THAT explains it...

Discussion in 'General Discussion' started by Albtraum, Oct 30, 2012.

  1. Albtraum

    Albtraum Member

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    So I'm taking a physics class, and throughout the semester, we've gone over a variety of topics that can be related to shooting, like: dropping an object, and throwing that object parallel to the ground, which lands first? (Dropping a bullet, and simultaneously firing a bullet straight ahead 90 degrees to the ground) Both will land at the exact same time, gravity exerts the same force on both objects equally, and brings them down simultaneously.

    Now for a concept the professor covered last night, dealing with energy and heat... if you heat up a metal ring, imagine a steel washer, does the hole diameter increase, decrease, or stay the same? You should know that metal expands with heat, and the molecules expand in all directions.







    Well, the hole will actually grow bigger, since more mass is on the outside, so the inside and outside diameters both increase...

    After this, I thought of a rifle barrel heating up as more and more rounds go through it... the bore expands ever-so-slightly, that the bullets are not gripping the rifling in the same way as when the bore/barrel is cool, and that is how accuracy diminishes with rapid fire/ shooting with a hot barrel...

    Kinda of a simple concept, but if you look at the details it, you really get a grasp of how and why it happens.
    Just thought I'd share.
    Last edited: Oct 30, 2012
  2. firefighter1635

    firefighter1635 Active Member

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    Neat hearing it the physics way, thanks.
  3. Millwright

    Millwright Active Member

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    Waal, your example has a "yes and no" answer. Been a mechanic for more than fifty years and can cite countless examples counter-supportive. Your washer/rifle barrel example isn't very good due to mass/expansion ratio issues. Heating a large mass via the bore axis results in initial bore shrinkage due to the resistance of the surrounding cold metal. In the case cited, the bore will return to (calculable) "size" when the barrel becomes isothermal; that is of uniform temperature.. Anyone trying to remove/install pump impellers has learned this first hand. Manipulating/controlling dimensional changes is a mechanic's stock in trade. The same principles apply to your application. >MW
  4. armedandsafe

    armedandsafe Guest

    I've had two different rifles which shot progressively tighter groups as the barrel heated. As it was 1000 yard competition, the barrels never got THAT hot. Also, we were shooting in high ambient temperatures, usually in the 90 - 115F range.

    Which doesn't disprove or prove either of the two above posts.

    Pops
    Last edited by a moderator: Oct 30, 2012
  5. WHSmithIV

    WHSmithIV Well-Known Member

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    This is actually only true in a vacuum and provided the dropped bullet starts it's fall at the exact same instant that the shot bullet leaves the barrel. The bullet travelling through the air builds a measurable amount of lift due to it's shape and therefore actually has a flight characteristic. The falling bullet does not benefit from the lift during travel so therefore will hit the ground first.
    Last edited: Oct 30, 2012
  6. deadin

    deadin Well-Known Member

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    I don't see how a spinning symmetrical object could generate any "lift". All of the aerodynamic forces would act equally on all surfaces and therefore no lift would be present. The only forces that come into play are velocity, drag and gravity. Velocity and drag vectors are in horizontal opposition and would eventually cancel each other out.
    That leaves gravity which has a perpendicular vector. The only catch is that in the real world the surface of the earth is curved so a bullet fired horizontally actually will have slightly farther to fall then the one dropped at the muzzle.
  7. WHSmithIV

    WHSmithIV Well-Known Member

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    Bullets do indeed get a lift factor - the tail end of the bullet has more weight than the nose so the back side of the bullet goes through the air a minute fraction lower than the nose. The bullet isn't going through the air parallel to the surface below and picks up a small amount of lift due to the angle of its flight very much like a flat wing would get at an angle.
  8. WHSmithIV

    WHSmithIV Well-Known Member

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    I just did a check on my own common sense - Mythbusters actually proved this, then another science site made independent calculations and proved it again. The dropped bullet will hit the ground first every time. One of the factors they point out is air resistance. The dropped bullet only suffers air resistance in the Y direction that is not happening to the fired bullet until its lost momentum in the X direction from air resistance.

    So, what the professor was teaching is indeed true only in a vacuum.
  9. WHSmithIV

    WHSmithIV Well-Known Member

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    Albtraum - thanks for sharing this - you got me to do some research into some areas where I learned a few things I had never thought of. Here's a link to some really good descriptions of the forces involved. I really enjoyed the long range factors section :)

    http://en.wikipedia.org/wiki/External_ballistics#Long_range_factors
  10. Blackhawk Dave

    Blackhawk Dave New Member

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    As an aside about the heat factor, when I was at Boy Scout camp, a counsellor demonstrated the effect of heat from both the firing of a rifle and friction by shooting a gallon jar of gas. Resultant explosion was quite impressive to a bunch of Scouts.
  11. deadin

    deadin Well-Known Member

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    But the back end of the bullet will have more surface area then the front end and therefore should drop slower because of air resistance allowing the front end to catch up with it.
    Besides gravity isn't picky, it affects everything the same regardless of weight. (Remember the old pound of feathers vs. pound of lead theory? Gravity will work the same on each. Only surface area will change the equation.)
  12. GunnyGene

    GunnyGene New Member

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    To add to the question: If you fire a standard rifle bullet - say 7.62 - straight up, when it falls will it fall nose first, tail first, or side first? And how fast will fall (approximate terminal velocity)? :) ;)
  13. WHSmithIV

    WHSmithIV Well-Known Member

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    The main factor influencing everything is of course that the projectile is travelling at speed through air whereas the dropped bullet has very little air influence by comparison.
  14. Buckshot

    Buckshot Active Member

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    I enjoy Mythbusters a great deal, but their science is frequently faulty. (Or more often incomplete.) Take them as gospel at your own risk.

    The bit about the vacuum is to remove the differences in aerodynamic properties. Dropping a feather vs. a cannon ball for instance. Take two identical objects of the same mass - two identical bullets - and they WILL hit the ground at the same time. One being fired and one being dropped makes no difference.

    The key to the whole thing is firing the bullet in such a way as to be the same height above the surface as the dropped bullet, and to never RISE ABOVE that relative distance. Extremely difficult to do, or judge the success of without effecting the path.

    The idea that anything about the shape of a standard bullet producing lift in a vertical plane away from the ground is specious at best. I've NEVER seen or heard of any evidence in ballistics of that effect, nor does anything suggest it.

    As for GunnyGene's question, Adam and Jaime did explore this very subject. Keeping in mind the aforementioned caveat about MB, they determined that once the bullet loses the stabilizing effect of the spin imparted by the rifling - likely near apogee - the bullet tumbles, and while it can land with enough force to give you a nasty thump it likely does not have enough oomph to deliver a fatal wound. I wouldn't want to be the one to test that however. There are stories...
  15. NRA_guy

    NRA_guy New Member

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    Yeah, but you have to think in terms of vectors: friction force (from the air) only works in the direction opposite to the velocity. So the 2 bullets see the same friction force in the vertical direction.

    The horizontal friction force acting on the fired bullet has no influence upon the vertical acceleration (rate of fall).

    Obviously they also see the same gravitational force, too.

    F=ma is the same in the vertical direction: F is the same, m is the same, and a (gravity) is the same.

    So, if the fired bullet takes longer to hit the surface, it has got to be lift causing the difference.

    That, of course, assumes the Earth is flat. :)
  16. goofy

    goofy Well-Known Member

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    WOW! This is over my head.
    So what is being said is that if I hold a bullet 4' off the ground and let it drop the same time I shoot one that is 4' from the ground(level with the ground) both will hit the ground at the same time?
    And the effect of the twist, speed,of the shot round does not change the answer?
    Mike
  17. 76Highboy

    76Highboy Well-Known Member Supporting Member

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    I have to go with deadin on this one. I have a degree in Electronics and know the theory. A spinning bullet will not increase in altitude at all. It is subject to the effects of gravity no more ofr no less then a bullet that is dropped from the muzzle.

    Myth busters is entertaining but they are not 100% correct on everything.
  18. tcox4freedom

    tcox4freedom Well-Known Member Supporting Member

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    All you smart guys make my head hurt! :D
  19. Appliancedude

    Appliancedude Active Member

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    :yeahthat: Way over my feeble little mind
  20. Old Grump

    Old Grump New Member

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    Tail first, Read Hatcher, the only way it will come down at any other angle will be if it hits something or air turbulence makes it tumble and that would be a lot of turbulence. The spin of the bullet stabilizes it in the position it was fired in.

    http://www.amazon.com/Hatchers-Notebook-Julian-S-Hatcher/dp/0811707954
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