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Homework Help: Check my work

  1. Aug 26, 2005 #1
    Started another thread, but no one seems to be reading it:

    So, simply put, here's what I've done so far and just need someone to glance over it and check for the most glaring (I already know I messed some things up) mistakes:
    http://img388.imageshack.us/img388/3677/egrproblem5dq.jpg [Broken]
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Aug 26, 2005 #2

    Chi Meson

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    Why are you using 1 s for time?
  4. Aug 26, 2005 #3
    Just as a stopgap kind of, until I get the recording of what the actual times were. It's only temp, I just wanted to try and get the calcs right and in order, then fix the # inputs later.
  5. Aug 27, 2005 #4
    So, how's it look?
  6. Aug 27, 2005 #5
    Anyone? :frown:
  7. Aug 27, 2005 #6
  8. Aug 27, 2005 #7
    Please, will someone just look over what I did and comment?
  9. Aug 28, 2005 #8


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    Well, there's no diagram, so I can't really tell,
    but I expect that the stretch of the spring was NOT in the x-direction only,
    and that the y-component of velocity was a direct result of the spring's PE.
    [ie, 1/2 k s^2 = 1/2 m v^2 , not just (v_x)^2].

    you could solve horizontal eq'n for landing time t
    as a function of variable v_0 and measured x_f
    then insert in vertical equation, so you solve directly for (v_0)^2
  10. Aug 28, 2005 #9
    Okay, here's a quick diagram:
    http://img267.imageshack.us/img267/3533/diagram6gl.jpg [Broken]

    How could I solve for time using v0 if I don't have v0? I used the xf=x0 + v0x*cos(theta)*t +(1/2)*a*t^2 to find the v0x, and the same deal for v0y. But was that right or not? I still have not gotten the correct times, but is there any way to figure them out with what I have?

    All I need in the end is the spring constant for the system and to show equations describing it's motion/flight (like how far it would go with the particular v0 it shot with). But I keep confusing myself on what it is I'm actually calculating...
    Last edited by a moderator: May 2, 2017
  11. Aug 28, 2005 #10
    Wow, the more I go over this problem, the more confused I get....especially since I don't have v0 to begin with, which is just throwing me off.

    Could someone just help me with a little more in-depth layout of the steps? I've done several projectile problems before, but I always seem to mess up one or two steps and throw the whole thing off...
    Last edited: Aug 28, 2005
  12. Aug 28, 2005 #11
    Bump...just need someone to help with an outline of the steps/equations I should have to get what I need :smile:
  13. Aug 28, 2005 #12
    This is an AWESOME resource....well, as long as someone pays attention to your question....

    Anyone? Just take a look over my work and help me straighten out the steps to get what I need :smile:
  14. Aug 28, 2005 #13

    Chi Meson

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    Hey. If it took 2.5 seconds to travel 6.2 m horizontally, why are you using 1 s in your calculations? Your formulas look correct, now find (initial speed) (cos 30)= (horizontal distance)(2.5 s).
  15. Aug 28, 2005 #14
    So I'm finding initial speed again using:
    v0*(cos 30)=dist*(2.5s) ?

    Is that all?

    What about the spring constant, k, at the end? What velocity figure do I use? Since I have like 10 of them, that was confusing me, as to which one I actually plug in down there...
    Last edited: Aug 28, 2005
  16. Aug 29, 2005 #15
  17. Aug 29, 2005 #16
    Yet again....
  18. Aug 30, 2005 #17

    Chi Meson

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    I don't know why no one else is helping here.

    After finding the actual initial velocity, calculate the initial kinetic energy of the projectile. This energy must be equal to the stored elastic potential energy before release. The elastic potential enery follows (approximately) the formula PE=1/2 k x^2, where x is the amount of stretch in the rubber band.
  19. Aug 30, 2005 #18
    Okay, thanks! That helps me get closer to finishing this!
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