351–360 of 451 posts
I'm trying to understand what is meant by this. When I drop an item on the floor it goes there in a straight line, not a parabola. Same if I drop something from a helicopter. Obviously I'm missing something here. Can someone with more insight ELI5 this to me please?
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I thought that the Earth curved spacetime, and since an inertial observer approaching the earth would follow a straight line through curved spacetime, they'd appear to be following a curved line through space towards the earth. What's confusing me here is the notion that when two objects collide, they accelerate into each other. Why and how is force constantly applied after the collision? My intuition is falling down…
1st sentence is correct and is the same as what I said, sorry if I wasn't clear. Remember that spacetime = space + time dimensions. The object is always travelling through time, and the curvature of spacetime is converting some of that speed through time into speed through space. That's what you perceive as motion (caused by gravity). Time and space are linked together. The faster you go through space, the "slower" y…
Your explanation makes sense, and it sounds like I'd need to understand the maths behind relativity to be able to really understand how objects behave in spacetime.
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> Physicist here, gravity is a force, just a different one. Isn't gravity an acceleration, and weight a force? Otherwise gravity would accelerate heavier objects more slowly.
A person who has studied physics in a good school here. Weight is the force a body exerts on its support. Force of gravity is a force exerted by each body in a system onto every other body.
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> if you are inside of the sphere, at least some of the mass will be pulling you away from the point at its center If the mass is spherically symmetric, this will not be the case; all of the Newtonian forces from the masses further away from the center than you are will cancel out. This is called the "shell theorem", and it turns out to hold even in General Relativity.
I don’t think this is correct. I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. A perfect sphere of uniform density would not meet the shell theorem assumptions.
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Why would acceleration not be constant? (Assuming the ship isn't moving from the center of the earth outward or something like that.
> Why would acceleration not be constant? Look up the Bell Spaceship Paradox. In relativity, two spatially separated objects (or two ends of a spatially extended single object) that have the same proper acceleration in the same direction do not stay at rest relative to each other; they move apart, as seen in each of their own frames. This is different from the behavior predicted by Newtonian mechanics. In order to ha…
Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…
However, that concept is not available in arbitrary manifolds, you need additional structure: The covariant connection, which allows you to parallel transport velocity vectors, enabling you to define the concept of straight lines (autoparallels, which will be geodesics if the connection is 'metric').
According to general relativity, gravity hooks into that. So from that perspective, the gravitational force on a test particle will be a consequence of (the generalization of) the first law instead of the second one, making it into a pseudo-force like the Coriolis force.
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> I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. No, it's stronger than that. It says that any spherically symmetric distribution of matter outside a certain radius exerts no "gravitational force" on anything inside that radius, whether there is matter inside that radius or not.
What if my distance to one point on the shell was zero? Would I not feel infinite acceleration towards the massive point on the shell that I was infinitely close to?
How is it not a force though? Regardless of curvature, a ball starts moving if you let it go without applying any force. Curvature alone can't account for that could it?
Imagine a bowling ball and a marble at rest from your perspective, and an apparatus with a pair of pneumatic guns that eject pistons with the same precisely-calibrated amount of force. You arrange the guns so that they will hit the marble and the bowling ball at the same moment, and you trigger them together. The marble and the bowling ball are hit at the same time by the same amount of force. The marble, being much lighter takes off much faster than the bowling ball.
Now take the same marble and bowling ball to a place a mile above the moon (so that there is no confounding atmosphere to complicate things) and release them next to one another at the same moment.
If gravity is a force, then we should expect that the marble will fall much faster than the bowling ball, because the same force is acting on two different masses; the lighter mass should be accelerated more, just as it was when the source of the force was the pneumatic gun. F = ma, after all. If the force is the same and the mass is less, then the acceleration must be more.
That's not what happens, though. The marble and the bowling ball fall together at the same accelerating rate.
Gravity acts like an acceleration, not a force.
Lo these many years ago when I was an undergraduate physics student, my advisor told me that we should say "the force due to gravity", not "the force of gravity".
In every day colloquial speech it doesn't matter, of course.
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> So if you want to be really pedantic, it's never an ellipse because the Earth is not a point mass. This turns out to be not pedantic but very important if you're guiding an ICBM. And when landing on the Moon, Apollo had to deal with irregularities in the Moon's gravity due to mass concentrations, called mascons. (If you're interested in missile guidance, take a look at the book Inventing Accuracy . Among other thin…
Do we have to use this knowledge to toss nukes at each other? Surely there's something else we could be doing instead?
ICBMs were never used to toss nukes at people.
Pretty sure their existence saved the world from WW3.
> Surely there's something else we could be doing instead?
The application most useful in everyway life is GPS.