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Homework Help: Simple 2d kinematics help

  1. Sep 13, 2009 #1
    A projectile is fired straight upward at 135 m/s. How fast is it moving at the instant it reaches the top of its trajectory?
    My answer: 0 m/s

    Suppose instead it were fired upward at 35°. What would be its speed at the top of its trajectory?

    I do not know how to approach this. Does it want vertical speed or horizontal speed? How do I calculate this?


    Consider an airplane that normally has an air speed of 95 km/h in a 105 km/h crosswind blowing from west to east. Calculate its ground velocity when its nose is pointed north in the crosswind.
    ______ km/h
    ______° (counterclockwise from east)

    I have been staring at this problem for the last hour, I do not know how to set this up. Any help would be much appreciated!
  2. jcsd
  3. Sep 13, 2009 #2
    It wants both horizontal and vertical. But because it said the top of the trajectory, one of these is zero. And you know which one based on your first answer. So find the other. It should be easy as it does not involve acceleration. But it might involve knowing something about right triangles.
  4. Sep 13, 2009 #3
    so i would first need to find the time in the air. This can be determined by vertical motion, but would it be at the same 135m/s even though it is at a 60 degree angel, or would i need to use vectors/trig in order to find the velocity at 60 degrees?
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