The effect of gravity on a speeding bullet

Discussion in 'Physics & Math' started by Magical Realist, Jan 31, 2017.

  1. sculptor Valued Senior Member

    Wow, some folks idea of "fun" absolutely amazes me.

    entertaining link origin
    however, the bullet ain't "falling"
    It starts out supersonic--M3 in the example above-- ----then slows.............initially, it meets resistance which pushes against the nose of the bullet, while negative pressure at the back of the bullet are both working to slow the bullet down.

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    Then as it approaches the earth, the atmosphere gets denser and adds more resistance, while gravity is working as an opposing force.
    Assuming that all of that is correct:
    I ain't got a clue as to how to configure an equation which would quantify any of it.
    If you do, my hat is off to you!

    When I was in Florida(circa '72-'73) there was a young man who was fond of jumping out of helicopters without a parachute. He remarked that his terminal velocity was slow enough to mitigate the dangers of free falling into water. So, he kept jumping from higher and higher.
    I wonder what ever happened to him?
    Last edited: Feb 7, 2017
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  3. Michael 345 Looking for Bali in Nov Valued Senior Member

    He probably won a Darwin Award
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  5. sculptor Valued Senior Member

    better'n "splat"
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  7. Kittamaru Never cruel nor cowardly... Staff Member

    Yet the answer is still simple, despite your attempt at obfuscation (and by all appearances, attempting to provoke and incite without cause) - the bullet WILL eventually reach terminal velocity. Unless some force continues to act upon it to propel it faster, it will eventually slow to this speed. Unless something acts upon it to increase drag, it will not fall below this speed (until impact, of course).

    Your statement makes it seem as though you are questioning this - this is vexatious and borderline trolling.

    So, I ask again - why are you demanding he go through with this? Are you suggesting that the bullet will NOT end up at terminal velocity?
    exchemist likes this.
  8. sculptor Valued Senior Member

    May be misleading in this case.
    Terminal velocity is normally used to describe the action of a falling object.
    This thread ain't referring to a falling object. It is describing the action of a bullet which starts out much faster than the terminal velocity of a like falling object.
    Will the bullet actually slow down to the (falling) terminal velocity of a like object?
    (some) variables
    initial speed of the bullet
    shape of the bullet
    mass of the bullet
    varying density of the atmosphere
    height from which the bullet was fired

    I dare say that "
    " is something that this scenario ain't.
  9. Kittamaru Never cruel nor cowardly... Staff Member

    So you contend that the bullet will not actually slow to terminal velocity?

    From the OP:
    If we assume "high flying" to mean around 30,000 feet (the lower end of most jet liners cruising altitudes), I think it is quite safe to say the bullet will slow due to air resistance well before impacting the ground - it will, ultimately, have two forces acting on it - Gravity and Air Resistance. The longest confirmed sniper kill on record is:
    8,120 feet

    We are talking over triple that range - I see no reason why air resistance would not bring the bullet to its terminal velocity in that span (note, the faster the object is moving, the higher the wind resistance - as a result, as the bullet slows towards terminal velocity, it will bleed speed at a slower rate) - I don't believe, however, that it will result in an asymptote.
  10. RajeshTrivedi Valued Senior Member

    Please let me know which part of my post #51 is obfuscation or trolling? If you fail to find ny traces, then withdraw your allegations pl.

    Please refer to origin post #60, he is talking about terminal speed for a drop falling object (u=0), which surely is simple and high school physics. But shooting a bullet supersonic @u = 800 m per seconds from an aircraft at 40000 ft travelling @200 m per second is not a simple stuff.

    If you read and attempt to grasp my #51, then I stated that bullet has a horizontal speed of 200 m/seconds and vertical speed of around 800 m/seconds, so the effective velocity is not vertically down, its at some angle from vertical at t = 0. The bullet cannot achieve constant terminal velocity in downwards direction till first the 200 m/sec, that is horizontal component, becomes zero. Sculptor has summed up the complexity.

    And if it is simple, why not put it on paper. What is stopping you or origin. Seriously I would love to know the exact equation, not some elementary drag maths. Let me give you an idea for a bullet, the drag is dependent on speed squared not on just the speed. Mapping the air drag between 40000 ft to 0 altitude that too when fired at an angle is not simple. If it was so, we would have had a very pleasant turbulence free air travel. Have fun. Leave it, its more of a numerical problem, by punching mindless data in computers, not analytic. If any one tells you, its simple then either he is a genius or he has just not understood.
  11. sculptor Valued Senior Member

    I did find the
    part rather entertaining.

    As origin said, that would require solving a "non linear" equation.
    I suspect that that would require a familiarity with the calculus (at which i was an abismal failure --- I passed the required course with an a, but it was all monkey see-monkey do--I never developed a feel for nor complete understanding of the calculus)
    I can suggest variables. But I ain't got a clue as to how you could figure this out.
    (can one "contend" ignorance?)
  12. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

    Sculptor, to expand on your earlier post....

    Whether something reaches terminal velocity will depend upon the initial velocity, the rate of change of velocity, and the time it is travelling for.
    Terminal velocity is the velocity at which the forces of acceleration (I.e. Gravity) are balanced by the drag force. If an object, such as a sky diver, can alter its shape then it can alter its drag coefficient as well as its reference area, and thus its force, thus speeding up and slowing down while in the air.
    If something is travelling faster than the terminal velocity it will slow down (drag force is in excess of gravitational pull), and if it is travelling slower then it will speed up (Gravitational pull is in excess of drag etc).

    Obviously, if an object starts with a velocity above a certain point, there may be insufficient time for it to attain a terminal velocity before impacting (e.g. meteors hitting the earth) but likewise there may be insufficient time for an object to speed up to a terminal velocity if the height is not sufficient.

    While the acceleration due to gravity can be assumed constant, things that effect the drag force will be the drag coefficient of the body, air density, it's current velocity, and the reference area. So the rate of change of the velocity will be some function that takes those into account, which themselves may change over time (such as air density).
  13. RajeshTrivedi Valued Senior Member

    Kittamaru, your post #66 shows that you are talking from an assessment point of view, some kind of personal (or read) experience, I am talking from the mathematical perspective. You know its like this, we know that an apple falls, but is the gravity so simple? Well your #66 is good for chit chat, but not good contains no science.
  14. exchemist Valued Senior Member

    This is untrue. Qualitative answers have a perfectly legitimate place in science. (cf. use of Group Theory in chemistry, for example) The OP question which was being addressed does not require a quantitative answer.
  15. sculptor Valued Senior Member

    GEE darn, just when it could get interesting......................(sigh)

    The above mentioned bullet would have a maximum range of about 5 miles or 26400 feet. fired from the ground to the ground.
    It seems most likely that the maximum range when fired from the stratosphere to the ground would be in excess of the actual distance to the ground, ergo would be traveling at a faster speed than a like object falling from the same height upon impact..............
    (all is a guess)
  16. exchemist Valued Senior Member

    By all means pursue it!

    My only quibble is with the assertion - not by you, obviously - that the qualitative answer to the original question contains no science and is mere chit-chat.
  17. Michael 345 Looking for Bali in Nov Valued Senior Member

    I'm with you but puzzling

    Why 10,000 feet?

    By George I got it

    The plane was flying at 10,000 feet

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    I'm also with you but think there is science in working out the exact answer which some post appear to be taking waaay to serious
  18. RajeshTrivedi Valued Senior Member

    Yes, you are right regarding the legitimacy of qualitative response, if it looks plausible and verifiable to certain extent.

    1. In this case I feel that 40000 ft may not be sufficient for the bullet to acquire the terminal speed.
    2. It is not clear without maths when the horizontal component of 200 m/sec will become zero due to drag, because only after that the possibility of terminal speed can arise.

    Moreover origin seems to have bullet drop in mind while referring to terminal speed which is easier and available in literature, for a bullet shot with 200i+800j with many other uncertain variables, the qualitative aspect may or may not hold. Thats what i said in my post #51.
  19. Kittamaru Never cruel nor cowardly... Staff Member

    You still have yet to show any reason why anyone should undertake the effort of running the math to its full extent - either you agree that the bullet will slow towards terminal velocity, or you are arguing it will not.

    That you are refusing to make an actual statement and stick to it is one of the ways you are trolling.

    Also - why are you introducing horizontal speed? It was not part of the original post...
  20. RajeshTrivedi Valued Senior Member


    Please refer to OP and my first ppost at #51.

    OP says "high flying aircraft" it hovering? I am sorry but you are giving me an impression that you lack insight on the subject.

    1. You still have not answered why it is needless to provide maths. when so much of actual needless crap is around, at least detailed maths in science section would be welcome.

    2. Why are advocating for origin's time, he made a claim that it is simple and terminal velocity is achieved. Let him demonstrate. I have very clearly stated that variables involved are too many for a simple brushing aside of this problem.
  21. paddoboy Valued Senior Member

    Obviously you have a problem.
    The question asked was simple, and the answer given was simple as well as correct.
    So why are you demanding some mathematically complicated superfluous answer and at the same time, demeaning anyone that dares question your totally unjust and uncalled for stance.
    If I didnt know better, I would say you sound like.......ahh never mind!

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  22. iceaura Valued Senior Member

    If a bullet from a sniper rifle will carry five horizontal miles before hitting the ground, it clearly is not feeling its muzzle velocity reduced to 0 by air drag within 25,000 feet even at low altitude air density.
  23. origin In a democracy you deserve the leaders you elect. Valued Senior Member

    That is not correct. The horizontal velocity can be ignored in the problem.
    Here is another example that may help you to understand; if I shoot a bullet it will have a large horizontal veloctiy but it will accelerate towards the earth at the same rate as a bullet with no horizontal velocity, in other words the horizontal velocity can be ignored.

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