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Homework Help: Projectile Motion Firing Question

  1. Jun 13, 2010 #1
    1. The problem statement, all variables and given/known data
    A projectile is fired with an initial speed of 65.2m/s at an angle of 34.5deg above the horizontal on a long flat firing range. Determine
    (a) the maximum height reached by the projectile,
    (b) the total time in the air,
    (c) the total horizontal distance covered (that is, the range)
    (d) the velocity of the projectile 1.50s after firing




    2. Relevant equations
    v = vo +at
    x = xo + vot + .5at2
    v2 = vo2 + 2a(x - xo)
    x=vt



    3. The attempt at a solution
    First I wanted to find out the initial velocity in the x and y direction so i made a right triangle with the hypotenuse as 65.2 m/s with an angle of 34.5 from here I solve for opposite (sin34.5 = opp/65.2) and got 36.929. Then i solved for adjacent (cos34.5 = adj/65.2) and got 53.733. the adjacent is my Xo and the opposite is my Yo.

    a. the maximum height reached
    Equation v2 = vo2 + 2a(y - yo)
    0= 36.9292 + 2(-9.8)(y)
    -1363.75 = -19.6y
    y = 69.579 i rounded to 69.6m

    b. total time in the air
    Equation y = yo + vot + .5at2
    0 = 0 + 36.929(t)+ .5(-9.8)(t2)
    -4.9t2 + 36.929t + 0 = 0
    form here I did a quadratic and got t = 7.536 I rounded to 7.5s

    c. the total horizontal distance covered (that is, the range)
    Equation x = xo + vot + .5at2
    x = 0 + 53.733(7.5) +0
    x = 402.99 I rounded to 403m

    d. the velocity of the projectile 1.50s after firing
    Equation vy = voy +at
    vy = 36.929 + -9.8(1.5)
    vy = 22.229 I rounded to 22.2m/s

    Are my solutions and use of formulas for this problem correct?

    Thanks for your help!
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jun 13, 2010 #2
    Yes, correct. When you find the time easiest is
    v(y)=v(0y)-gt
    -v(0)sin(theta)=-v(0)sin(theta)-gt
    t=[2v(0)sin(theta)]/g
     
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