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Speed of projectile after t seconds

  1. Sep 14, 2008 #1
    1. The problem statement, all variables and given/known data
    A projectile is fired with an initial speed of 68 m/s at 37 degrees above horizontal on a flat firing range. c) Determine speed of the projectile 2.3 seconds after firing. d) Determine direction of projectile 2.3 s after firing.

    2. Relevant equations
    c) Vox=Vx=68m/s
    Vy=Voy*sin(theta)-gt
    V=sqrt(Vx^2+Vy^2)

    d) tan^-1(Vy/Vx)= theta

    3. The attempt at a solution

    I got that Vx=68m/s
    Vy=68*sin(37)-(9.8*2.3)
    Vy=18.38

    V=sqrt(68^2+18.4^2)
    V=70.4
    but the velocity is not 70.4 what am I doing wrong??
    Thanks for any help!
    The second part I get theta= 15.1 degrees but I don't want to submit it because if the first part is wrong then this is probably not right either.
     
  2. jcsd
  3. Sep 14, 2008 #2

    LowlyPion

    User Avatar
    Homework Helper

    You need to take the horizontal component of velocity in the RSS which is 68*Cos 37, not 68.

    Then you should be fine.

    You will need to get another calculation for theta.
     
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