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Work/Kinetic energy, running a race with mass and speed

  1. Mar 3, 2013 #1
    1. The problem statement, all variables and given/known data
    You run a race with your friend. At first you each have the same kinetic energy, but then you find that she is beating you. When you increase your speed by 21%, you are running at the same speed she is. If your mass is 81 kg, what is her mass?
    Your mass: 81kg
    Your speed is increased by 21%
    Her speed:?

    2. Relevant equations
    KE=1/2mv2



    3. The attempt at a solution
    1/2mv2=1/2mv2
    1/2(81)v2=1/2?v2
     
  2. jcsd
  3. Mar 3, 2013 #2

    haruspex

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    That's not a terribly useful equation. Try using different symbols for different variables, e.g. the two masses and the two speeds. If you find subscripting a pain, combine upper and lower case, like Ma, Mb.
     
  4. Mar 4, 2013 #3


    Apart from the fact that you should give different names, you are considering both kinetic energy and speed equal at the same time... but it is not what the problem asks you...

    When kinetic energy is the same, speeds are different; speeds are equal after you increasing speed. You will need two equations for solving this
    Hint: write in equations the two different informations I gave you... it will solve by itself then :smile:
     
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