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"Consider a skydiver. normally we think of sitting still on the ground as the default mode and the skydiver who is deviating. The force of gravity reaches out from Earth to the skydiver and reals her in.
relativity flips ths reasoning on its head. freefall is the default mode. leaving aside air friction, the skydiver feels weightless- that is to say, she feels no gravity acting on her, artificial or otherwise. Only when the ground gets in the way does she feel any force, and even then, its not gravity but the force of whatever stopped her."
this and other confusing claims such as gravity itself produces gravity are reasons why (to me) gravity is so confusing.
Ok let's talk about Gravity for a second. something amazing no doubt. but why? why did GOD make it like that...to spin on earth to hold us down? can anybody chime on this one?
Simple enough, but God had nothing to do with it..
It is possible to describe gravity in the framework of quantum field theory like the other fundamental forces, such that the attractive force of gravity arises due to exchange of virtual gravitons, in the same way as the electromagnetic force arises from exchange of virtual photons. This reproduces general relativity in the classical limit. However, this approach fails at short distances of the order of the Planck length, where a more complete theory of quantum gravity (or a new approach to quantum mechanics) is required. Many believe the complete theory to be string theory, or more currently M-theory, and, on the other hand, it may be a background independent theory such as loop quantum gravity or causal dynamical triangulation.
"Consider a skydiver. normally we think of sitting still on the ground as the default mode and the skydiver who is deviating. The force of gravity reaches out from Earth to the skydiver and reals her in.
relativity flips ths reasoning on its head. freefall is the default mode. leaving aside air friction, the skydiver feels weightless- that is to say, she feels no gravity acting on her, artificial or otherwise. Only when the ground gets in the way does she feel any force, and even then, its not gravity but the force of whatever stopped her."
this and other confusing claims such as gravity itself produces gravity are reasons why (to me) gravity is so confusing.
I would argue for a different interpretation of what is happening in the skydiver analogy. Rather then the early reeling the sky diver in both the skydiver and the earth are being pulled together by gravity. Gravity is more apparently acting on the skydiver, but since the skydiver does have mass she is ever so slightly pulling the earth towards herself as well.
I would also say that leaving air friction aside is kind of like saying I was shot by a 12 gauge shot gun but aside from the slug I did not feel anything. Air friction is a direct result of the the potential energy of gravity being turned into kinetic energy via falling. Another notable side effect the skydiver is bound to feel from falling unimpeded is acceleration, which I would imagine is quite apparent until terminal velocity is reached at which point the all additional gravitational potential energy is converted into friction. Another notable sign of gravity in this situation is the person's awareness that they are falling quite fast. The reason for the sensation of weightlessness is because one's body is passing through a less organized bunch of molecules, that does not provide as much resistance as a more organized bunch of molecules would. The same is true on a lesser scale for the feeling weightlessness in a swimming pool.
As to what causes gravity it is unknown. Though it has been argued that mass curves space and that causes gravity.
Last edited by Randomstudent; 01-27-2010 at 03:42 PM..
You'd think so...but I have the strange feeling that the OP wasn't kidding--or rather, that he had an idea what kinds of responses his comment was going to generate, and all he was trying to do was start some arguments. (You can say the same thing for the other threads he's started recently).
"Consider a skydiver. normally we think of sitting still on the ground as the default mode and the skydiver who is deviating. The force of gravity reaches out from Earth to the skydiver and reals her in.
relativity flips ths reasoning on its head. freefall is the default mode. leaving aside air friction, the skydiver feels weightless- that is to say, she feels no gravity acting on her, artificial or otherwise. Only when the ground gets in the way does she feel any force, and even then, its not gravity but the force of whatever stopped her."
this and other confusing claims such as gravity itself produces gravity are reasons why (to me) gravity is so confusing.
Gravity is considered one of the fundamental forces of the universe. As stated earlier, everything physical has a mass. And everything with mass has gravity. The greater the mass the stronger the force of gravity. That doesn't mean an object must be larger though. Rather, it depends on the density of the object's mass. In your scenario of the skydiver, you have to understand that both the skydiver and the planet have their own "center" of gravity. The reason the skydiver falls to the ground is because the gravity of the planet is vastly greater than the gravity of the skydiver. It's pulling the skydiver in. Gravity doesn't just stop at the hard surface of the ground. It extends all the way into the center of the planet. In other words, if there was a deep hole that went clear to the core of the planet, the skydiver would (disregarding being vaporized from the heat) continue falling in toward the core. Everything is being pulled in toward the center point of the Earth's core. The International Space Station is in orbit around the Earth which would seem like it must be beyond the pull of Earth's gravity. But that's not true. Gravity is weaker there, but still strong enough to pull things in. Without periodic adjustments, the ISS would eventually crash into the planet.
Consider too NASA's training aircraft, the "Vomit Comet", which is used to simulate weightlessness. The passengers are not actually weightless. Both the aircraft and the passengers fall at the same rate. The illusion is caused because the passengers (who are inside the pressurized craft) don't feel the friction of air against their body they'd otherwise feel if they were free-falling outside of the aircraft. and because the interior of the aircraft appears to be stationary even though it isn't.
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