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Negative angle projectiles

  1. Feb 15, 2012 #1
    1. The problem statement, all variables and given/known data
    Trying to work out final velocity of a projectile that is initially fired at a negative angle. Initial velocity is 30m/s at a negative angle of 16 degrees. Initial height is 0.44m. Can't get my head around the initial horizontal velocity. I figure its Vox =30 sin 16 =28.8m/s. What's Voy? Its not just gravity because its fired at an initial speed. How do you combine them?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Feb 15, 2012 #2


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    Staff: Mentor

    Hi Eube, welcome to Physics Forums.

    Did you make a drawing of the scenario? The X and Y components of the velocity can be found by applying the usual Vx = V cos(θ) and Vy = V sin(θ) method.

    Remember that gravitational acceleration applies only to motion in the vertical direction. Acceleration starts once the projectile is in motion, in this case once it has its initial velocity.
  4. Feb 16, 2012 #3
    The attached image shows the problem. Basically its a ball being fired at 30m/s at a downward angle of 16 degrees. It is fired at a distance of 1.6m away from the intended target. I am trying to work out the height it needs to be fired at? My thinking is that it won't be a straight path due to gravity acting on it so the height will be higher than if it was a constant velocity along the path. So the height needs to be increased slightly to compensate for gravity. Just not sure how to account for that.

    I am trying to find the angle and path the ball will follow after it bounces. Ball mass = 0.057kg coefficient of restitution = 0.7. I figure I just need to know the final velocity when the ball hits the ground and the angle will still be 16 degrees which can be used to find the initial speed and angle after it bounces.

    Attached Files:

    Last edited: Feb 16, 2012
  5. Feb 16, 2012 #4


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    Staff: Mentor

    Is the firing angle fixed at 16° below the horizontal, or is the impact angle that is 16°? Your diagram seems to indicate the latter, but your previous post stated the former...
    You're right that the projectile path is going to be curved; in fact it'll be a section of a parabola; the launch and landing angles will not be the same in general. So the firing height is not predetermined. Is the horizontal distance from the firing point to the target (1.6m) fixed?
    Depending upon what are truly your fixed parameters, the remaining conditions should be solvable. The 16° incidence angle needs to be clarified.
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