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Homework Help: Algebra question

  1. Jun 22, 2008 #1
    1. The problem statement, all variables and given/known data
    A projectile is fired with an initial speed of 30 m/s at an angle of 60 degrees above the horizontal. The object hits the ground 7.5 s later.
    knowns: x0=y0=t0= 0, v0=30 m/s, t2= 7.5 s, angle= 60 degree, ay= -9.8 m/s, v1y=0, and v1x=v0x=v2x.....find: y2-y0 y1-y0 and v2 angle

    2. Relevant equations
    1# vy2= y0 + v0y(t2-t0)+ 1/2ay(t2-t0)^2
    2# v1y^2=v0y^2 + 2ay(y1-y0)

    3. The attempt at a solution
    I've found y2=-80.8 meters and am currently trying to figure out y1. I am using equation #2 and have this 0m/s^2= (30sin60m/s)^2 +2(-9.8m/s^2)(y1-0m) = y1= 34.4 m. I know the answer, but am having trouble isolating y1, i.e. not sure how to do the algebra. Everytime i attempt it, i get the wrong answer. any help will be appreciated, thanks.
  2. jcsd
  3. Jun 22, 2008 #2


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    Welcome to PF!

    Hi doucette1717! Welcome to PF! :smile:

    (I assume y1 is the maximum height.)

    Your y2 = -80.8 and y1 = 34.4 seem to be correct.

    So I don't understand what answer you're not getting. :confused:

    Show us the algebra you're having trouble with. :smile:
  4. Jun 22, 2008 #3

    Well the problem is with this 0m/s^2= (30sin60m/s)^2 +2(-9.8m/s^2)(y1-0m) = y1= 34.4 m.
    i was given the answer, i myself don't understand how to isolate y1 and do the math to get 34.4 m.
  5. Jun 22, 2008 #4


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    oh I see …

    well, you have 0 = (30sin60m/s)^2 +2(-9.8m/s^2)(y1-0m)

    ie 0 = (30*0.866)² + 2*-9.8*y1,

    so y1 = (30*0.866)²/2*9.8 = 34.4. :smile:

    what was worrying you? :confused:
  6. Jun 22, 2008 #5
    i have a test coming up soon. My math skills aren't the greatest. Just trying to improve them so i can do better on my test. Thanks.

    so i tried the calculation you gave me and i got 3307. m. I'm not sure what im doing wrong either, i plugged those exact values right into my calculator.
    Last edited: Jun 22, 2008
  7. Jun 23, 2008 #6


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    Hi doucette! :smile:

    That's because you plugged in ((30*0.866)²/2)*9.8 instead of (30*0.866)²/2*9.8. :rolleyes:

    Tip: If you're using a cacluator, it's easy to go wrong, so always check that the result is roughly correct …

    in this case, you know it's roughly 30*30/2*10 = 45 … so 3307 is obviously about 100 time too much! :smile:

    Another tip: if I'm using a calculator, I do the bottom line first, then I press "1/x", then I do the top line! :smile:
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