bullets and physics?

[njrich]

having no knowledge of physics i couldn't refute the debate at the dinner table today.my nephew who is really into it told us he read a article written by MIT scientists stating a bullet leaving the barrel will hit the ground at exacly the same time the ejected case will.

your thoughts guys please.

[lndshrk]

Would a 10 pound brick or 10 pounds of feathers hit the ground first?

If the bullet was fired flat (read no upward trajectory) and the brass was ejected perfectly horizontaly, then the statement is probably true.

[SmittyFL]

Well he's on the right track but not exactly right. The theory is one of gravity. If the barrel (not the sights) is parallel to the ground and you drop a bullet (or a case) at exactly the same time as the bullet leaves the barrel they would hit the ground at the same time.

The theory is gravity has the same effect regardless of the fact the bullet is projected forward by the gunpowder. Gravity still pulls it down at the same rate as an object dropped straight to the ground.

His theory wouldn't work because the case doesn't come out of the chamber the same time the bullet leaves the barrel plus the case is ejected upwards or has other factors affecting it. But he has the right idea.

[MikeFoley]

No way. The ejected case varies in how high and right it will go. The mass of each case is slightly different, and the speed and weight of the bullet determine the flight time. Try this, shoot a 300 yard steel target and time the hit. Shoot another round and time the brass hitting the pavement. I would like to read this article if anyone can find it. This sounds like one for Mythbusters. Now with that said, I can't even spell MIT.

[MikeFoley]

Throw a baseball and drop a baseball at the same time...distance from point a to points b and c are different. If the target were three feet away, and so was the ground, then the propellant would still be faster or slower than gravity. The fired round has two forces acting on it, and the dropped case has only gravity.

[njrich]

i'll ask him to give me a copy of the article and post it.

i understand the point about gravity having the same effect but can't grasp it for some reason.

just for example lets use a 22 rifle.

on a flat plane and that was part of the debate.

i can't see how the case could possibly hit at the same time when a 22 lr. can travel up to a mile(i've read?)

[mcoliver]

Hey, shouldn't you guys be out practicing instead?

[MikeFoley]

Hey, shouldn't you guys be out practicing instead?

If one is not shooting, I can only think of one other activity where this is not a fair question, and I hope to never hear it at that time.

[Yardbird]

If you fire a bullet perfectly level over perfectly level ground and drop a bullet from the same height at exactly the same time they will hit the ground at the same time. But I agree with the others here about the case ejecting at some angle other than straight down. Ain't gonna happen.

[MikeFoley]

We forgot the words "in a vacuum".

[njrich]

on a lighter side we had over 30 people at the table today.

doctor,lawyer,teachers,at least 10 college grads and students.

of all the possible conversations that could come up in this group of very educated people and some like me not lol.

the most heated one was this one. gotta love it bullets and shooting while passing the bird around and drinking good wine.

[EricW]

On the moon...maybe with all the previous conditions as stated. On Earth in a standard atmosphere....no way Jose. I know better than that and I didn't even go to MIT.

Idle eggheads are the Devil's workshop.

[Skywalker]

In an ideal case (vacuum, no air, thus no drag force acting against the motion), given that g acceleration is the same for everything on the earth (but this is an approximation as well), a feather and a rock, dropped straight on the ground from the same height, will land at the same time.

The fact that the bullet is travelling downrange, while the brass is not, doesn't add to this: to compute the time they need to touch ground you only consider initial velocity and acceleration components perpendicular to the ground, the rest doesn't have any influence.

But we're in real world, we have air (thus there is drag force - different for bullet and brass - acting against the fall), the bullet might not be fired exactly parallel to the ground (thus there will be a vertical initial component of velocity and acceleration), the brass is not dropped straight towards the ground (it usually has a climbing initial velocity and acceleration).

All in all, there is a very slight chance that all these jamming factors sum up to give the result your nephew reported, and my pick is that they won't.

Probably, the article will list a serie of initial assumptions/semplifications that will lead to the conclusion that bullet and brass are going to land at the same time.

[Skywalker]

Related to this: I once saw a physics demonstration of the above principle (feather and rock dropping at same time) on a tv show.

They had set up a crossbow, loaded with an arrow, aiming at a ballon filled with water, suspended at some 2m from ground; the crossbow was levelled with the ballon.

Both crossbow and ballon were wired to electromechanic actuators to ensure ballon was released and crossbow fired at exactly the same time.

The aim of this was to demonstrate that the fall of the ballon and the arrow (due to the gravity) was the same, and even if the crossbow was aiming at initial ballon position, the arrow would have eventually hit the ballon (which punctually happened).

In this case, different air drag was compensated by the different sizes between batloon and arrow tip (arrow tip had a bigger surface to hit than its own size).

[Pierruiggi]

I believe Luca explained it perfectly. Good job!

[Tri Burst]

Everything falls at the same rate.

If you could drop something from the same level line of the barrel just as the bullet left the barrel and the barrel was completely level, NO upward tilt as would be normal, both the bullet and whatever you dropped would fall at the same rate and hit the ground, if the ground was totally level, at the same time.

The object you drop does not have to be the same weight as the bullet but it needs to have the same, or very similiar, drag coefficient. If you dropped a feather the feather would not hit the ground at the same time because it has more surface area, drag coefficient, and the air would restrict it's fall.

As to whether the brass would hit the ground at the same time all depends on the upward movement of the bullet and the brass and whether or not they both reached the apex, top, of there arcs at the same time. Nothing stops GRAVITY.

[Pierruiggi]

The point is that you CAN, by artificially setting up a series of variables (like the barrel's angle, the ejection timing and angle, etc.) make bullet and brass hit the ground at the same time. But only as an experiment.

It is not something that happens regularly and everytime, like njrich's nephew says he read.

[John Dunn]

No Physics for you! Come back...one year! [Physics Nazi Mode/ off]

Do a search for the thread on angled shooting if you really want to be bored to tears by a related problem!

[Pierruiggi]

HEY!

I take that as a personal attack, John

[zorba]

Hey !!! GO SHOOT MAN !!!

[shred]

Around here (planet Earth), it's probably safe to say that unless the barrel is pointing significantly upwards, the bullet will impact something before the brass does (excepting Glock shooters and their foreheads, of course)

[mcoliver]

For all your Physics needs: Physics Forum

I took up Applied Physics in college but all I remember now are those silky legs of my Social Studies teacher.

[Sam]

Take the little nerd out to the range and show him how you can whip out yer blaster and put two holes in a 7 yd target before any brass hits the ground.

[dajarrel]

It is interesting to hear the discussions that can occur when discussing theory vs real life. Theoretically, the feather and the rock will hit at the same time under controlled conditions It seems that air messes everyting up. The same can be said about the bullet and the brass.

OBTW, who is the ijit downrange with a stopwatch catching the bullet???? I can probably time the brass, but you will be one lucky sucker to catch the bullet hitting the ground. (If it doesn't hit you )

FWIW

dj

[blackdragon]

After reading this thread, my head hurts, I think I will have a beer and read my physics book.

Ivan