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Homework Help: Projectile Motion ball velocity

  1. Feb 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A physics student on Planet Exidor throws a ball, and it follows the parabolic trajectory shown. The ball's position is shown at 1.00 s intervals until t=3.00 s. At t=1.00 s, the ball's velocity is v=(2.73i+1.78j) m/s. Determine the ball's velocity at t=0.00 s. (Enter your answer in component form. Input the x-component first, followed by the y-component.)
    http://capa.physics.mcmaster.ca/figures/kn/Graph06/kn-pic0608.png [Broken]

    2. Relevant equations
    I know that v0x=vcos∅ and v0y=vsin∅

    3. The attempt at a solution
    I'm not sure how to begin without being provided with the launch angle. I tried to find the angle using the given velocities (at t=1 s), using tan-1= vy/vx=1.78/2.73, which gave me 33.1. But this angle, at t=1, will not be the same as the launch angle, right?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 6, 2012 #2
    From the diagram you know that the velocity in the y direction is 0 at t=2
    You can you this to work out the acceleration due to gravity g from which you can find the y component of velocity at t=0
  4. Feb 6, 2012 #3
    I'm not sure what you meant about working out the acceleration due to gravity, but I used the fact that Vy =1.78 at 1 sec to find the y component of velocity at t=0. So, I did V0y=Vy-at
    =1.78- (-9.8) (1)=11.58 m/s
    Am i on the right track? How would i go about finding the acceleration in the x direction to do the same for v0x?
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