Anyone see the Mythbusters the other day?

[Cabinetman]

Mythbusters have begun airing their new shows and their first one had an interesting firearms scenario.

The "myth" they tried to bust was this: That a bullet dropped from a specific height would hit the ground at precisely the same time as a bullet fired from a gun at the same height. According to them, the laws of gravity prevail even though one if dropped and one is fired.

Whatdaya know. They were right. In a very large military warehouse, they had horizontally mounted and fired what appeared to be a .45 . The muzzle was 36" off the ground. At the same time the handgun was triggered, the drop bullet was released. The fired bullet and the dropped bullet both hit at precisely the same moment.

I know I shouldn't be surprised at this but it was cool to see it actually happen. Cool.

Rome

 

[C-pher]

It's cool when they do physics problems that screw with your head. Much like the one with the plane and pulling the rug out from under it.

 

[Bob P]

Physics is cool!

 

[BenDover]

Did they do it at heights higher than 36 inches? Something smells wrong.

If I fire a bullet at 1000fps it takes 1 second to travel 1000 feet (up down sideways whatever). If I drop a bullet from 1000ft its gonna take much longer than a second to reach the ground....the accel rate is 15fps/fps if I remember correctly?

I could be missing something my head is cloudy with Nyquill at this moment.

 

[lupis42]

Did they do it at heights higher than 36 inches? Something smells wrong.

If I fire a bullet at 1000fps it takes 1 second to travel 1000 feet (up down sideways whatever). If I drop a bullet from 1000ft its gonna take much longer than a second to reach the ground....the accel rate is 15fps/fps if I remember correctly?

The bullet was fired sideways. The point is that regardless of how fast it's moving in the horizontal plane, the downward acceleration due to gravity is the same, and so the time to fall is the same. Firing the bullet upward or downward would affect that time, because the downward acceleration would not be solely from gravity at that point.

 

[Dmn0166]

I agree with Chaynes.

 

[BenDover]

got it...it sounded to me like they were firing the bullet from 36 inches into the ground...makes more sense.

EDIT: wonder if the results would be the same with a golf ball? The dimples creating lift and all.

The bullet was fired sideways. The point is that regardless of how fast it's moving in the horizontal plane, the downward acceleration due to gravity is the same, and so the time to fall is the same. Firing the bullet upward or downward would affect that time, because the downward acceleration would not be solely from gravity at that point.

 

[Gonzo]

Physics is cool!

You can't sit at the cool lunch table anymore.

 

[Blitz1]

You can't sit at the cool lunch table anymore.

What do you mean "anymore"

 

[Bob P]

Awe c'mon guys! I'll let you beat me up!

 

[Gonzo]

Awe c'mon guys! I'll let you beat me up!

Pants him!

 

[MisterHappy]

I wonder if they'll do the "monkey in the tree" Problem?

( for those who don't know , the premise is that since the bullet and the monkey are falling at the same rate, when the muzzle flash scares the monkey, he lets go; the bullet, also under the influence of gravity, hits him...and yes, I know that there are many other factors that make this not work, e.g. reaction time of the monkey, but this is one of the "simplified" problems, like the ones that use the frictionless bearings.)

When I was presented with the problem, first I solved it the way it was "supposed" to be solved; I then pointed out that that was for a bore-sighted rifle, and that a real shooter would have the sights adjusted to strike at point of aim at monkey-distance. Fortunately, this was long enough ago that understanding this sort of thing in high school merely made me weird, and not a terrorist!


Considering how many monkeys they'd go through, I'm guessing that Mythbusters will pass on this one!

And you're ALL welcome to sit a my table at lunch.

 

[C-pher]

I wonder if they'll do the "monkey in the tree" Problem?

( for those who don't know , the premise is that since the bullet and the monkey are falling at the same rate, when the muzzle flash scares the monkey, he lets go; the bullet, also under the influence of gravity, hits him...and yes, I know that there are many other factors that make this not work, e.g. reaction time of the monkey, but this is one of the "simplified" problems, like the ones that use the frictionless bearings.)

When I was presented with the problem, first I solved it the way it was "supposed" to be solved; I then pointed out that that was for a bore-sighted rifle, and that a real shooter would have the sights adjusted to strike at point of aim at monkey-distance. Fortunately, this was long enough ago that understanding this sort of thing in high school merely made me weird, and not a terrorist!


Considering how many monkeys they'd go through, I'm guessing that Mythbusters will pass on this one!

And you're ALL welcome to sit a my table at lunch.

Not if Grant makes a robot Monkey...

 

[Bob P]

Not if Grant makes a robot Monkey...

I, for one, welcome our new Robot Monkey Overlords!

 

[MisterHappy]

I'm sure it will just angry-up the "Save-the-Robot-Monkey" crowd.

Best stay clear of THAT can of worms!

 

[Polibious]

Objects fall at 32 ft./sec. squared (mass isn't a variable in the equation). You could drop me and a sherman tank from the same heigh at the same time and we would hit the ground at the same time, not accounting for obvious resistance (a parachute would hit the ground much later). You could drop almost any object from the height the gun was fired and it would hit the ground at the same time as the bullet.

 

[Fixxah]

I could catch the bullet that was dropped in my hand while someone else could catch the one fired from the gun. It should work, right?

 

[Big Daddy 45acp]

Of course, they forgot to factor in the lock time in their experiment.
Had they done this, their results would have been closer.
I mean, come on, 39 milliseconds apart?

 

[jdubois]

I mean, come on, 39 milliseconds apart?

That was my thought. Since we all knew the physics (I hope!), the only thing left for the story was to try and get it as exact as they could. The math works out to a 432ms fall time from 36", so 39ms is about 9% error. They could have gotten it closer than that!

I was also disappointed that they didn't discuss inertial mass vs frictional force in the other, knock your socks off, segment to explain why the socks weren't getting knocked off.

 

[TomH]

got it...it sounded to me like they were firing the bullet from 36 inches into the ground...makes more sense.

EDIT: wonder if the results would be the same with a golf ball? The dimples creating lift and all.

I thought that too when I first read it. The OP wasn't very clear. This is basic physics, vertical and horizontal components of travel are completely seperate (in normal earth gravity situations.) Another thing that is really cool is that a feather and a heavy metallic object will both reach the ground at the same time in a vacuum.

 

[TomH]

Objects fall at 32 ft./sec. squared (mass isn't a variable in the equation). You could drop me and a sherman tank from the same heigh at the same time and we would hit the ground at the same time, not accounting for obvious resistance (a parachute would hit the ground much later). You could drop almost any object from the height the gun was fired and it would hit the ground at the same time as the bullet.

Sorry, but this is wrong. This would be true absent the atmosphere. However since you and the tank have different size and wind resistance you will not hit at the same time. What they are talking about on the mythbuster's piece are two objects with the same dimensions, and the same mass. Therefore they travel vertically through the air at about the same. The slight variation they found could be the difference in orientation in each of the bullet's travel.

 

[Cabinetman]

At one time on the Moon, I believe, one of the Astronauts did just that: drop a feather and a rock or some other heavy object in the vacuum on the Moon to demonstrate that principle. Since there is no air resistance, the feather did drop at the same speed as the rock as expected.

Regarding the "knock your socks off" segment, I've read reports of airplane crashes when I worked for Delta back in the 70s. If the cabins were intact after a crash, many times passengers were found still strapped in their seats, deceased, of course. However, the seat, itself, was stripped of all cloth and the passengers were many times naked with one exception: the elastic bands in their socks remained on their ankles. I know this is a graphic description of a tragic situation. But, the reason I mention it here to demonstrate the massive amount of force that those passengers were subjected to in an instant; strong enough to rip their clothes off and remove the fabric from the chair they are sitting in.

Rome

 

[vellnueve]

Sorry, but this is wrong. This would be true absent the atmosphere. However since you and the tank have different size and wind resistance you will not hit at the same time. What they are talking about on the mythbuster's piece are two objects with the same dimensions, and the same mass. Therefore they travel vertically through the air at about the same. The slight variation they found could be the difference in orientation in each of the bullet's travel.

I think he noted that air resistance would play a factor.

 

[jdubois]

At one time on the Moon, I believe, one of the Astronauts did just that: drop a feather and a rock or some other heavy object in the vacuum on the Moon to demonstrate that principle. Since there is no air resistance, the feather did drop at the same speed as the rock as expected.

http://www.youtube.com/watch?v=5C5_dOEyAfk

Looks pretty convincing too, I wonder how they managed to do that on that fake moon set .

 

[Cabinetman]

Ha! Thanks for finding that video. I didn't recall the hammer but that works. And, yes, it would have been hard to recreate that experiment on a fake moon set. Dummies.

Rome

 

[zeppelinfromled]

There was a cool video (what I think was a cool video) of dropping a feather and a hammer at the same time, but on the moon. On earth, they wouldn't hit the ground at the same time because of air resistance, but since the moon doesn't have an atmosphere, it works.

http://video.google.com/videoplay?docid=6926891572259784994#

ETA: I also love how he calls him Mr. Galileo, although Galileo was his first name.

ETA2: What? People beat me to it and I didn't see it. That's sad.

 

[derrick_lui]

I remember my high school physics teacher asking the class this question. Everyone including myself raised our hands saying the bullet not fired from the gun would hit the ground first.

Then he started explaining to us why we were wrong.