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Projectiles, 2D Motion, Bullet falling

  1. Oct 22, 2009 #1
    1. The problem statement, all variables and given/known data
    From the top of a tall building, a gun is fired. The bullet leaves the gun at a speed of 340 m/s, parallel to the ground. As the drawing shows, the bullet puts a hole in a window of another building and hits the wall that faces the window. (y = 0.48 m, and x = 6.0 m.) Using the data in the drawing, determine the distances D and H, which locate the point where the gun was fired. Assume that the bullet does not slow down as it passes through the window.
    acceleration: 9.8m/s^2
    2. Relevant equations
    sin, cos, tan. a^2+b^2=c^2
    Vfy2=Viy2+2ady
    3. The attempt at a solution
    tan-1(.48/6)=4.57392126 degrees
    .48^2+6^2=6.019169378^2 meters
     
  2. jcsd
  3. Oct 22, 2009 #2
    The bullet falls at a constant acceleration (gravity = 9.8m/s^2). Its horizontal velocity is constant, since it does not accelerate due to gravity. A little birdie seems to have told you the horizontal velocity of the bullet (tweet tweet, it's 340 m/s.) How does its horizontal position D relate to the time it has traveled and the velocity in which it is moving (look up "kinematic equations" in your books index, or ask yourself: if I travel v miles per hour for t hours, how far have I traveled?

    Now you have to answer the question of how high the bullet was fired from. You can find the time that the bullet traveled from the first question, so all you have to ask is "how does position relate to constant acceleration?" Again, consult "kinematic equations" in the index of your textbook. If your textbook doesn't address these equations, then you are being cheated and you should either PM me or post on this thread, and I will help you further.
     
  4. Oct 22, 2009 #3
    You could make life easier by just spelling things out for me since I am dumb and have a 50% in IB Physics
     
  5. Oct 22, 2009 #4
    Maybe you have a 50% because you've convinced yourself that you are dumb. We aren't even halfway through the semester here, and hard work proves to be a miracle maker. I start almost every semester with 65% or less on tests in my classes, but I have managed to turn every one of those 60%'s into at least an 80%. I started Calc I with a 57% on my first test, and I finished with an A. I started E&M with a 59%, and Mechanics with a 69%. I finished all these classes with A's. I got a 57% on my first probability exam, then, in a fit of incredible stupidity, I misread the final exam time and got a 0% on my final exam. Somehow I got a C (70-79%) in the class.

    The bullet is moving across the ground at a CONSTANT speed of 340 m/s. It eventually moves 6.0 m after traveling for some time t. Now ask yourself this: If you travel on the freeway at some CONSTANT velocity v (let's say 60mph) for some time t (let's say 1 hour), then how far have you traveled? Now how far would you travel if you were going 60mph for 2 hours? 3 hours? 3.1415926535 hours?

    Now you have to ask yourself, "what if I was traveling at 60mph, and I eventually went 60 miles? How long did I travel? If I traveled 120 miles at 60mph, how long did I travel? 188.49556 miles? At some known constant speed, how long would I have to travel to go this far?"

    Again, look up "kinematic equations in your textbook."Does your textbook address these? (if it does not, I suggest suing the person that made the book.) If you still have questions, please ask me by PM or respond to this thread.
     
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