Mythbusters: bullet fired vs. bullet dropped video...

[68wj]

What I find most interesting about this thread is the focus on some of the variables that only shooters think of. Otherwise this is the exact same conversation that I remember in high school, college, and played out in other settings. Those usually portray the idea of a musket ball with no talk of spin or bullet profiles.

In this example, a .45 is about as close to a ball as you will get, so a falling bullet will present to the air about equally. I do wonder if over a long enough horizontal distance, and from high enough, the spin will have any effect (Magnus effect inducing lift?).

 

[PowerG]

Feather vs. ball bearing.

http://www.bing.com/videos/search?q=feather+vs+ball+gravity&FORM=VIRE15#view=detail&mid=A0FC6352DA57A5941B2AA0FC6352DA57A5941B2A

 

[ironworkerwill]

Velocity does matter to a point. If the bullet was to travel 186k miles in one second, impossible I know, it would clear earths gravity entirely and never touch the ground. But theoretically it would have an arch as earths gravity would have an effect matching that of a dropped object during the time it was affected by gravity. As the projectile travels further away from the planet the less gravity will pull on the projectile. So it would end up is space somewhere.

Forward resistance will have no effect on how long it takes to hit the ground. Gravity has the same effect on all objects: 32.17xxfps squared. Weather it be the most ballistically sound bullet fired from the fastest rifle cartridge, it makes no difference. If a projectile is launched parallel with the earths surface, say at a height of 32 feet, it will hit the ground in one second.

A bullet cross section(length wise from tip to base) is a symmetrical airfoil. It creates no lift at 0 angle of attack.

 

[DanTheFarmer]

Actually a body in free fall falls 16 feet in the first second, not 32.
It starts at 0 fps and accelerates to 32 fps in the first second. The average velocity for the first second is therefore 16 fps. 16 fps * 1 = 16 ft.

The general formula is d = 1/2 * g * t^2 where d is the distance fallen and g is the local acceleration due to gravity or 32 fps near the Earth's surface.

The velocity at any given instant is v = g * t, again where g is the local acceleration due to gravity and t is time since the free fall began.

These formulae apply to vacuums.

Dan

 

[Nature Boy]

Velocity does matter to a point. If the bullet was to travel 186k miles in one second, impossible I know, it would clear earths gravity entirely and never touch the ground.

Only as it relates to the "ground". If you were positioned in space at the exact point to observe the drop the results would still be the same and match the 32 ft/sec squared. But you knew that, LOL!

Can we stipulate that until Mythbutsers proves otherwise, Newton's laws apply here on earth as well as the rest of the know universe?

 

[MachIVshooter]

Acceleration is the result of force.

Each object (as soon as it starts falling) will have two forces acting on it in the vertical direction. The downward force due to gravity and the upward force due to drag.


Each will accelerate at a rate based on the magnitude and direction of the net force.

The object with the high drag, at any given downward velocity greater than zero, will have a much larger force in the upward direction generated by drag and that will result in a much lower net force in the downward direction. It will accelerate slower due to the smaller net force in the downward direction and will reach a lower terminal velocity. At terminal velocity, the drag force will equal the force of gravity and there will no longer be any net force. At that point the balanced forces will mean no acceleration and the object will continue to fall at terminal velocity.

The object with low drag will have very little force generated by drag which will mean that there will be a larger net force in the downward direction. It will accelerate at a higher rate and reach a higher terminal velocity.

Right. My submission is that those differences are imperceptible and inconsequential outside of theory and precision scientific calculations. Hence why I state 32 ft/sec squared, rather than specifying a precise figure several places to the right of the decimal that is calibrated to air density, altitude, distance to ground, etc.

Anyone who doesn't believe me needs to take themselves out to their garage, find a couple objects of very different shape and density (say, a steel bolt and a foam earplug). So long as neither is able to reach it's terminal velocity from the height it is dropped, you will not be able to detect different impact times visually or audibly.

The super low density and high drag objects like feathers simply reach their very low terminal velocity almost instantaneously, while the high density and low drag bodies continue to accelerate for quite some time in free fall.

 

[Sunray]

Sir Isaac Newton did the apple thing long after Galileo was dropping cannon balls off Italian towers. This is kind of neat.
http://thekidshouldseethis.com/post/the-hammer-feather-drop-in-the-worlds-biggest-vacuum-chamber
The Mythbusters gang shouldn't be allowed near any kind of firearm. The one clown claims expertise as the result of growing up on a farm. So did Elsie the Cow.

 

[wally]

I do wonder if over a long enough horizontal distance, and from high enough, the spin will have any effect (Magnus effect inducing lift?).

The Magnus effect can produce either a lift or a drop force but it depends on a wind component at right angle to the direction of flight. Its small compared to the normal drop and wind deflection, although I think artillery tables may account for it.

Bigger consequence of the Magnus effect is its effect on bullet stability because it acts on the center of pressure instead of the center of mas and will thus produce a torque that can rotate the bullet.

Its negligible below about 10 mph crosswind.

 

[ironworkerwill]

Dan: you are right, the average is the distance it would travel. With the eventual velocity of 32fps at the end of the first second. Nice catch. So we'll have to make that a 16' drop

Nature boy: if it was fired parallel to a continuous plane with 1 G of gravity it would strike that plane at the same time as the dropped object.

 

[wally]

Mythbusters: Proof that a 10 minute show can be made to last an hour.

LOL, boy is that spot on!

Gotta love the DVR!

 

[Corn-Picker]

I had a paintball gun barrel shaped like a banana (Tippman Flatline). Imparted a backspin to impart a Magnus Effect force opposite gravity. It certainly worked, though that was a different phenomena than what the mythbusters were testing.

 

[Goosey]

As a fired bullet noses downwards, this happens...​

Large red arrow is the force of air resistance, and note the vertical component circled in blue. Since the fired bullet is moving far faster than the dropped bullet, this vertical component acting on the fired bullet may be larger than the vertical component acting on the dropped bullet.

​

 

[Aragon]

No. They both accelerate at 32 ft/sec squared. The reason they won't hit the ground at the same time is that the object with lower density and/or higher drag attains a lower terminal velocity. Both accelerate to their respective terminal velocities at the same rate, vacuum or not. It's the disparity in terminal velocities that results in different times of descent.

Not true. If I drop a feather right now, it's not going to drop at a rate of 32 f/s/s. If would if I dropped it in a vacuum.

 

[Aragon]

Right. My submission is that those differences are imperceptible and inconsequential outside of theory and precision scientific calculations. Hence why I state 32 ft/sec squared, rather than specifying a precise figure several places to the right of the decimal that is calibrated to air density, altitude, distance to ground, etc...

Yet they still exist. That's all some of us are saying.

 

[highlander 5]

In high school physics class we launched and dropped items of equal size and mass using a spring loaded fixture . Every item hit the floor at the same time every time. Unfortunately cell phone and digital cameras didn't exist in the 1960s,would have been cool to watch in slow motion.

 

[JohnKSa]

My submission is that those differences are imperceptible and inconsequential outside of theory and precision scientific calculations. Hence why I state 32 ft/sec squared, rather than specifying a precise figure several places to the right of the decimal that is calibrated to air density, altitude, distance to ground, etc.

In the course of accelerating to terminal velocity, the net force starts out 1 G and decreases to zero as the drag force increases, due to increasing velocity, to equal the force of gravity. As a consequence, the acceleration of the object starts out at 32 ft/sec squared and decreases to zero at terminal velocity.

The change from 1G to zero acceleration is significant, and even if the object doesn't reach terminal velocity over the course of the drop, it's going to be significant unless the drop is very short and the terminal velocity of the object is relatively high.

Practically speaking, that means that objects with very different terminal velocities will show perceptibly different acceleration levels even if neither one reaches terminal velocity. The shorter the drop distance is, relative to the drop distance needed to attain terminal velocity, the less the difference will be. Or, one could say, if the objects don't achieve sufficient velocity for drag to be significant over the distance of the drop then there won't be a significant difference in the acceleration of the objects.

It will be a significant difference unless the comparison is limited to objects with pretty similar and relatively high terminal velocities and drop distances that are kept quite small.

Anyone who doesn't believe me needs to take themselves out to their garage, find a couple objects of very different shape and density (say, a steel bolt and a foam earplug). So long as neither is able to reach it's terminal velocity from the height it is dropped, you will not be able to detect different impact times visually or audibly.

I tried the experiment as you described it. I dropped a steel bolt and a foam earplug from waist height. It was quite easy to note the difference in impact times for the two objects.

I certainly agree that the comparison of two different bullets is unlikely to show any difference over a drop of a 5 or 6 feet because they both will have roughly similar and relatively high terminal velocities. It would definitely be reasonable to assume that they are being accelerated at identical rates over the entire drop distance and assuming that the acceleration was equal to 1G over the entire drop distance would provide accurate results.

 

[MachIVshooter]

I tried the experiment as you described it. I dropped a steel bolt and a foam earplug from waist height. It was quite easy to note the difference in impact times for the two objects.

If I do it from as high as I can hold them facing downward (7'4"), I cannot. Tried it several times, and switching hands with each piece. And I have a faster-than-average reaction time at 170ms, the corollary of which that there is also lower-than-average lag between stimuli affecting my senses and brain perceiving it, so one would think I'd pick that up, especially giving twice the distance.......

You sure that your senses aren't tricking you, with the earplug making no detectable sound at impact? The brain is likes senses to agree on inputs; if the eyes see something our subconscious tells us should make a sound, the ears expect to hear it, just the same as if we put our hand down and feel a prick, our brains are confused if we look at that point and do not see something sharp. Maybe try it with another object that will also make noise at impact, like a milk jug cap or chapstick tube?

I'd video it, but unless someone can tell me a way to make my 2 minute videos upload in less than two hours, I'm just gonna have to ask ya'll to take my word for it.

 

[JohnKSa]

Dunno what to tell you. I just tried it several more times after your last post using different methods for releasing the objects to make sure they're both being released at the same time.

Each time it was obvious that the bolt was getting to the ground before the foam earplug was. In fact, it was usually pretty easy to see that the bolt was ahead of the earplug in the air before they hit.

Using this kind of earplug.
http://www.fishandsave.com/products...sher-Softest-Foam-Ear-Plugs-29-Decibels-Qty-5

Bolt is a pretty generic bolt about 1.5" long with a half inch hex head.

Dropping the objects from about waist height.

 

[rcmodel]

That would be the expected result.

The surface area might be close to the same ( drag).
But the mass of the objects favored the bolt outrunning the earplug by a wide margin.

Try them in the bathtub and see witch one floats longest.

Water & air isn't that much different really.

rc

 

[MachIVshooter]

Perhaps your reaction times are that much faster than mine.

I kept the video to just 25 seconds, so it only took 18 minutes to upload It's a relatively high resolution digital camera, but is not a high speed camera, so a frame-by-frame yields pretty blurry images of the objects. Still, the different objects (once an M12x45mm bolt vs. foam earplug, twice the same M12 bolt vs. chapstick, switching hands) have not hit in one frame, and are both bouncing in the next all 3 times. This camera videos at 22 FPS, so each frame is 45ms.

FTR, these are dropped from a slightly lower height of about 6-1/2 feet, as I'm reaching past the camera.

https://youtu.be/o7gLlGThRJw

There is audio, but it may help to mute it. At least with the ear plug.

Now, my hands are not electric solenoids, which is why I switched the objects and repeated several times, even though only 3 drops are recorded. Though there may be a few ms difference, I cannot perceive it, the 45ms frames cannot catch it, and we don't how much of any measurable difference might be my own nerves or muscles being slightly slower in one arm than the other. So no, this isn't a truly scientific experiment, but it really needn't be; my only goal is to show that the calculated differences in acceleration of these bodies in free fall are not detectable to our senses if neither achieves terminal velocity before impact. You need elecromechanic releases and a camera with many more FPS recording to see different impact times. My guess? There's less than 10ms between impacts. That's a very small amount of time in human terms, since the literal blink of an eye is 100-150ms.

 

[HammsBeer]

Think of why parachutes work. When it's folded in the bag, you fall like a rock. When it's deployed, you float to the ground. You didn't change the weight of you or the parachute, you just changed the surface area for air drag. Density vs surface area. Bolt vs earplug. Put a big enough parachute on a bolt and the earplug will beat the bolt to the ground.

 

[MachIVshooter]

But the mass of the objects favored the bolt outrunning the earplug by a wide margin.

Watch my little video I know you can't see my hands, but I'm absolutely not cheating. I'm doing the very best that is humanly possible to hold them at identical heights and release simultaneously. Even if I were inclined to rig the experiment, I'm not an engineer or physicist; it would take me hours to calculate the differential heights required to sync impacts if there truly were a noticeable difference.

Try them in the bathtub and see witch one floats longest.

Water & air isn't that much different really.

Except that we humans float in water; last I checked, we do not float through the air except in movie magic or specially built chambers. Air at sea level averages 784 times less dense than water, which has a bit of an effect on buoyancy.

 

[rcmodel]

But, we humans are not as light as an ear plug for our size.

If we were foam ear plugs we might weigh 5 pounds.

But our size ( drag) falling through the air would remain unchanged.

A 5' 6" long bolt as big around as you or I might weigh 2,500 pounds.
With the same surface area (drag).

rc

 

[MachIVshooter]

But, we humans are not as light as an ear plug for our size.

If we were foam ear plugs we might weigh 5 pounds.

But our size ( drag) falling through the air would remain unchanged.

A 5' 6" long bolt as big around as you or I might weigh 2,500 pounds.
With the same surface area (drag).

Not debating that. It is established that more dense and aerodynamic objects have higher terminal velocities, and that they accelerate more quickly at 1G. My contention, once again, is that the difference in acceleration will be imperceptible to human senses if neither object reaches (or nearly reaches) terminal velocity prior to impact. Yes, we all know that if I drop aragon's feather with my bolt, there will be a huge and easily perceptible difference in the chronology of the impacts. But that is because the feather reached it's extremely low terminal velocity almost instantly, probably less than an inch from release.

The questions you would have to answer are 1) what is the feather's terminal velocity and 2) how long did it take to accelerate to it. I have neither the mathematical education nor the inclination to figure that out right now, but I strongly suspect the rate of acceleration is still pretty dang close to 32 ft/sec squared, that the terminal velocity is <2 ft/sec, and that it takes this very low density and highly buoyant body fewer than 50 or 60 ms to reach it.

 

[rcmodel]

I thought the OP's question was if bullets 'fly' upward when shot??
(Aerodynamic Lift)

The answer is still no, they don't.
They start falling at the acceleration of gravity as soon as they leave the muzzle.

rc