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Homework Help: Angle of bullet when fired

  1. Feb 24, 2008 #1

    Could somebody please tell me what formula(s) to use to find the answer to the following problem?

    A bullet is fired from a gun at 300 m/s. It hits the ground 3s later. At what angle (in degrees) above the horizon was the bullet fired? (Assume no air resistance)

    Thank you in advance
  2. jcsd
  3. Feb 24, 2008 #2
    well you have velocity and a time. Think about breaking things up in the x and y components.

    In this case, does the x component of velocity ever change?

    The Y component of your velocity changes and it changes at a rate of 9.8m/s^2

    Since its constant acceleration, use a constant acceleration equation that relates the time, starting velocity, final velocity, and acceleration
  4. Feb 24, 2008 #3
    OK, how 'bout this?

    V(y-final) = V(y-initial) + a(y)t

    V(y-final) = -9.8m/s^2 * 3

    V(y-final) = -29.4 m/s

    then find angle using inverse tangent

    Angle = tan-1 (-29.4/300)

    Angle = -5.52

    Is this correct?
  5. Feb 24, 2008 #4
    Let me know if I understand the question correctly. Your gun is set on the ground and mounted at an angle that you need to find. The bullet is shot out at 300 m/s and stays in the air for 3 seconds.

    If that is the case, is your initial y velocity correct?
  6. Feb 24, 2008 #5
    here's a question to ask based on the work you just did, is the initial y-velocity zero? Do you know the initial y-velocity?
  7. Feb 24, 2008 #6
    I assume the initial y-velocity is zero.

    No other informtation other than what has been posted is given for the problem

    Did I use the correct formula?
  8. Feb 24, 2008 #7
    Can you let me know if my interpretation of the question is correct? If so, that would mean the bullet was shot out at some initial velocity, which it is true you don't know. However, because you know the acceleration acting on the bullet (9.8 m/s^2) at every point during its flight, and you know how long it stays in the air, those two things tell you exactly what vertical velocity the bullet needs to start off with.

    If you can find that initial vertical velocity, and you know the total beginning velocity, which is 300 m/s, you can find the angle.
  9. Feb 25, 2008 #8
    It's obvious that no one is able to solve this problem because they only ask questions (that lead nowhere) and all the information to the problem has been posted. Thank you all for your help anyway.
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