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Homework Help: Length contraction equation help

  1. Jan 28, 2016 #1
    1. The problem statement, all variables and given/known data

    After year of over-eating and no exercise, Uncle Joe's is overweight, with a waist 50cm wide. He's also out of shape and can only hold his breath for 20 seconds. Worse, he can only jump 20cm high. But at his high-school reunion, he'd like to fool his old friends who haven;t seen him in years, into thinking otherwise by using special relativity. If he speeds by them on a vehicle that's traveling horizontally at (3/5)c, and he's standing vertically, holding his breath, and then jumping vertically, what will they observe for Uncle Joe's waist size (width), breath-holding time, and jumping height.
    2. Relevant equations

    L=L0 Sqrt(1-(v/c)^2)
    3. The attempt at a solution
    where v=3/5c c= 3.0*10^8 and L0= 50

    I want to know if this is the right equation and if im using it right
  2. jcsd
  3. Jan 28, 2016 #2

    Simon Bridge

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    Depends on your reasoning ... that is certainly the length contraction equation: ##L = L_0/\gamma## this means that ##L<L_0## - is that what you want?

    note: ##v \neq 3/5c## you are told that ##v=(3/5)c = 3c/5## (pedantic I know, but pedantery is important in relativity)
    ... this means that ##(v/c)=3/5 \implies (v/c)^2 = 9/25##, so now you don't need a value for ##c## for your equation... in fact the numbers have been chosen so you don't need a calculator.
    Also - what about how long he can hold his breath for?
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