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Homework Help: Projectile Motion with few variables

  1. Oct 10, 2008 #1
    1. The problem statement, all variables and given/known data
    A cannon shoots a ball from the origin. There is a wall of 17.58 meters 67.5 meters away. The ball needs to clear the wall just barely and hit a person sitting on the ground 12 meters behind the wall. No air resistance. What is the degree the ball is launched at. No max height or initial velocities are given. Just two various lengths and height of the wall.

    2. Relevant equations

    position= x+vt+1/2at^2 (applies to both x and y)
    gravity is 9.8 m/s^2 in the negative y direction

    3. The attempt at a solution

    Using properties of parabolas I know I have 4 points on the curve and I used those four points to get the quadratic equation. Using that I found the max height and used the basic formulas to find the initial velocity and used the two vectors of x and y velocity to find the angle that the ball is launched and got 59.9 degrees which is the correct answer. My professor however told me that i couldn't do it that way and had to find a different way to do this problem using what I know about projectile motion.
  2. jcsd
  3. Oct 11, 2008 #2


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    Homework Helper

    Welcome to PF!

    Hi Rebelpyr7! Welcome to PF! :smile:

    hmm … I have some sympathy with you …

    but the object of the exercise is to give you practice in using a particular method …

    so you need do it the other way as well. :smile:

    (in other words: I think you're both right! :wink:)
  4. Oct 11, 2008 #3
    Problem is I don't know how to solve it any other way. The prof hasn't done anything like this in class so I was out of options.
  5. Oct 11, 2008 #4


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    Hi Rebelpyr7! :smile:
    "using what I know about projectile motion"? :confused:

    in other words, nothing?

    your professor obviously thinks (s)he has told you something!

    Can you start by showing us exactly how you actually did it? :smile:
  6. Oct 11, 2008 #5
    well what i ended up doing was doing a linear regression with my calculator and finding the quadratic formula of the parabolic curve. i then solved for max height and use that in the formula for initial velocity in the y direction. v=(2gh)^1/2 i then just plugges it into the original equation and solved for t. then I used the t to solve for the velocity in the x direction. with those two velocity vectors i solved for the degrees and got 59.9 degrees. He said what i did was wrong although it is completely sound method of doing the problem. And about him telling me something, hes an extremely bad professor. he hasnt done a single problem like this. every problem hes done had more variables. Thats where my problem is, i dont know how to solve these types of problems with virtually no information.
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