# Thread: OT: Question regarding gravity

1. ## OT: Question regarding gravity

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

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. ## 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. ## 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 <brian@isi.edu>
The Astronomy Corner at http://astro.isi.edu/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

4. ## OT: Question regarding gravity

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

Brian Tung wrote:

5. ## 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).

Regards,
Martin Brown

6. ## 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 ;-)

http://www.phys.unsw.edu.au/explorat...es/hunter.html

Al

7. ## OT: Question regarding gravity

"Al" <null@null.com> wrote in message
news:452b96e9\$0\$22937\$afc38c87@news.optusnet.com.a u...
of

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
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. ## 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.

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
physics.

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.

-John

"Brian Tung" <brian@isi.edu> wrote in message
news:egfjn5\$30k\$1@praesepe.isi.edu...

9. ## 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. :-)

Jim

10. ## 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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