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  1. #1
    nawt2smart's Avatar
    nawt2smart Guest

    Default OT: Question regarding gravity

    Long time lurker and I know you guys/gals will be able to answer this

    No I am not a trolling. I was talking to my dad on the phone tonight
    and somehow the concept of gravity came up.

    Scenario: I am shooting a high powered rifle at eye level directly
    parallel to earth. The round is travelling at 3000 fps from the
    barrel. Say 308 win. No wind, but air friction. This is a flat
    ground area, say salt flats.

    At the same exact time I drop a marble at eye level.

    Which will hit first?

    I said the marble because velocity throws the equation out of balance.
    Thanks for the help!

  2. #2
    Sam Wormley's Avatar
    Sam Wormley Guest

    Default OT: Question regarding gravity

    nawt2smart wrote:

    They hit the ground at the same time... ignoring air friction all
    objects fall at the same rate. Flat is not required... works
    around the curvature of the Earth.

  3. #3
    Brian Tung's Avatar
    Brian Tung Guest

    Default OT: Question regarding gravity

    Sam Wormley wrote:

    First of all, OP is explicitly *not* ignoring air friction. He wants it
    taken into account. Secondly, what you say obviously cannot work around
    the curvature of the Earth, or else satellites could not orbit. I know
    you know this, so you must mean something other than what you wrote, but
    I can't figure out what.

    Taking into account air friction is difficult, especially with rifling.
    My guess is that the marble might beat the bullet by just a little, but
    I really don't know. Anyone else have any bright ideas?

    Brian Tung <>
    The Astronomy Corner at
    Unofficial C5+ Home Page at
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  4. #4's Avatar Guest

    Default OT: Question regarding gravity

    >From the frame of reference of the bullet, the marble or the eye at eye

    Brian Tung wrote:

  5. #5
    Martin Brown's Avatar
    Martin Brown Guest

    Default OT: Question regarding gravity

    Brian Tung wrote:

    Passes through the tangent plane at the feet of the rifle user maybe?

    But even this is wrong since if you use a fast enough projectile near
    to escape velocity then the influence of gravity is no longer
    (approximately) constant over the entire trajectory.

    Basic air viscosity is something like 1.7x10^-5 Ns/m^2. But at nearly
    Mach 3 the Reynolds number will make it much higher drag, slowing the
    bullets velocity over the flght duration. This will affect how far it
    flies, but not so far as I can see how long it will fly for.

    My instinct is that the bullet will see ground effect forces from its
    own reflected shockwave when it gets near to the ground (provided that
    the surface is very smooth - like a salt flat) and so skim along on a
    cushion of air for a little bit longer. Since it is spin stabilised by
    rifling it shouldn't tumble until it finally touches something. The
    marble will drop like a stone.

    I doubt if the experiment could be done in practice even with high
    speed cameras and a sniper rifle - tiny pointing inaccuracies would be
    hugely magnified. And if you set the bullet trajetory parellel 1mm
    above a steel plate you still need around 10ms of flight time to the
    predicted collision at 10m (perhaps 20m to allow for observing any
    interesting effects).

    Martin Brown

  6. #6
    Al's Avatar
    Al Guest

    Default OT: Question regarding gravity

    "Martin Brown" wrote

    I think I see what you're getting at but, by the same token, the bullet is
    also "falling like a stone" from a certain frame of reference. The effect of
    the world (and all its atmosphere) rushing past shouldn't change the fact
    that that it is falling like a stone...

    This link won't help, but it is on-topic ;-)


  7. #7
    Greg Neill's Avatar
    Greg Neill Guest

    Default OT: Question regarding gravity

    "Al" <> wrote in message
    news:452b96e9$0$22937$ u...

    The usual high-school analysis of the horizontally
    fired projectile versus the dropped one makes
    certain simplifying assumptions, the primary one
    being that the gravitational field is uniform
    and vertical. It also assumes that the "playing
    field" is flat and horizontal and of unlimited
    extent (with respect to the experiment at least).

    This leads to the conclusion that the horizontally
    fired projectile will trace out a parabolic arc with
    respect to the fixed horizontal ground surface, with
    the vertical and horizontal components being entirely
    independent. If these simplifying assumptions are not
    made, the arc is instead a section of an elliptical
    orbit, and the ground is a curved surface (section of
    a sphere).

    It thus becomes possible to fire a projectile
    horizontally with sufficient velocity that it can
    escape the Earth entirely, or go into an orbit that
    would not strike the surface barring energy lost
    due to air friction. The reality will, of course,
    be something in between, with the projectile
    traveling much further and longer than it would
    in the simplified model.

  8. #8
    John Banister's Avatar
    John Banister Guest

    Default OT: Question regarding gravity

    This is an old "gotcha" from the shooting world. If you take the problem
    simply, i.e. the horizontal plane is flat and the bullet is fired perfectly
    levelly, then there is only one force causing both to drop: gravity. And
    gravity works equally on both so both hit the ground at the same time.

    In high school physics we did an experiment to "prove" this: We had a
    simple machine that dropped a ball bearing at the same time that it shot
    another (via a spring) horizontally off of a table. Both hit the ground at
    the same time.

    However, if you think too much about this, you will miss the simple message.
    For instance, any bulllet fired at 3000 fps will have some instablity in
    yaw. This will cause the point of the bullet not to be in line with the
    flight path for some of the time. During that time there will be a bit more
    lift on one side of the bullet than the other, causing it to rise or fall
    due to air pressure. If the initial yaw is upward (positive lift) the
    bullet path will be slightly above horizontal, increasing the time of fall.

    There are many other things which make this problem not so simple, but the
    simple problem does indicate the counter-intuitivity (?) of the basic

    BTW, rifle bullets are rarely fired horizontally. Normally, they are fired
    so that they cross the line of sight a few yards downrange and remain above
    the line of sight for a while before dropping below.


    "Brian Tung" <> wrote in message

  9. #9
    Jim Klein's Avatar
    Jim Klein Guest

    Default OT: Question regarding gravity

    Same time.

    Sometimes it is caused by gravity, except in Guam.

    In Guam there is no gravity, the Earth just Sucks there. :-)

    It Sucked harder the longer I was stationed there. :-)


  10. #10's Avatar Guest

    Default OT: Question regarding gravity

    nawt2smart wrote:

    Both hit at the same time. Even when you add aerodynamic drag to the
    equations. Even when you use the rotational-stability of the bullet to
    keep the nose of the bullet above the plane of flight--this only ends
    up contributing to drag. Even when you use nutation carve some latteral
    distance due to aero and spin on the bullet.

    The only thing that would make the bullet hit later is the curvature of
    the earth over the path of the bullet (over the salt flats).


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