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Homework Help: Simple Projectile Motion Problem. Unknown initial speed.

  1. Nov 4, 2013 #1
    A ball is thrown at an unknown speed at an angle of 30 degrees.

    The initial and final height from the ground is 0 meters.

    What speed should the ball be launched at in order for it to land on the ground 3 meters from the launch point?

    Here is what I tried:


    I got 5.829 m/s but I feel this is wrong.
  2. jcsd
  3. Nov 4, 2013 #2
    "I feel this is wrong" is OK, but then you should do something about your feeling. Why does that feel wrong? Can you check your solution somehow? In the end, we are not magicians here, you can do whatever we can.
  4. Nov 4, 2013 #3
    It feels wrong because I'm not sure if my strategy was legitimate. I tried to check it, but I didn't really know how to.
  5. Nov 4, 2013 #4
    You have used the kinematic equations to find the initial velocity. Now that you have the initial velocity, you can also find the time; then substitute the time and the initial velocity back into the equations and see whether you get what you should.
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