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Homework Help: Space travel, mass increase, and length shorteneing

  1. Jan 15, 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
    the problem is that you dont have the right definition of mass. it isn't the quantity of matter, thats the mole. mass is just an intrinsic property of matter. Even the new generation of scientist dont know anything about mass. (see higgs boson).
    You have to understand that formula in this way: No physical system with a mass M different from zero can reach the speed of light c. Otherwise you need an infinite ammount of energy.
    In any case i think you are confusing the rest mass Mo wich is always the same and the relativistic mass M=Mo/sqrt(1-beta^2). what remain constant and gives you the conservation (lorentz invariant) is the square of the 4 momentum. wich is
    p^2 wher p=(E,P) P is the common 3-dim momentum.
  4. Jan 16, 2008 #3
    im sorry i have just check your ID.
    i think that what i have told you in the last part is a little bit too much 4 high school.
    well in any case try on wikipedia maybe it can be more clear.
    especially it is written in english. i think i cannot comunicate in this language as well as in italian.

  5. Jan 16, 2008 #4
    lol..... it is fine.... i think i am starting to get it.... there are two mass types... one is relativistic, moving mass, and the invarient mass is resting mass...... i think i found out the spave travel one... time slows down relative to you because your body adjusted to that speed, and that is going to be normal speed for you, and your heartbeat will adjust to it too.... relative to people on earth, you are moving very fast (ver very very fast XD) and there body is set for that speed.... so ten minutes to you is going to be like three months to them...... the mass is going to increase, the relativistic mass, because i think that the force needed to push it using the equation F=MA will increase, and the acceleration will stay the same, therefore making the mass have to increase
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