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Homework Help: Kinetic Energy Speed Increase

  1. Oct 14, 2008 #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 77 kg, what is her mass?

    2. Relevant equations


    3. The attempt at a solution

    K(initial me)=1/2*77*.79v
    K(initial friend)=1/2*mass*v


    the v's cancel out so the mass should be 30.415/.5 or 60.83 but this is not the right answer. Does anyone know what I did wrong?
  2. jcsd
  3. Oct 14, 2008 #2
    You started out with the right idea for setting up the problem, but there are a few errors.

    1. Check the math for the relative velocities. If you started out 21% slower than your friend, you were not going 0.79Vfriend.

    Try using Vfriend = 1.21Vme.

    2. Check the formula you used in your step 3. Velocity should be squared. Remember to calculate the entire term (1.21V)2.

    Once you make these changes, you should get the correct answer.
  4. Oct 15, 2008 #3
    Okay i got it. Thank you so much!!
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