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Baseball pieces

  1. Mar 17, 2013 #1
    exploding baseball

    1. A 150 g trick baseball is thrown at 59 km/h. It explodes in flight into two pieces, with a 41 g piece continuing straight ahead at 85 km/h. How much energy do the pieces gain in the explosion?



    2. Relevant equations



    3. The attempt at a solution
    is it asking for kinetic energy? or am I wrong?
     
  2. jcsd
  3. Mar 17, 2013 #2
    What indicates it is kinetic energy?
     
  4. Mar 18, 2013 #3
    the gain in motion ? it was accelerating at 59 km/h and the at 85 km/h
     
  5. Mar 18, 2013 #4
    Lets just say kinetic energy is defined by "motion" at this point. Do you have a formula for kinetic energy?
     
  6. Mar 18, 2013 #5

    jedishrfu

    Staff: Mentor

    okay so it looks like one piece is traveling in the original direction with an increased velocity and the second piece must be traveling in the oposite direction. The 41g piece has a delta velocity of 85-59=16 km/h and from that you can get its KE.

    For the 150-41g piece you could assume it got the same amount of energy and from that compute its delta velocity to subtract from the 59km/h original velocity.

    I'd also convert everything to MKS units for consistency ie no kilometers use meters, no hours use seconds and no grams use kilograms.
     
  7. Mar 18, 2013 #6
    so basically, i have to get the KE of the whole thing and then get the KE of the 59km/h and subtract it from the whole thing? It should give me the same for both pieces right?
     
  8. Mar 18, 2013 #7
    the formula for kinetic energy is KE= 1/2 m v^2 right?
     
  9. Mar 18, 2013 #8
    1. A 150 g trick baseball is thrown at 56 km/h. It explodes in flight into two pieces, with a 38 g piece continuing straight ahead at 85 km/h. How much energy do the pieces gain in the explosion?



    2. Relevant equations
    KE= 1/2 m v^2


    3. The attempt at a solution
    converted everything to meters kg and seconds then i got the kinetic energy of the 38g piece and got the velocity of the other piece, this is where i got lost.
     
  10. Mar 18, 2013 #9
    Something that helps me work physics problems is to write down all of the given information before I start working on the problem. For your example, I would write:

    Initial Values
    mi=150 g
    vi=59 km/hr

    Final Values (2 pieces)
    mf=41 g
    vf=85 km/hr

    mf= 109 g
    vf= ?


    Then I would look at my "cheat sheet", where I have written all these formulas down to reference quickly while I learn the material. Notice kinetic energy use m and v in the formula and the ball is in motion and energy is the desired answer. Another one I see is the conservation of momentum.

    I write formulas I'm thinking about:
    KE = (1/2)mv2
    ρ = mv (ρ=momentum)

    Do you understand the concept of conservation of momentum?
     
  11. Mar 18, 2013 #10

    TSny

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    Can you show how you got the velocity of the other piece?
     
  12. Mar 18, 2013 #11
    v=sqrt( (1/2 m)/KE)
     
  13. Mar 18, 2013 #12

    TSny

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    How did you get a value for KE to plug into this equation? Can you state what physics principle you are using? (Edit:The equation v = sqrt((1/2 m)/KE) isn't correct.)
     
  14. Mar 18, 2013 #13
    first I did the 38g piece and it gave me 1.23 or something near that, and I assume it is the same for the other piece of the ball.
     
  15. Mar 18, 2013 #14

    TSny

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    How did you get the number 1.23? What does the number 1.23 represent? It will be very helpful if you can show the individual mathematical steps.
     
  16. Mar 18, 2013 #15
    KE = 1/2(.038g)(8.05m/s)^2 = 1.23J
     
  17. Mar 18, 2013 #16

    TSny

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    OK. The units for the .038 should be kg, right? The speed of 8.05 m/s is not correct. Can you show how you converted 85 km/h to m/s?
     
  18. Mar 18, 2013 #17
    sorry, i got confused hahah it should be like this

    KE= 1/2(.038kg)(23.611)^2 = 10.59J
     
  19. Mar 18, 2013 #18
    now I think what is my mistake, 85000m/3600s= 23.611m/s
     
  20. Mar 18, 2013 #19

    TSny

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    Right. So, that's the KE of the 38 g piece. If you could figure out the KE of the other piece, then you could find the gain in KE of the system due to the explosion. You're going to need the velocity of the other piece. Can you think of a way to get it?
     
  21. Mar 18, 2013 #20
    the concept of conservation of momentum, is the sum of the momentum on both objects?
     
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