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Accurate Measurement

  1. Sep 2, 2004 #1
    Today, our AP physics class had a lab.
    The task was to caclulate the velocity of a projectile gun, then the next day place the gun at an angle, which we measure and calculate the distance away to place a street cone with a hole cut through the top so that the projectile would land directly in the hole. If it did, we would score a 100. If it didn't we would score a 70.
    Although we did the calculations correctly (they have been verified), we missed the target. The reason we presume is inaccuracy of the instruments of measurement. We had a wooden meter stick and a plastic protractor, both of which had been used for several years, including summer school. We were very cautious in taking measurements but it proved to be in vain. :cry:

    I was wondering if anyone knew of any devices worth buying or techniques to improve accuracy of measurement.
    If you would, please email me at estoydemoda@gmail.com.
    Thank you very much
  2. jcsd
  3. Sep 2, 2004 #2
    How far away was it, was it on the same level ground, does that gun produce nice predictable fireings time and time again, or does it shoot things in any which way? It could be the nature of the very device your using thats the problem. Your calculation should not steer you wrong if your not shooting too far, since you wont have to worry about drag that much. Were you sure you set the gun up at the right angle with the right starting velocity, and made sure the wind was calm and did not produce any sideways forces on the projectile?
  4. Sep 4, 2004 #3
    yes the gun was producing precise shots, we used carbon paper to see where they were landings and they were mostly overlapping.

    we were inside so there wasnt much worry about wind. we measured the angle to the best of our ability, and we were right on horizontally just not vertically.
  5. Sep 4, 2004 #4


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    Instead of a protractor, you might try measuring with a ruler
    the legs of the appropriate right triangle, then calculate the angle from that. You really don't need the angle... just the various ratios of sides to get sine, cosine, and tangent.

    Have you properly accounted for where the zero is in the vertical direction?
  6. Sep 4, 2004 #5
    hmm, if you were good horiztonally and not vertically, I might suspect that you made the mistake of calcuating the required angle in order to land the distance that the cone was without considering the fact that the cone is not on the same level as the gun. If this is the case, it WILL land where the cone is, but remember that the cone is now a little bit higher than the gun, (I dont know if this was the case but from what i take it it seems to be.) If that was the case, then the ball will be on its way down as it approaches the cone. So since the cone is at a higher level, it would hit the front side of the cone facing the gun and bounce away. I dont know if this is what happened to you guys or not. If that happened, you should calculate the angle so that it will land at that distance, at the same time, it will also be at the right height. The reason I say this is that it will be following its curve on the way down to that distance the cone is at, but since its no longer level, because the cone is higher than the level of the gun, its going to see this big cone in its way. When its at the point where it meets the cone, it wants to contiune arching down until it meets the same height the gun was at. So it will be just short of the cone and hit it. thus bouncing back. I attached a picture of what im talking about it should be alot better than my confusing text. See how the ball is calculated to land at that spot, butt you forget that the cone is higher, so it "runs" into the front of the cone facing the gun. Notice the higher trajectory. This time you account for the different heights. So when its at that distance x away, it is also at the right hight, and so it will land inside the cone.
    Last edited: Dec 26, 2005
  7. Sep 4, 2004 #6
    Well, what I think you did not take into account is the conditions of the surrounding, like gravitational pull, air drag and stuff.
  8. Sep 5, 2004 #7


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    I don't know the answer, but I can ask some questions. Well, lots of questions....

    What sort of gun was it, and how did you determine the velocity of the projectile?

    How did you determine the local acceleration of gravity?

    Is there any chance that tilting the gun changed the velocity of the projectile significantly?

    How did you account for air resistance?
  9. Sep 5, 2004 #8
    Did u see my picture pervect, do you think that might have been the problem? Its the only thing that I can think of that would calculate out correctly, but not work in practicality, because it will hit the cone if you dont account for the differeing heights and only calculate for distance away.
  10. Sep 5, 2004 #9

    If there only going a couple feet, this should not have major implications on their calculations should it. Its only a couple feet mind you.
  11. Sep 5, 2004 #10
    you guys were probably right about te bad measurement of the angle.

    i did take into account the fact that they were on different levels, and the air resistance was generally negligible
  12. Sep 5, 2004 #11
    it looked like this

    Attached Files:

    • lab.jpg
      File size:
      6.3 KB
  13. Sep 5, 2004 #12
    by the way, thanks for helping me out
  14. Sep 5, 2004 #13


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    I saw a picture, but it didn't have a lot of info. Like, is this a bb gun, or what? How was the velocity measured? And all my other questions :-)
  15. Sep 6, 2004 #14
    I was just depicting what might have happened to get an understanding of what went wrong with is trial pervect. Its really a 20 foot long circus cannon that shoots me out! :-) Thats a good question too, I wonder how did the gun work. I would think it to be a spring of some sort. But again, he said the teacher said his calculations were corect so I wonder if that was really the problem?
  16. Sep 6, 2004 #15


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    The only way that anyone will be able to help find your error is if you post the measured velocity and the distances to the cone as well as your calculations. Without this information we can only provide guesses.
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