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Where do you aim at the target if it falls when you pull the trigger?

  • Above the target.

    Votes: 0 0.0%
  • At the target.

    Votes: 17 73.9%
  • Below the target.

    Votes: 3 13.0%
  • You can't know; It depends on velocity.

    Votes: 3 13.0%

Ballistics Question from Engineering Course

11K views 53 replies 26 participants last post by  miestro_jerry 
#1 · (Edited)
I am taking Mechanical Engineering Technology and will be finished this spring. One question that was asked is ballistics related - how many of you can get it right?

Keep in mind that this is more of a theoretical question, not a real-world question. Things like wind, air resistance, and time from trigger pull to bullet exiting the barrel is neglected.

You are going to shoot at a target that will fall as soon as you pull the trigger. Where do you aim? Above the target, below the target, at the target, or it depends on velocity?

Please don't explain you answer until the poll is done in one week (let others put in their vote first), then we can talk about it.
 
#2 ·
Are we assuming the sights are aligned to be parallel with the barrel, and not designed to raise the barrel slightly for a trajectory? That's how this type of question is usually put.
 
#5 ·
Sorry, I thought about this later. This isn't where the scope is aimed, but rather where the bore is aimed (if you were to pull the bolt out and look down the barrel).

And may we assume that the target was placed at the same height as the barrel before it started to fall?
That can be part of the discussion on Sunday. Make The assumption if it helps you pick an answer.


I'm not trying to toot my own horn here - I've taken a total of 2.5 years of engineering classes and I still got the question wrong. :rolleyes: This isn't so much a competition but an exercise in physics.

Later we'll talk about the feasibility of perpetual motion. :p (jk)
 
#6 ·
If the bore is aimed at the target, then I know the answer. However, I'll wait and see what gets posted before I say anything. I had a quite similar issue come up in my physics class many moons ago in college. The professor actually had a device that demonstrated the answer. Interesting question. :)
 
#9 · (Edited)
Assuming the target is 100yards away, Assuming the average projectile is moving 3000 ft/sec. the difference in time is .1 sec for the projectile to leave the end of the barrel and reach the target. At the time the bullet strikes the target. Accelleration due to gravity is 9.8m/sec/sec or 9.8m/sec2(squared)
At the end of the .1 sec the target would be falling at 9.8m/100 or .098m/sec but due to constant acceleration from 0m/sec the target would have only fallen 1/2 of the velocity of which it is traveling at that moment in time. 1m=39.67" so 9.8m/100 is 3.89"/2(due to accelleration from 0) means the target would have only fallen 1.94" So if you are using a scoped rifle zeroed at 100yards or an iron sighted rifle your point of aim should be 2" below, however if the rifle is bore sighted to the target all things fall at the same speed (neglecting wind resistance) and you should just aim at the target. I'm guessing that if the the professor is neglecting wind resistance, Ballistic coefficient, distance from target, velocity of projectile, humidity, temperature, elevation above sea level, elevation difference between rifle and target. Due to the extreme lack of vital information in the proposed question. I am goinng to go with the most simplistic answer.

FLAME ON
 
#15 ·
If the rifle is zeroed at the same range, shooting AT the target, where drop has already be adjusted for, will put the bullet exactly where the target was when the fall started.

With any moving target we must aim at where it's going to be when the bullet arrives at the same point in space.
 
#18 ·
Well thanks for everyone for voting, it seems the majority of you was right. Since both fall at the same rate, if you aim the bore at the target, you will hit the target.

I answered "it depends on velocity" in school, and subsequently got flamed for choosing wrong (guess I should have known :oops: ).

Thanks again for playing. :)
 
#21 · (Edited)
I remember those stupid questions in college physics, made you think for a second, but they were all usually really easy. I didn't get to vote, oh well. I would have said dead on due to the fact that I remember how the Professors teach it, and how physics works on paper.

As you said the trigger pull time to barrel exiting is neglected, but if they failed to mention that you would have to aim below your target just a bit due to the fact that the bullet doesn't start falling when you pull the trigger, but when it exits the barrel. In all reality, most bullets usually have an arch after they leave the barrel so they are launched up above the bores line of aim, then part way start falling. That's why if I zero my .243 rifle @ 200 it will also be zeroed at 50 yards. It arches above the bores line of site between those two, so then you would need to aim lower yet. So theoretically the bullet only falls 1/2 to 2/3 the time that the target will fall due to the arch.

The answer to this question is assuming that the bullet starts falling as soon as it leaves the barrel. In real life you would have to aim where the target will be when your given bullet at a given speed will reach it being that the rifle is zeroed at that range.

The question is flawed.
 
#19 · (Edited)
The simultaneous drop thing usually only works in a vacuum. In air, the correct answer for most bullet shapes is you will have to bore sight slightly below the target, but not nearly as far below as the target is going to fall during the bullet time of flight. That is because most bullet shapes pitch slightly upward in flight, creating a small lift component. Thus, they do not fall quite as fast as they would if simply dropped from a stationary position.

An example of an exception would be a round ball projectile, though it must be spin stabilized to prevent any rotation on a horizontal axis not coincident with the trajectory. If it gets that, it will either have lift or act like a sinker ball, depending on which way the rotation is going? Not to mention veering off to the side.

Of course, this can all work out backward if the target is paper and is far enough away? If you are at, say, 1000 yards, and the bullet is going to take well over a second to arrive, then air pressure may have time catch the bottom edge of a paper target sliding down the backer and start it turning onto its back, in which case it may be the slower falling object of the two? But this won't be a consistent problem. Bullet lift will.
 
#20 · (Edited)
"You are going to shoot at a target that will fall as soon as you pull the trigger. Where do you aim? Above the target, below the target, at the target, or it depends on velocity?"

"it seems the majority of you was right. Since both fall at the same rate, if you aim the bore at the target, you will hit the target."


Your intitial premise didn't specify that you/we were aiming "the bore" at the target. That, vs. the line of sight on a zeroed rifle which is corrected for drop over the 'as fired distance' do make a difference, don't it?
 
#23 · (Edited)
The dimples are to break up boundary layer drag. Theoretically should improve BC, but won't affect gravity's influence.
 
#24 ·
That question was on a seventh grade general science test that I took about 50 years ago.

Does this mean that they are teaching seventh grade general science in college engineering now? I guess they are too busy teaching the politically correct agenda in the lower grades now.
 
#33 ·
And it was demonstrated and simplistically explained in a grade-school assembly I attended in the late 70s or early 80s. They used balloons and arrows for simplicity's sake, but it's the same question and demonstration. It was fun! but I was later surprised at how many of my classmates had forgotten it once we got to high school and college.
 
#26 ·
Don't post here much...but lurkin' around and saw this....and couldn't resist.

Assuming a perfect 100-yard zero and assuming no deviation between POA and POI...the bullet will strike high.

Why?

Time. Nothing happens instantaneously. It takes time for the striker to fall. Time for the primer to ignite and light the powder. Time for the bullet to reach the muzzle.
(Gravity doesn't start to pull the bullet down until it gets loose from the barrel)

Meanwhile, the target is accelerating toward the Earth's center at a constant rate.

So...It'll hit high. Not much...but it'll hit high.

Cheers
 
#27 ·
That and the small lift component I mentioned. Both will make it strikes slightly high. I watched the Mythbusters try to resolve this. They ran some extreme slow motion images of the .45 ACP hitting the ground just a fraction after a dropped bullet did, but they apparently thought their instrumental resolution was poor enough that they called that slight difference a simultaneous hit. But it wasn't.
 
#28 ·
Nick...I dunno about tyhe lift question. In order for an object in flight to create lift, there has to be a pressure differential between the top and bottom...which requires an asymmetrical shape, like an aircraft wing. Since the bullet is symmetrical, and spinning...there could be no lift. More likely an instrument glitch or operator error.

Those Mythbuster guys are pretty entertaining...but I've noticed that they miss the mark a lot.

F'rinstance...the experment that they did on a thrown recap blasting through a windshield and decapitating the car's occupants. They shot the tread straight backward into the target at an impact velocity of 60 mph. In the real world, the tread wouldn't impact at the same speed of the vehicle that it was thrown from, because the tread would be moving in the same direction as the target vehicle, at roughly the same speed. The impact speed of the tread would be the difference between the target vehicle's speed and the decelerated speed of the tread. With only a 3-4 car-length lead...the difference wouldn't be very much. I was a little surprised that they missed something that obvious.
 
#29 · (Edited)
Tuner,

Yes. They miss a lot.

As to the lift, asymmetry isn't required, it's just the best aerodynamic way to get lift with the least drag. You can, instead, as many cheap model airplanes do, use a flat wing with a lot of elevator so the wing keeps slightly pitched up into the oncoming air, and air deflects off the bottom of it, creating lift between it the underside of the wing and the partial vacuum drawn by its top side. This is what happens when you stick a flat hand out the window of your car at highway speed, and tip the leading edge of your fingers up. It is how the flaps on an airplane add lift at slow speeds, though you need to be willing to waste fuel on the extra power it takes to pull the airplane forward against the extra drag they create.

In the case of the bullet, it may be symmetrical, but because it pitches slightly upward, it doesn't penetrate the air symmetrically. As a result, a small portion of the vacuum at the base of the bullet is displaced toward the top rear side of the bullet, creating lift by reducing opposition to the air resistance encountered by the front underside of the nose.

By the way, if you spin a bullet too fast, assuming it doesn't come apart, you are increasing the gyroscopic forces at work, so the pitch increases. The lift then also increases, but the net drag from that greater pitch and lift slows the bullet sooner. Thus, if over-spun, the bullet has a reduced ballistic coefficient because it is presenting a slightly larger profile to into the air.
 
#30 ·
Aerodynamic lift, gravity fall rate, scratches arm pit and yawns loudly. Hey guys the OP said aim at the target above or below. You got to aim below, aim where the target is going to be. If you aim at the target it is going to hit where the target was.
 
#31 ·
"Pitches up?"

I don't understand how a bullet would pitch up except from the "rise" due to having the rifle itself angled upward in order to compensate for drop...or from the weapon rising upward in recoil.

Many people...including the Mythbusters...don't seem to understand that the gun is in recoil before the bullet leaves the barrel. If it torques upward...the bullet will exit at a higher angle, delaying its descent to terra firma.

The only way to really test the "Fired vs Falling" bullet for ground strike is to have the rifle or pistol in a machine rest that holds the gun parallel to the ground, and doesn't allow the weapon to move when it fires...and to drop the other bullet at the same instant that the fired bullet clears the muzzle and comes under the effects of gravity. That's gonna be the tricky part...calculating the bullet's exact moment of exit.
 
#35 · (Edited)
The barrel would also have to be completely ridgid. I got to see a test one time, where a .30-06 sporter was fired and filmed with high speed photography (Maybe something like 5,000 frames per second, over about .75 seconds), then played in extremely slow motion. It honestly look like the barrel bends about 1/2" in an "S" shape as the bullet travels down it. We were actually able to trace the bullet down the barrel. The barrel in front of the bullet actually compressed (in all planes), and then "sagged" into the S shape. As the bullet travelled down the barrel, the barrel straightened out and stretched (again, in all planes) behind the bullet.

This also appears to be how a barrel stabilizer works, by adding more mass to the end of the barrel. This reduces the "wagging" effect as the bullet reches the last few inches of the barrel, and tries to wag it faster.
 
#32 ·
Hey grand you need to get the book Ballistics:Theory and design of guns and ammunition. Then after you read and understand it all.. you can translate for me and the rest of us non engineer types. Thanks! :)
 
#36 · (Edited)
Grump: " You got to aim below, aim where the target is going to be. If you aim at the target it is going to hit where the target was."

Correct.

The whole premise of the "engineering" questions is that gravity works on bullets exactly as it does on everthing else; if bullet is hand held and dropped at the moment a fired bullet exits a perfectly horizontal barrel both bullet will hit the ground at the same time.

One of the principles of engineering tests is to answer the question as it's presented, not as you may think it should have been worded. Problem here is the question was not correctly presented to establish the parameters of the "experiment" properly so the posters desired "conventional" answer of shooting at the target is NOT correct. Nothing was said of "aiming the bore" nor that the bore must be firmly held nor that the bore and target are on the same vertical plane. Thus, as the question is posed, the shooter would have to aim low, at the point where the paths of the bullet and target will intersect.
 
#40 ·
As a teacher for over 30 years, I have come to believe that "trick questions" are not good questions. A question that admittedly does not establish the parameters of the experiment cannot be answered correctly.

The truly correct answer to the question, as posed, is that there is not enough information to answer it.
 
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