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Homework Help: H=rt-4.9t^2 questions

  1. Jan 6, 2008 #1
    t=number of seconds
    r=initial speed
    How do you solve these two problems?
    1. A projectile is fired upward with an initial speed of 2940 m/s. After how many minutes does it hit the ground?
    2. A ball is thrown upward from the top of a 98m tower with an initial speed of 39.2 m/s. When does it hit the ground?
    I don't understand how you can find the answers to these types of problems
  2. jcsd
  3. Jan 6, 2008 #2
    Help PLEASE!!!
    I got this far on the second one...
    h=rt- 4.9t²
    0=-(t²-8t -20)
    (t-10)=0 (t+2)=0
    t=10 t=-2
    Last edited: Jan 6, 2008
  4. Jan 6, 2008 #3
    For the first:
    If you don't need the height then why are you calculating it ?
    Try Vf = Vi + at
    Where t would be time of flight & Vf,Vi final and initial velocity respectively
    Last edited: Jan 6, 2008
  5. Jan 6, 2008 #4
    Well, I'm supposed to put the problem into that formula, that's what I'm confused about.
  6. Jan 6, 2008 #5
    Don't forget to convert time to minutes b/c the answer you will get is in seconds.
  7. Jan 6, 2008 #6


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    For the first, if it's fired from the ground and lands on the ground, what is h (often times, more appropriately written as [tex]\Delta[/tex]h)?
  8. Jan 6, 2008 #7


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    Homework Helper

    For the first question: you can use the same equation as you did for problem #2. If the projectile returns to the ground, what is its displacement? What does that make h in your equation?
  9. Jan 6, 2008 #8
    Ok, so what I have so far is
    0=2940t/60 -4.9t^2/60
    0=-4.9t(t/60 -1)
    Is that right?
  10. Jan 6, 2008 #9
    60 60
    -t=0 (t-600)=0
    t=0 t=600

    or is this right?
  11. Jan 6, 2008 #10
    Just a little tip, you might wanna work through things in algebra before you number plug :)
  12. Jan 6, 2008 #11
    Would time 0 make sense as an answer?

    And what units are your answers in?
  13. Jan 6, 2008 #12
    0 wouldn't make sense as an answer, my answer is in minutes~I think... but now i'm more confused...
  14. Jan 6, 2008 #13
    Is it 10 minutes...?
  15. Jan 6, 2008 #14
    Stop guessing. What equation are you using?

    And before plugging values into your equation, let's SOLVE for the variable we want.
  16. Jan 6, 2008 #15
    The one I posted as the title!
  17. Jan 6, 2008 #16
    Well looks right to me. You have it in terms of minutes, so there you go.
  18. Jan 6, 2008 #17
    So was my answer correct? 10 minutes? Because 600/60=10
  19. Jan 6, 2008 #18
    oh ohkay, thank you i didn't see the second page
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