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Speed ratio

  1. Nov 2, 2008 #1
    Particle A has three times the mass and twice the kinetic energy of particle B. what is the speed ratio Va/Vb
     
  2. jcsd
  3. Nov 2, 2008 #2

    hage567

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    You need to show some work before you can receive help. What have you tried so far?
     
  4. Nov 2, 2008 #3
    Well i am working on it and I came up with equations

    2k= 1/2*3M*Va
    k=1/2*M* Vb

    but I got stuck I really dont know what to do with this equations
     
  5. Nov 2, 2008 #4

    hage567

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    Your equation for kinetic energy is wrong. The velocity is supposed to be squared.

    So you have two equations. How do you think you might put them together?
     
  6. Nov 2, 2008 #5
    equal to each other i was thiking maybe dividing the firt equation by 2 so that both equations equal to K
     
  7. Nov 2, 2008 #6

    hage567

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    OK. Then what?
     
  8. Nov 2, 2008 #7
    equal each other and solve for Va/Vb?
     
  9. Nov 2, 2008 #8
    it gives me 1/6
     
  10. Nov 2, 2008 #9
    sorry 1/36
     
  11. Nov 2, 2008 #10

    hage567

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    Can you show more of the steps on how you got that number?
     
  12. Nov 2, 2008 #11
    ok so i did it again and i notices i had skipped a step and got a new answer which is square root of 1/3

    here is the work

    2k= 1/2 3M Va^2 divide by 2
    k= (3M Va^2) / 2 set it equal to the other equation
    (3M Va^2)/ 2 = 1/2 M Vb^2 muiltiply by 2
    3M Va^2 = M Vb^2 divide by Vb^2
    3M Va^2/Vb^2 = M divide by 3M
    Va^2/ Vb^2 = M/3M M's cancel and take square root
    Va/Vb = square root of 1/3!~
     
  13. Nov 2, 2008 #12

    hage567

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    You are almost right, you are missing a factor of 2 somewhere.

    You had

    2k= 1/2*3M*Va^2
    k=1/2*M* Vb^2

    so if you divide the first equation by 2, like you said you were going to do in post #5, you get

    k = 1/4*3M*Va^2

    right? I think you forgot about the 1/2 on the right side. Do you see that?

    So now if you set the two equations equal to each other and solve, what do you get?
     
  14. Nov 2, 2008 #13
    right!!! i was saw that error
    in that case the ratio is square root of 2/3!
     
  15. Nov 2, 2008 #14

    hage567

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    That looks much better!
     
  16. Nov 2, 2008 #15
    thank you !!!!
     
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