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Need help on projectile

  1. Dec 19, 2006 #1
    need help on projectile!!

    1. The problem statement, all variables and given/known data
    A projectile is fired at an angle of 38 degrees with respect to the x axis and with an initial velocity in the x direction of 3.3 m/sec. (gravity=10m/sec^2)

    a)whats the initial velocity of the projectile?

    2. Relevant equations
    a) i think Vy=Vi(sin38 degrees)
    and a^2+b^2=c^2

    3. The attempt at a solution
    I first want to get the Vy by using: Vy=Vi(sin theta)
    so, Vy=3.3m/sec(sin 38 degrees)
    Vy=2.03 m/sec

    now, (2.03)^2 + (3.3)^2= r^2
    so r or the initial velocity of the projectile =3.87 m/s ????????
  2. jcsd
  3. Dec 19, 2006 #2


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    Science Advisor
    Homework Helper

    Your reasoning is circular and incorrect. First you are using Vi to find Vy, and then you are using Vy and Vx to find what you called r, which is in fact Vi. You are not getting r = Vi because you did not know Vi when you used it to find Vy. What you are given in the problem is Vx and a launch angle. How do you find Vi from the given information?
  4. Dec 20, 2006 #3
    let me try again, with the right variables and numbers:


    Viy=Vi/(tan theta)
    Viy=3.3 m/sec / (tan 52 degrees)
    Viy=2.58 m/sec

    so then i do the phythagorem (sp?) theorm:
    4.19 m/sec=Vi !!!!!!!!?!??!?!

    i believe thats correct
  5. Dec 20, 2006 #4


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    Staff Emeritus
    Science Advisor

    Your answer is correct. However the lines in bold should read Vi2=Vix2+Viy2

    The Theorem to which you are referring is the Pythagorean Theorem (named after Pythagoras).
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