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Thread: MythBuster question.

  1. #1

    MythBuster question.

    If a bullet in free fall, and a bullet fired from a gun both hit the ground together, if fired level and from the same height.
    A/ Does the bullets BC effect the outcome, i.e. 55 grain bullet fired, 55 grain ball dropped.
    B/ Does the height above ground that it is fired/dropped from effect the answer, i.e. dropped bullet reaching terminal velocity or not.
    I have been thinking about all evening and now my brain hurts

    Neil.

  2. #2
    A/ I'd say the bullet with the higher BC will travel further in the given time, but still hit the ground at the same time.
    B/ Yes&No. So long as the two bullets leave at the same height and the trajectory is such that the bullet can reach terminal velocity then it matters not whether from a height of 1m or 100km. If you fired the gun 1cm from the ground then there would not be the opportunity to reach a terminal velocity.

    Section 161 of the Highways Act 1980 (England & Wales) makes it an offence to discharge a firearm within 50 ft of the centre of a highway with vehicular rights without lawful authority or excuse, if as a result a user of the highway is injured, interrupted or endangered.

  3. #3
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    Assuming the gun is fired completely level, you can consider that the only force exerted on them in the vertical plane is that of gravity- an acceleration of 9.81ms-2. This is because you can resolve the force acting on the projectiles into a horizontal and vertical component, completely eliminating the horizontal velocity from the equations. after that simply use s=ut+.5at^2 to establish the time required for them to hit the ground. BC will have no effect and neither will any other variable which influences the horizontal velocity, as this was disregarded when we resolved vertically. Both will have similar terminal velocities due to their similar mass and air resistance and assuming that air temp etc. is constant will attain it within a fraction of a second of each other, thereby proving that the height dropped from has no influence on the results.

  4. #4
    I might tentatively suggest that the bullet fired horizontally would be stabilised by its spin (to begin with anyway), whereas the dropped one would tumble, increasing its drag and slowing it down, and also giving it a lower terminal velocity.

  5. #5
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    Hmm, you may be right, although objects, such as a dart or a pencil, sometimes stabilise in the attitude which yields lowest resistance, i.e point down which may actually swing it in favour of the dropped bullet as the fired one will never be completely vertical as it will fly in a parabola. Something to think about anyway, although I doubt it would have much of an effect over the distance and time periods involved

  6. #6
    I suspect this is how Neil's brain began to hurt in the first place

  7. #7
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    lol, you may be right!

  8. #8
    Top answers so far to packrat and Alistair

    Neil.

  9. #9
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    Thanks Neil, i knew my A level Physics revision would serve some useful purpose, and it certainly isn't remembering the f**king syllabus!

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