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Homework Help: At what speed must I throw vertically an object.

  1. Sep 18, 2015 #1
    1. The problem statement, all variables and given/known data
    At what speed must I throw vertically an object so that it will lose 25 % of its speed during the first second.

    2. Relevant equations
    V final = v initial + a * t
    3. The attempt at a solution

    v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

    That's pretty much what I figured out only ...
     
  2. jcsd
  3. Sep 18, 2015 #2

    Bystander

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    "Only,..." what?
     
  4. Sep 18, 2015 #3
    Is there a problem ?
     
  5. Sep 18, 2015 #4

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    You haven't answered the original question, what's the original vertical speed?
     
  6. Sep 19, 2015 #5
    Well that's the thing I have no idea of how to obtain the answwer...
     
  7. Sep 19, 2015 #6

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    You're interested in the rate at which the object loses speed, and specifically asked for a 25% loss in one second. What's "vfinal" for 100% loss?
     
  8. Sep 19, 2015 #7
    Well it's 0.
     
  9. Sep 19, 2015 #8

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    Substitute that value, and rearrange. What's "v(t=1s)" relative to "vinitial?"
     
  10. Sep 19, 2015 #9
    Vi=9,8 m/s
     
  11. Sep 19, 2015 #10

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    What's "a?"
     
  12. Sep 19, 2015 #11
    -9.8 m/s^2
     
  13. Sep 19, 2015 #12

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    9.8 is what percentage of 9.8?
     
  14. Sep 19, 2015 #13
    100%
     
  15. Sep 19, 2015 #14

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    In one second, the speed changes by how much?
     
  16. Sep 19, 2015 #15
    it goes from 9.8 to 0.
     
  17. Sep 19, 2015 #16

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    No. It changes by 9.8.
     
  18. Sep 19, 2015 #17
    Yes, that's what I said. ok, it changes by 9.8.
     
  19. Sep 19, 2015 #18

    Doc Al

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    Fix the left hand side of that equation. Then you should be able to solve for vinitial.
     
  20. Sep 19, 2015 #19
    I put it that way so that the speed would be 75%... it's not correct?
     
  21. Sep 19, 2015 #20

    Doc Al

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    No, it's not correct.

    This is the formula you are trying to use:
    Hint: express Vfinal in terms of Vinitial.
     
  22. Sep 19, 2015 #21
    v initial = Vfinal/(a * t)

    Vinitial = V final / (-9.8* 1)

    Vinitial = V final / -9.8

    This is what you want ?
     
  23. Sep 19, 2015 #22

    Doc Al

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    No. I want you to fix the left hand side of your equation, just like I asked.

    First step: Can you tell me what's wrong with it?
     
  24. Sep 19, 2015 #23
    Well I multiplied the left side by 75%
    So we have v final = v initial + a* t
     
  25. Sep 19, 2015 #24

    Doc Al

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    But the left side of your equation was 0.75vfinal, not vfinal. Fix that!
     
  26. Sep 19, 2015 #25
    Ok, so I had v final * 0.75 = v initial + -9.8 m/s^2 * 1 second

    and I change it to v final = v initial + -9.8 m/s^2 * 1 second
     
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