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A not so simple question

  1. Mar 15, 2004 #1
    Hi, I'm very new to physics and I was just wondering if any of you might be able to awnser a question for me. Its probobly a question only someone who doesn't know pretty much any physics would ask, so if you all could humor me I would appreciate it :) Oh and sorry if I posted this in the wrong section.

    Ok here goes. I believe it was Einstein who said that gravity and accerleration are equal. Acceleration can not reach the speed of light (your mass would approach infinity?) so shouldn't it also be true that gravity can not reach the point where it's gravity (measured in acceleration) would equal the speed of light? And for light to be stopped or trapped by any kind of gravity, wouldn't the gravity have to be equal, in acceleration terms, to the speed of light? And yet isn't that exactly what a black hole does, or do black holes just affect light in a different way?

    Sorry if the above seems a bit disjointed, I'm having an attack of insomnia and lack of sleep tends to make me a little less coherent then normal. Anyway thanks for bearing with me.
  2. jcsd
  3. Mar 15, 2004 #2

    Chi Meson

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    first of all, Einstein did not ever say that gravity and acceleration are equal. While it is true that (in absence of air resistance) the gravitational field strength of a planet is equal to the acceleration of an object near that planet's surface, this is not the same as saying "gravity equals acceleration."

    Second of all,Special Relaativity suggests that the velocity of an object can not reach the speed of light. Acceleration is not the same as velocity; acceleration is the rate of change in velocity (how quickly you change how fast you go, such as "zero to 60 mph in 10 seconds" means an acceleration of 6 mph/sec.

    You are thinking some interesting thoughts, but what you suggest is unfortunately nonsensical because you don't have the definitions of these concepts entirely straightened out. Keep thinking new thoughts, but study Newtonian physics some more before taking on Einstein's relativity
  4. Mar 15, 2004 #3
    Agree with the Chi Meson.

    What you heard or read about "gravity equals acceleration" is Einstein's fundamental post of general relativity. I don't know much more about this subject yet, but here's what it actually means: a system free of gravitational forces which moves with constant acc. can't be distinguished of a inertial system in a grav. field of the same amount. In other words, you can't tell whether you sit in a rocket far from earth that travels with acc. g or you sit in the same rocket standing still on earth.
  5. Mar 16, 2004 #4
    Got it, thanks for clearing that up for me.
  6. Mar 17, 2004 #5


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    This only holds localy.
    As long as the rocket is not just a point it's possible to distinguish between acceleration and gravity.
    Last edited: Mar 17, 2004
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