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Homework Help: Projectile Motion and Vectors

  1. Jan 24, 2008 #1
    1. The problem statement, all variables and given/known data

    A. If a bullet that leaves the muzzle of a gun at 281 m/s is to hit a target 77.0 m away at the level of the muzzle, the gun must be aimed at a point above the target. How far above the target is this point?

    B. The coach throws a baseball to a player with an initial velocity of 27.0 m/s at an angle of 38.0° with the horizontal. At the moment the ball is thrown, the player is 73.10 m from the coach. At what velocity must the player run to catch the ball at the same height that the ball was released? Your result should be negative if the player is running towards the coach and positive if he is running away.

    2. Relevant equations

    A. d = v/t

    B. R = (V0^2 * sin(2 * theta)) / g
    t = 2v0y / g

    3. The attempt at a solution

    A. I'm really not too sure what to do with this one after I find the time. The only thing I could think of what be to relate the acceleration to that time, but I didn't come out with the right answer.

    B. I'm pretty sure I have the right answer on this one, but the computer thinks otherwise. I found the range of the ball given the above equation, and then found the time after finding the velocity of v0y. From here, I subtracted the 73.10 from my range, and then divided that number by the time I got for the average velocity.

    Keep in mind these were evidently wrong, but here were my numbers:

    R = 72.1781
    t = 3.3924
    Velocity of the player = -0.272m/s

    Any help would be greatly appreciated.
  2. jcsd
  3. Jan 25, 2008 #2
    A.Why should the gun be aimed above the target if the target does not move at all>>>???


    try doing both the ques using the above concepts and if ya are still not able to do it..

    then i shall do it for ya..
  4. Jan 25, 2008 #3
    A. It needs to be aimed above the object due to gravity, of course, I'm just not sure how to incorporate it into an equation with the other givens.

    B. Even using relative velocity I don't see how the answer could be different from the one I got. Maybe I just don't fully understand the concept of relative velocity, though, as we haven't really covered it all that much.

    Thanks for the help.
  5. Jan 25, 2008 #4
    my frnd..it should be not be aimed above..

    how can ya say that???

    If the bullet is not fired at an angle ..then where does ''g" come into play???


  6. Jan 25, 2008 #5
    It's not fired along the ground. If you read the question it says it hits the object 77m away at the exact level that the muzzle was fired. Therefore, gravity will be acting on it in context of the problem.
  7. Jan 25, 2008 #6

    understand the ques better...
  8. Jan 25, 2008 #7
    So, you're saying gravity doesn't play a part in a horizontal launch assuming there is a height that the object is launched at? I'm pretty sure it does.

    Although, I did try putting in that it did not need to be aimed above, but the answer was wrong.
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