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Homework Help: Electromagnetic waves and astronaut

  1. Apr 10, 2007 #1
    1. The problem statement, all variables and given/known data

    I've been trying to do this problem for about 2 hours now. I can't seem to find the right equations to use. Any help would be appreciated

    A spacewalking astronaut servicing an orbiting space telescope has run out of fuel for her jet pack and is floating 20.0 m from the space shuttle with zero velocity relative to the shuttle. The astronaut and all her gear have a total mass of 150kg. If she uses her 220w flashlight as a "light rocket," how long will it take her to reach the shuttle?

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 10, 2007 #2

    Dick

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    How about this one, E=c*p. Where E is the energy of the light beam, p is it's momentum and c=speed of light.
     
  4. Apr 10, 2007 #3
    the answer is supposed to be 25.1 hours. When i used E=c*p i ended up getting 1250000 hours. Am i doing a simple calculation wrong?
     
  5. Apr 10, 2007 #4

    Dick

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    I get the 25 hour figure as well. Did you compute the force exerted by the beam? What are some of your intermediate results?
     
  6. Apr 10, 2007 #5
    well i first calculated p 220w/3x10^8 which was 7.33 x 10^-7
    then i divided that answer by 150kg to get 4.89 x 10^-9 which i'm assuming is my velocity
    from there i divided the distance from the ship which was 20m to get 2.44 x 10^-10
    and then i divided 1/2.44 x 10^-10 to get my answer in seconds
     
  7. Apr 10, 2007 #6

    Dick

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    Keep track of units. Then you won't have to 'assume' that something is a velocity. Power/c is a force. (Power/c)/mass is an acceleration. Not a velocity.
     
    Last edited: Apr 10, 2007
  8. Apr 10, 2007 #7
    so I took the square root of the 2.44 x 10^-10 1/s^2 to get 1.56 x 10^-5 1/s and i flipped the sign by 1/1.56 x 10^-5 1/s to get 64018.4 but then when i convert it back to hours it's only 17.8 hours when it is supposed to be 25.1 hours. Where did i make the mistake? thanks for the help
     
  9. Apr 10, 2007 #8

    Dick

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    The relation between distance, time and acceleration is d=(1/2)*a*t^2. Looks like you dropped the (1/2).
     
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