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Homework Help: Theory of relativity help

  1. Jan 16, 2008 #1
    Alright, i have a final and i have to explain just three parts to einstein's theory of relativity...... they are space travel (time slowing nearing the speed of light), increase of mass with speed, and length contraction..... the space travel makes no sense, and the mass one, i have mixed feelings about.... i know that mass is the amount of matter something has, but as somethign speeds up, it gets more matter? it doesn't make sense..... and the length contraction seems backwards too.... normally when you see a car fly by really fast, it looks a little longer.... man if i were einstein, then i would know, but seeing as how i am just in high school, it doesn't make to much sense... please help
  2. jcsd
  3. Jan 16, 2008 #2


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    First of all, mass is NOT "the amount of matter something has". Mass is a measure of inertia- it measures how much force it takes to change the velocity of an object. That is what increases with speed.

    I'm not sure at all what you mean by "normally when you see a car fly by really fast, it looks a little longer". I've never noticed that! In any case I assure you that you have never seen "a car fly by" at speeds where relativistic effects would be noticed! In order for the contraction be as much as a millimeter, say, on a car of length 4 meters, the car would have to be going about 97% the speed of light or about one 1,700,000,000 miles per hour!

    I am puzzled why you would have to write three parts of Einstein's theory of relativity if you have not been taught any of this and there is nothing in your text book about it.
  4. Jan 16, 2008 #3
    well, our whole class is taking it apart and split it all into groups... we were chosen for those three..... now that i think about the car, it doesn't quite make sense.... but why does the length contract? i think i have figured out the space time thing, and i think i am grasping hold of the mass increase, but length contraction is what we need to work on now
  5. Jan 17, 2008 #4
    I'm assuming that you aren't actually being asked to explain 'why' these things happen, but merely 'how' things will happen.

    If you are at rest relative to the car:

    (1) time passes more slowly in the car than for you
    (2) yardsticks in the car are shorter than yardsticks are for you
    (3) two equal masses when both are at rest will be different if one of them is put in the car (that one will have greater mass than the one that you keep)

    The weird thing is, for someone in the car, all of those effects are reversed. If I'm in the car:

    (1) Time passes more slowly for people standing on the road
    (2) Distances will compress (the road itself will get shorter)
    (3) Masses on the roadside will increase

    This dual effect is one of the hardest things to understand about relativity.
    Last edited: Jan 17, 2008
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