If we compare it to a rope pulled by two equal forces in each end. The net forces would be zero and you would have static equilibrium, but the rope would still break if pulled hard enough.
Additionally, wouldn’t a metal ball uniformly surrounded by powerful magnets be pulled apart as well?
I am not sure why I can’t wrap my head around this, when it comes to the center of the earth.
In: 83
Yes and no, the absolute centre would experience net zero force, but anything off centre would experience a marginal gravitational pull in that direction.
Tension happens when different parts of an object are being pulled apart, and the internal binding forces of the object have to hold it together. When it happens with gravity, we usually call it a tidal force.
The thing is, when you’re inside a uniform spherical shell, there is exactly zero gravitational force from the mass in the shell. No matter where you are inside, each particle in your body will feel a net force of zero, so there’s nothing pulling it away from any other part of you.
So if you imagine the Earth as one or more uniform spherical shells that are outside you, and you’re living in a small cavity in the middle, there isn’t any gravitational force on you at all (from the Earth itself). So there’s no different force on different parts of you to pull you apart.
The shell thing is even cooler with electricity, because there are negative electric charges and they can move around. So you don’t even need a complete shell, or a net charge on the shell, and the charges in the shell can rearrange themselves to cancel out any field from anything else outside the shell too. This is called a “Faraday cage”
To be at the center of the Earth you would need to be in a hollow chamber right? So you’re in a hollow space surrounded by the rest of the Earth. Maybe we just simplify that to you’re inside a large spherical shell made of some dense material.
You know the rubber sheet model of gravity, where you put a heavy ball on a rubber sheet and it bends representing kind of what’s happening to spacetime due to gravity?
Lets imagine that spherical shell you’re in as 2D, so you’re inside a circle right? Lets apply that to the rubber sheet gravity and take a pipe to represent the circle you’re in and press it against the rubber sheet.
What happens? Everything outside the pipe bends down, representing how objects outside the original spherical shell would be pulled towards its surface due to gravity. But inside the pipe, the rubber sheet is just pulled flat. No curvature = you’re weightless inside the center.
So it’s not just that you’re being pulled equally in 2 directions by gravity, there is no curvature of spacetime so no force due to gravity at all so no kind of tension or any other force. It would be as if there was no gravity at all.
This should hold true even for very strong gravity fields (say if you were in the dead center of the sun), but I don’t know if going as far as say a sphere made of neutron star material would start to break things. Maybe a physicist here could help with that.
The force gradient is important. Most of the gravity is being cancelled out molecule by molecule.your body parts and being pulled evenly.
It’s believed that a smaller black hole would stretch and pull you apart because your feet would be getting pulled much harder that your knees and the other parts of your body. But then a significantly larger black hole could have a smoother transition where you would barely notice crossing the event horizon because while it’s pulling hard on you the different parts of you are being pulled just as hard.
Gravity just isn’t that strong of a force. If some massive force accelerated you into the ground, you would be crushed against the ground. Gravity is trying to do that to your right now – feel anything? At then center of the earth you would theoretically feel tension in every direction at once as your body is simultaneously pulled in all directions at the same time. But, it just wouldn’t be that much, you wouldn’t be pulled apart.
Tidal forces due to gravity can create tension and compression in objects, but the key here is scale. The object in question needs to be big enough to feel a difference in gravitational force across it’s volume, and your body just isn’t big enough, nor is the Earth’s gravitational field strong enough. But large bodies like Jupiter or Saturn can crush (weakly-held-together) asteroids and would-be moons with their tidal forces. An object with a really crazy gravitational field like a neutron star or a black hole could cause tidal forces across the size of your body that would kill you.
I guess if you were standing at a place equidistant from two neutron stars, that’s kind of like being inside a neutron star. And theoretically the gravity could be so great that you would be ripped in half.
In order for there to be tension, there needs to be different forces acting on opposite sides of the object. Inside a hollow sphere, all points have the same net force of zero due to the [shell theorem](https://en.m.wikipedia.org/wiki/Shell_theorem). It’s not just that the total force on the object as a whole is zero, the force is zero *for every point* on the object.
I think a common source of confusion is people thing the earth on the left only pulls on your left, and the earth on your right only pulls on your right. It doesn’t work that way.
The earth on the left pulls on both sides of your body, but the left side a little bit moreso. The earth on the right pulls on both sides of your body, but the right side a little bit moreso — it exactly cancels out the “moreso” on your left side, though.
So each part of you is pulled by all around, simultaneously, so the net force is negligible
If you were at the center of a hollow cylinder, you would indeed be pulled apart. This is because the force from a hollow cylinder is only zero at the exact center. Anywhere away from the center is pulled toward the cylinder wall, with a stronger pull the farther it is from the center.
A hollow sphere is special because the forces exactly cancel out everywhere on the inside, not just at the center.
Gravity pushes not pulls. Imagine a ball in space and space is opened up and pushing back on the ball the bigger and ball and the more dense then the more gravity is pushing back on it.
I don’t know if this is how it is usually done on this subreddit. But I just wanted to share what I have learned and I also believe I can explain it with the rope analogy.
Let us say we have a rope, pulled by 2N on each end.
Example 1;
2N <———————-> 2N This would give us Fnet = 2N-2N = 0 and a tension of 2N in the rope. Or a body, it would be pulled apart with enough force to overcome to tension.
BUT! Inside of the earth, or a hollow sphere. The forces that pull you, act everywhere at once so our rope would look like this instead;
Example 2;
2N-2N <———————-> 2N-2N This would give us Fnet = 2N-2N+2N-2N = 0 and a tension of 0N in the rope.
This is what I couldn’t wrap my head around at first. There is nothing pulling since everything cancels out. Since all the force are uniform and acting everywhere and not just two opposite forces on each end. I hope this helps others that were confused about this as I was.
Edits; Spelling and trying to make it looks cleaner.
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