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Homework Help: Projectile Motion question

  1. Oct 15, 2005 #1
    I just wanted a second check on this. This is a mortar fired at a mountain to cause an avalanche. What is xf and yf given:

    Vi=300 m/s
    theta= 55 degrees
    t = 42 secs

    so I found
    Vxi= 300cos(55) = 172 m/s
    Vyi= 300sin(55) = 245 m/s

    Xf = Xi +Vxi*t + (1/2) ax*(t^2)
    = 0 +172(45) + (1/2)(0)(45^2)
    = 7227 m
    = 7.23 X 10^3 (this was correct w/the book)

    Now for Yf
    Yf = Yi +Vyi*t + (1/2) ay*(t^2)
    = 0 + 245(45) + (1/2) (-9.80)*(45^2)
    = 11058 - 9922
    = 1136
    = 1.14 X 10^3 ... book says 1.68 X 10^3

    What am I doing wrong to not get Yf correct? Thanks for the help.
     
  2. jcsd
  3. Oct 15, 2005 #2

    Chi Meson

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    what's t? 42 or 45 s?
     
  4. Oct 15, 2005 #3
    Oh shoot, it is 42, but for some stupid reason I wrote 45 in finding Yf. I redid it with t=42 and I got the right answer, woohoo, thanks!!
     
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