size of the 2 tidal bulges

Yes, to some degree, but not for exactly the reason you're imagining. Even in the idealized model you're using, the size of the tides is not controlled by the strength of the moon's gravitational ...

### Does bottle water rise a little bit on full moon days?

Even the most rewarded answer here has missed out on the fact that the entire bottle of water will rise by up to a meter due to the phenomenon of earth tides. The body of earth is deformed by the same ...
• 25.6k
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### Tidal force in equivalence principle

The trick is in the definition of the word "local". What physicists typically call inertial coordinates or freely falling coordinates, mathematicians will call normal coordinates. The thing ...
• 22.3k

### If water is essentially incompressible, why are there tides?

I think you have started from a picture of the tides making water go up and down that is a natural picture to have, but it is wrong: they don't. If that picture were right, you ought to be able to ...
• 2,000

### Do you really die if spaghettified by a black hole?

When you are being stretched by the tidal effect of gravity, you really are being stretched. You can estimate this already using Newtonian gravity, and then G.R. simply adjusts the details of the ...
• 59.8k

### How come the Sun's gravity can hold distant planets in orbit, but cannot rip humans off Earth?

The sun is pulling you up (towards its center) with a force $$\rm F_{sun}=G \frac{M_{sun} × m_{you}}{R^2}$$ and Earth is pulling you down (towards its own center) with a force \rm F_{earth}=G \frac{...
• 2,241
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### If I jump into a black hole, will I see myself passing event horizon?

This answer is based on general relativity, which is currently the best foundation we have for understanding these things. For some round numbers, let's use $M\sim 10^{12}\,M_\odot$ for the mass of ...
• 54.5k
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### Possible non-negligible physical effects that relate planetary/lunar geometry and seismic activity such as earthquakes?

The planets do not affect Earth seismicity, but the Sun and Moon do. Earthquakes are caused by strains in the Earth's crust, and the gravitational strains produced by the planets, Moon, and Sun on the ...
• 11.7k

### Why do some location on Earth have only one tidal maximum per day instead of two?

OK, here's a theory: It's all (when I say all I mean mostly) got to do with the fact that the moon's plane of revolution is inclined about 20 degrees or so relative to the earth's equator. That causes ...
• 182

### If I jump into a black hole, will I see myself passing event horizon?

Nothing special happens at the event horizon. Time does not actually freeze at event horizons. There are two things that lead to this misconception: Things appear to freeze at event horizons, from ...
• 27.6k
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### Expanding Moon's orbit and newtonian mechanics

No. The moon and earth are not an isolated two body system. Specifically the reason the moon is getting further away is because of tidal friction - angular momentum is being taken from the Earth's ...
• 7,678