PDA

View Full Version : Maximum speed of falling objects?

Dinosaur
03-01-04, 11:15 PM
A recent TV program claimed that the maximum speed of a penny is between 35 and 65 miles per hour (56 to 104 km per hr). I guess that a penny does not fall edge first, or it would reach a higher speed.

I think that a falling human body reaches a maximum speed of about 125 MPH (200 km per hr).

How fast does a bullet fall? I have read about people being killed by bullets fired into the air and hitting a person when they fall.

Would a spherical steel ball bearing fall faster or slower than a bullet? I expect very small steel spheres to fall slower than large ones due to a larger area to mass ratio, but am not sure that medium size ones would fall slower than large ones.

Does anybody know how fast various objects fall?

CTEBO
03-02-04, 01:45 AM
A bowling ball and a walnut dropped from the same height would fall at the same speed. Although gravity has everything to do with mass, mass plays no part in determining which object falls faster. Both fall at the same rate, regardless of mass.

Technically, the only real top-speed of any falling object is the speed of light. When an object falls, it falls at a perpetuating accelerating rate. In free fall, you get faster and faster as you fall, no exception; be it a penny, bullet, person or grand piano.

FNG2k4
03-02-04, 01:57 AM
Yes all of that is true but the air gets in the way limiting how fast objects can fall. I guess to reach the fastest speed object needs to act like a wing flying straight down to cutt through the air. Perhaps an ellipse would work better then a sphere.

Or maybe your could design somethat that would try accelerating as it fell like a fixed helicopter blade designed to fly down but passively powered by the natural act of falling and then spinning.

What a contest if would be to design an object to fall to the ground the fastest. I will be betting on the rock.

Nasor
03-02-04, 02:50 AM
CTEBO: Like FNG2k4 said, all objects that fall through the air will eventually reach a maximum speed (called the 'terminal velocity') based on their mass and shape.

Dinosaur: I have also heard stories of people being injured or killed by falling bullets. The equations for calculating terminal velocity aren't very complicated. Maybe you could run them for a penny and a bullet and see how they compare?

John Connellan
03-02-04, 05:48 AM
A recent TV program claimed that the maximum speed of a penny is between 35 and 65 miles per hour (56 to 104 km per hr). I guess that a penny does not fall edge first, or it would reach a higher speed.

Well the penny would go ~35 mph when dropped face down (and it stayed face down) and it would travel 65 mph if it stayed edge down.

How fast does a bullet fall?

All objects should fall at roughly the same acceleration (unless they aren't very dense like feathers) so the speed will depend (nonlinearly) on the maximum height from which the bullet came up to a certain height. Beyond that height the bullet will not get any faster (terminal velocity).

Would a spherical steel ball bearing fall faster or slower than a bullet?

I would imagine it travels slower than a bullet (if the bullet is travelling the right way) since the bullet is aerodynamically optimised for air travel. It has a very sharp front which minimised turbulence.

Dinosaur
03-02-04, 10:22 AM
This is more complicated that you folks seem to think it is. You should not rely on intuition except for qualitative comparisons of various objects. Without some established formulae, you cannot get good numeric answers.

Doing calculations including atmospheric effects is a difficult task. Even if you assume no wind, constant density at all altitudes, and a constant amount of water vapor in the atmosphere, it does not seem easy. In a vacuum, it is easy to calculate final speed at impact with the Earth. For an object with no initial velocityDistance = Acceleration * Time<sup>2</sup> / 2

Acceleration = Approximately 32 Feet / Seconds<sup>2</sup> or 980 cm / seconds<sup>2</sup>

Time = SquareRoot( 2 * Distance / Acceleration )

Speed = Acceleration * Time

Speed = SquareRoot( 2 * Distance * Acceleration )I hope I did the above correctly. The acceleration is actually a variable, but the above is a good approximation for objects falling from less than ten miles above the earth. A reference book I have gives 32.1725 & 980.621 at sea level and 45 degrees latitude. I did a few calculations and rounded the results.From 5000 feet: 122 MPH
From 10,000 feet: 550 MPH
From 20,000 feet: 770 MPH
From 5000 meters: 1127 km / hourThe above are for falling in a vacuum.

I have heard estimates of 125 MPH as the maximum for a human body, but do not know what assumptions are made about being tucked up or spread eagled. The clothing (if any) would make some difference.

The falling penny is a very difficult problem. The lower estimate of 35MPH and the higher estimate of 65MPH are not the difference between falling flat or edge down. It is due to different assumptions about how the penny changes orientation as it falls. Falling exactly edge down is a theoretical possibility like the possibility of balancing a needle on its point. Edge down, I would expect a penny to fall faster than a human body. Note that a penny would not fall straight down. When it was at a angle with the veritical, it would have a horizontal component of velocity. It might tumble.

The TV program was debunking a myth about a penny tossed from a high building and killing somebody when it hit them in the head. It said nothing about a bullet fired upwards and hitting somebody when it descended. In a vacuum, it would impact at muzzle velocity if fired straight up, but I have no idea about the atmospheric effects.

I am not sure how a bullet would fall. I suspect that it would fall nose first after the first few hundred feet, but am not sure about this.

I was hoping that some body here knew formulae or a URL leading to some real information.

Nasor
03-02-04, 11:22 AM
It's really not that complicated. I recall having to calculate the terminal velocities of all sorts of things in my basic kinematics physics class, years ago. You can find the equations to do it here http://scienceworld.wolfram.com/physics/TerminalVelocity.html , although you'd have to look up the density for air at sea level, or wherever you want to do the calculation.

Interestingly, it doesn't matter what shape a falling object has; only the cross-section is needed for calculating terminal velocity. A pointy object would have the same terminal velocity as a flat object, so long as they had the same cross-sections pointing toward the ground and were of the same mass.

1100f
03-02-04, 11:30 AM
Interestingly, it doesn't matter what shape a falling object has; only the cross-section is needed for calculating terminal velocity. A pointy object would have the same terminal velocity as a flat object, so long as they had the same cross-sections pointing toward the ground and were of the same mass.

Are you sure of what you say here? You can look at the link that you gave and see that the drag force depends not only on the cross-section of the objects but also on the shape of the object through the drag coefficient C_d.
The smaller the drag coefficient, the higher the final velocity. (with all other parameters remaining the same, including the cross-section).

2inquisitive
03-02-04, 11:32 AM
Dinosaur, I don't know a where to find the info you are seeking, but the thread
reminded me of a program I once watched on the Science Channel, i believe.
It concerned a world record freefall jump from a high altitude balloon. I found a link to
the group that was to make the attempt, but am not sure if it has been completed
yet. The jump was to be from 130,000 ft. and velocities for the jumper were projected
to be much over Mach 1. I believe the record is Mach 1.7.
http://www.stratoquest.com/default.cfm?page=25

John Connellan
03-02-04, 12:47 PM
The falling penny is a very difficult problem. The lower estimate of 35MPH and the higher estimate of 65MPH are not the difference between falling flat or edge down. It is due to different assumptions about how the penny changes orientation as it falls. Falling exactly edge down is a theoretical possibility like the possibility of balancing a needle on its point. Edge down, I would expect a penny to fall faster than a human body. Note that a penny would not fall straight down. When it was at a angle with the veritical, it would have a horizontal component of velocity. It might tumble.

Well if the maximum speed of a penny is higher than 65 mph as u say then they're wrong right? Simple as that! The max speed will be when it is edge down all the way. Probably not as remote as balancing a needle on its head but gets more remote the further it has to travel.
Even though you're ignoring the effects of the mass/density and shape, I would also expect a penny to fall faster than a body.
Yes there might be a very small horizontal component of velocity even without a wind but this is the least of our troubles concerning average or max speed.

It said nothing about a bullet fired upwards and hitting somebody when it descended. In a vacuum, it would impact at muzzle velocity if fired straight up, but I have no idea about the atmospheric effects.

Well without a firepower behind it, the bullet is never going to fall with the muzzle velocity in an atmosphere. It is losing energy to the atmosphere all the time on the way up AND down.

I am not sure how a bullet would fall. I suspect that it would fall nose first after the first few hundred feet, but am not sure about this.

The bullet would have differential weight as well with most mass in the front to prevent deviations of the nose from pointing forwards all the time.

Dinosaur
03-02-04, 04:29 PM
2Inquisitive:Unfortunately, the link you provided mentioned exceeding Mach 1 without specifying speed in MPH or km/hour. I think the speed of sound decreases with density, approaching zero in a vacuum. What is mach 1 at 130,000 feet? This is above 99% of the atmosphere. While it seems strange, I would expect a person falling from 130,000 feet to initially accelerate and then start slowing down at some point. I have no idea of what the maximum speed would be or where it would occur.

1100f: You seem to be on the ball, realizing that the drag coefficient is affected by the shape of the object, making cross sectional area and mass only part of the equation. I have heard it claimed that a smooth golf ball would travel about half the distance of a dimpled ball due to the effect of the dimples on the drag coefficient. This effect would also apply to a falling object.

Another issue is the effect of shape on orientation while an object is falling. Assuming still air, a sphere with the same mass and cross sectional area as the face of a penny would fall vertically. I would not expect the penny to fall vertically with one face down. Without actually experimenting I cannot be sure, but I would expect the penny to change orientation as it fell. I would also expect a horizontal component of velocity, and perhaps tumbling. I have no idea of how orientation would affect the vertical component of velocity. As the orientation changed, the cross sectional area and perhaps the drag coefficent would change. A horizontal componet of velocity would (I think) be at the expense of the vertical component.

As mentioned in a previous post, it has been claimed that a bullet fired into the air would be potentially lethal when it came down. Obviously atmospheric friction would slow it down. Would it be slowed enough to make it non-lethal? Small caliber weapons with a low muzzle velocity would surely be less dangerous than a heavier bullet with a high muzzle velocity. There have been stories about people being killed by a rifle or hand gun fired into the air. Are such stories merely myths?

2inquisitive
03-02-04, 06:33 PM
There were some links on the page for more information. Joe Kittinger, who made the
current record jump in 1960 from 102,800 ft. reached a maximum speed of 614 mph,
"just under the speed of sound for his altitude." He opened his parachute at above
90,000 ft, but I don't know the exact altitude the speed was reached.
A excerpt from the Popular Science article linked to from the Stratoquest site:
"Twelve seconds later, her pace has picked up; the thin wind is howling and she's already descended 5,000 feet. Within another 15 seconds she's traveling nearly 700 mph, hits Mach and discovers how severe it really is. Another 10 seconds, and maximum speed: about 885 mph, Mach 1.3."
And yes, of course, a jumper's speed will slow as they move into denser air. At 20,000
ft. she will change to a horizontal, belly-flop position and slow to 150mph before releasing her parachute. I read Stratoquest is still trying to get enough sponsers
for the jump, which will cost over 6 million dollars for the specialized balloons and
equipment. A frenchman is planning a jump from the same altitude and there is a race
to see who can do it and survive. Very dangerous.
http://www.popsci.com/popsci/science/article/0,12543,409394,00.html

John Connellan
03-03-04, 10:33 AM
2Inquisitive:Unfortunately, the link you provided mentioned exceeding Mach 1 without specifying speed in MPH or km/hour. I think the speed of sound decreases with density, approaching zero in a vacuum. What is mach 1 at 130,000 feet? This is above 99% of the atmosphere. While it seems strange, I would expect a person falling from 130,000 feet to initially accelerate and then start slowing down at some point. I have no idea of what the maximum speed would be or where it would occur.

1100f:Another issue is the effect of shape on orientation while an object is falling. Assuming still air, a sphere with the same mass and cross sectional area as the face of a penny would fall vertically. I would not expect the penny to fall vertically with one face down. Without actually experimenting I cannot be sure, but I would expect the penny to change orientation as it fell. I would also expect a horizontal component of velocity, and perhaps tumbling. I have no idea of how orientation would affect the vertical component of velocity. As the orientation changed, the cross sectional area and perhaps the drag coefficent would change. A horizontal componet of velocity would (I think) be at the expense of the vertical component.

As we've said before the chances are that the penny will not stay the same orientation. It will fall faster with less drag (smaller x-sectional area).

As mentioned in a previous post, it has been claimed that a bullet fired into the air would be potentially lethal when it came down. Obviously atmospheric friction would slow it down. Would it be slowed enough to make it non-lethal? Small caliber weapons with a low muzzle velocity would surely be less dangerous than a heavier bullet with a high muzzle velocity. There have been stories about people being killed by a rifle or hand gun fired into the air. Are such stories merely myths?[/

Well since I heard that a penny thrown off the Empire State Building would kill someone if it hit their head, I presume a shot fired into the air definitely would as it would travel higher than the building.

Dinosaur
03-03-04, 08:38 PM
The TV show debunked the myth about a penny thrown from the Empire State building. Their mathematical analysis and some experiments showed that the penny would not do serious damage. It is too light and does not fall fast enough.

Either I missed part of the show or they did not analyze the bullet fired into the air and hitting somebody when it came down. I would guess that the bullet fired into the air could be lethal when it came down

John Connellan
03-04-04, 04:41 AM
I just worked it out. The velocity on impact of the penny would be 311 km/h which (even assuming zero air resistance and drag) is still a good deal less than half the muzzle velocity of a colt 45. It just might not kill u as u say!!!
Having said that, I think we can assume very little air resistance for a penny facing edge down most of the way.

Tophat
03-29-04, 03:56 PM
There seems to be a lot of speculation about the lethality of a bullet fired into the air on it's return to earth.

"A bullet fired into the air can climb two miles and remain in flight for more than a minute. As it falls, the bullet reaches a velocity of 300 to 700 feet-per-second. A velocity of only 200 feet-per-second is sufficient to penetrate the human skull."

http://www.mcall.com/news/nationworld/all-gunfire0118,0,819915.story

contrarian
03-29-04, 05:37 PM
I may be totally off base here, but wouldn't the density of the object be a major factor, as well. I keep thinking of Archimedes Principle.

John Connellan
03-30-04, 01:53 PM
I may be totally off base here, but wouldn't the density of the object be a major factor, as well. I keep thinking of Archimedes Principle.

Well according to that report (and to my own calculations), a penny dropped off the Empire state building would penetrate the human skull!

John Connellan
03-30-04, 01:55 PM
I may be totally off base here, but wouldn't the density of the object be a major factor, as well. I keep thinking of Archimedes Principle.

I don't think density would have a very important effect on falling objects in air. Now in a liquid medium (especially a viscous one) there might be a large effect.

Maia
03-30-04, 04:08 PM
I think it'd most likely depend on how the drag force is correlated to the velocity of the moving object. If you just simplify things and say that the drag force is proportional to the velocity, then my best guess would be:

F = ma
mg - b * (dx/dt) = m * (dv/dt) (b is a constant)

And solve to see how velocity changes over time.

Of course, in real life, the drag force would certainly also depend on other factors. I think an engineer or at least an applied physics major would know this kind of thing pretty well. :)

Repo Man
03-30-04, 09:18 PM
Dear Cecil:

I have this friend and he isn't playing with a full deck, if you know what I mean, and he said if you drop a penny from the top of the Empire State Building and it happened to hit someone in the head it would go through just like that. Is this true? --Joe D., Towson, Maryland

Dear Joe:

I'm explaining this only on condition you don't try the experiment yourself.

Given that the Empire State Building is 1,250 feet tall and ignoring such factors as wind resistance for the moment, a penny dropped from the top would hit the ground in approximately 8.8 seconds, having reach a speed of roughly 280 feet per second.

This is not particularly fast. A low-powered .22 or .25 caliber handgun bullet, to which a penny is vaguely comparable in terms of mass, typically has a muzzle velocity of 800 to 1,100 FPS, with maybe 75 foot-pounds of energy.

On top of this we must consider that the penny would probably tumble while falling, and that the Empire State Building, like all tall buildings, is surrounded by strong updrafts. As a result the penny's descent would be substantially slowed.

Thus while you might conceivably inflict a fractured skull on some hapless New Yorker (or, more likely, some cretinous tourist from Towson), the penny would not "go through just like that." I bet it wouldn't even penetrate the skin. Not that I intend to find out.

For the record, the Empire State folks claim no one has ever dropped anything off their building. Yeah, right.

http://www.straightdope.com/classics/a1_225.html

Dear Cecil:

Every so often you see it on the news: streets full of Middle Eastern men indiscriminately firing guns straight up into the air. If I learned anything from physics class, it's that what goes up must come down. I'm certain the returning projectiles don't float harmlessly to earth and wonder how often they plunge into bystanders. --Kathy Johnson, Madison, Wisconsin

Cecil replies:

Those Middle Eastern men. You want to shake them and say, guys! Is this the safe and sensible way to celebrate? Can't we just say "hooray!" and "whoa, baby"?

But you raise a good point. How dangerous is this really? The question is controversial. Let me lay it out point by point.

Datum 1. At first I thought being struck by a bullet falling straight down would be no worse than getting hit over the head with a two-by-four--not the average guy's idea of fun, but not fatal either. What goes up must come down, but it needn't do so at the same speed. You run up against what's known as "terminal velocity." A bullet fired straight up will slow down, stop, then fall to earth again, accelerating until it reaches a point where its weight equals the resistance of the air. That's its terminal velocity.

For further insight, we turn to Hatcher's Notebook (1962) by Major General Julian S. Hatcher, a U.S. Army ordnance expert. Hatcher described military tests with, among other things, a .30 caliber bullet weighing .021 pounds. Using a special rig, the testers shot the bullet straight into the air. It came down bottom (not point) first at what was later computed to be about 300 feet per second. "With the [.021 pound] bullet, this corresponds to an energy of 30 foot pounds," Hatcher wrote. "Previously, the army had decided that on the average an energy of 60 foot pounds is required to produce a disabling wound. Thus, service bullets returning from extreme heights cannot be considered lethal by this standard."

If 30 foot pounds doesn't mean much to you, the bullet made a mark about one-sixteenth of an inch deep in a soft pine board. About what you'd get giving it a good whack with a hammer. Note that we're talking about bullets shot straight up here. If the bullet is fired more or less horizontally, it may not lose much speed before returning to earth and could easily kill someone.

More... http://www.straightdope.com/classics/a950414b.html

JesseLeigh
07-19-08, 08:05 PM
Evening!

I'm new here, but have been thoroughly enjoying this thread. Great forum, BTW.

Re: falling/jumping into water, I find myself perplexed by the contradictory info on the net.

Forty-one years ago, as a child, I jumped from a ninety foot high diving board (in Aberdeen, Scotland) into (IIRC) twenty-seven feet of water. Some people dove off the board but I wasn't quite brave enough for that. I raised my arms straight in the air above my head, and after jumping, I pointed my toes to make myself as aerodynamic an object as possible. I hurt the bottoms of my feet when I landed on the bottom of the pool, and my lungs were bursting (I forgot to take a breath just before entering the water), but other than that it was certainly a survivable experience. The water was not 'Concrete' as the movie The Guardian said it would be.

I was only eleven at the time but I remember the incident well. I haven't seen the facility for decades so I don't know if it still exists, but my guess would be that it does.

If all the science I've read on the net (and seen in that otherwise terrific movie) is remotely true, I should be dead. I'm not, and can't scientifically find an explanation for that - not to mention the liability issues that would present due to the mere existence of the pool in Aberdeen. :shrug: Any thoughts?

Shalom aleikhem - Jesse.

Cannon
07-21-08, 05:08 AM
This is more complicated that you folks seem to think it is. You should not rely on intuition except for qualitative comparisons of various objects. Without some established formulae, you cannot get good numeric answers.

Doing calculations including atmospheric effects is a difficult task. Even if you assume no wind, constant density at all altitudes, and a constant amount of water vapor in the atmosphere, it does not seem easy. In a vacuum, it is easy to calculate final speed at impact with the Earth. For an object with no initial velocityDistance = Acceleration * Time<sup>2</sup> / 2

Acceleration = Approximately 32 Feet / Seconds<sup>2</sup> or 980 cm / seconds<sup>2</sup>

Time = SquareRoot( 2 * Distance / Acceleration )

Speed = Acceleration * Time

Speed = SquareRoot( 2 * Distance * Acceleration )I hope I did the above correctly. The acceleration is actually a variable, but the above is a good approximation for objects falling from less than ten miles above the earth. A reference book I have gives 32.1725 & 980.621 at sea level and 45 degrees latitude. I did a few calculations and rounded the results.From 5000 feet: 122 MPH
From 10,000 feet: 550 MPH
From 20,000 feet: 770 MPH
From 5000 meters: 1127 km / hourThe above are for falling in a vacuum.

I have heard estimates of 125 MPH as the maximum for a human body, but do not know what assumptions are made about being tucked up or spread eagled. The clothing (if any) would make some difference.

The falling penny is a very difficult problem. The lower estimate of 35MPH and the higher estimate of 65MPH are not the difference between falling flat or edge down. It is due to different assumptions about how the penny changes orientation as it falls. Falling exactly edge down is a theoretical possibility like the possibility of balancing a needle on its point. Edge down, I would expect a penny to fall faster than a human body. Note that a penny would not fall straight down. When it was at a angle with the veritical, it would have a horizontal component of velocity. It might tumble.

The TV program was debunking a myth about a penny tossed from a high building and killing somebody when it hit them in the head. It said nothing about a bullet fired upwards and hitting somebody when it descended. In a vacuum, it would impact at muzzle velocity if fired straight up, but I have no idea about the atmospheric effects.

I am not sure how a bullet would fall. I suspect that it would fall nose first after the first few hundred feet, but am not sure about this.

I was hoping that some body here knew formulae or a URL leading to some real information.

So that is in-fact is correct would the constance acceleration of 2g's be twice the speed, or is there a compounding factor involved?

Dinosaur
07-22-08, 07:02 PM
JesseLeigh: Water would act much like concrete if you fell from 90-100 feet Belly Flop style. Water would not act like concrete if your entry is like a diver doing a Swan dive or using the entry you describe with toes pointed.

Falling from 90-100 feet onto concrete would likely be fatal no matter how you were oriented. Falling onto a lawn, you might be better off with a spread eagle landing. I suspect that landing face up is better than face down. This might be better than landing feet first & trying to use leg muscles to decelerate more slowly.

Sci-guy
07-24-08, 09:55 AM
In an ideal enviornment there is no maximum velocity of an object because on earth the gravitational acceloration is 9.81 m/s2, but when accounting for the air resistance that an object would encounter you must observe the object geometric shape. It all depends on how areodynamic the object is.

I heard the same sorty on television of a bullet after being fired in the air that landed and injured someone. They tested these accounts on mythbusters, you might want to watch the episode, it was a good one! Well not really because they all are good episodes.

Tom13054
06-05-10, 06:32 AM
I am a retired Police Officer when we went to the range to shoot there was a test area behind the hill of another shooting range. Now I know this is only a ricochet so the bullet did not go as high up but on occasion we would get hit with a falling bullet it didn't kill but it sure hurt a lot. The speed of the ammo used was 950 ft a second at the muzzle.

I may be totally off base here, but wouldn't the density of the object be a major factor, as well. I keep thinking of Archimedes Principle.

three-brane
06-06-10, 12:16 AM
I would think the density of the object would have alot to do with it. i am thinking of how the mass of an object curves space. your mass curves space. not nearly as much as the earths mass. and the suns mass is even more so, right. your thinking "no shit".
well the mass of a collapsed star where it warps space to the point of tearing it. not even light escapes and it is traveling at the speed of light.
the mass of a photon.
my point is your falling into the curve of the space around the earth. if you were the mass of a star and the size of you, your density would be so great the earth would be actually falling into you. yet you could argue you fell to the earth.
so i say yes density would matter.

freziggity
06-06-10, 02:29 AM
The max falling speed of anything is directly relative to the mass/density of the objects and the gravitational field of the test environment. On earth, gravity accelerates an object 9.8meters per second per second. On jupiter it is 9.8 per second per second times 2.5 Since Jupiter has 2.5 times earth's gravity. Other than that, friction will diminish the rate of fall for any object, but in a vacuum, everything falls at the same rate of speed. Cool question.

Pinwheel
06-06-10, 05:39 AM
You can try chucking darts off the top of a skyscraper.

Syzygys
06-06-10, 06:51 AM
Alright people, just a few observations and facts:

1. It is a well documented FACT that bullets fired into the air can kill people. Happens quite often actually. To reach the highest speed at arrival, it is better to fire the gun at an angle, so the bullet would keep its spin, instead of tumbling down.

2. There should be a dropping height limit, somewhere where the air starts to get thin. Those record parachute jumps shouldn't count because they started in the thin air area, thus they had the advantage of speeding up before reaching normal air. Thus it is not really a drop.

3. Shape is everything in this matter, and bullets tumble coming down. Now we could talk about the penny coming down on its edge if we make it spin, before the drop. My guess is that the fastest shape is the raindrop shape, a spherical shape with a tail. That has the lowest natural air resistance ratio...

Syzygys
06-06-10, 07:03 AM
Interestingly, it doesn't matter what shape a falling object has; only the cross-section is needed for calculating terminal velocity. A pointy object would have the same terminal velocity as a flat object, so long as they had the same cross-sections pointing toward the ground and were of the same mass.

This is incredible silly missconception. Because of the airdrag, shape is everything in falling....

You are saying that a rocket-shape and a parachute would fall with the same speed assuming everything else (weight and cross-section) being the same...

iceaura
06-06-10, 05:37 PM
In an ideal enviornment there is no maximum velocity of an object There is, of an object accelerating only by gravity toward the earth.

The pull of the gravity gets smaller as the distance from the ground gets larger, and decreases faster than the distance increases - so eventually, falling from much higher up doesn't add much to the final speed.

My guess is that the fastest shape is the raindrop shape, a spherical shape with a tail. That has the lowest natural air resistance ratio... Arrows fall faster than raindrops - and I doubt raindrops have a tail when they are falling.

D H
06-06-10, 08:27 PM
I would think the density of the object would have alot to do with it. i am thinking of how the mass of an object curves space. your mass curves space.
True, but completely irrelevant to the problem at hand. The Schwarzschild radius of a feather is about 10-30 meters; for a Nimitz class aircraft carrier the Schwarzschild radius is about 10-19 meters. General relativity is not relevant to this discussion.

An object's density, shape and even surface texture are important because the object is falling through the atmosphere. Atmospheric drag builds with velocity. At low Reynolds numbers drag is proportional to velocity. Once turbulent flow kicks in the drag force grows with the square of velocity. As a falling object gains speed the drag force will grow to be equal to gravitational force. This velocity where drag force is equal to gravitational force is called the object's terminal velocity stops accelerating at this point, called terminal velocity.

Syzygys
06-06-10, 09:07 PM
Arrows fall faster than raindrops - and I doubt raindrops have a tail when they are falling.

Have you actually tried it? :)

What's your point by the way?

1. Arrows are NOT natural.
2. There is more to it, like density and cross-section.

Anyhow you are just proving that shape does count...

three-brane
06-06-10, 11:16 PM
True, but completely irrelevant to the problem at hand. The Schwarzschild radius of a feather is about 10-30 meters; for a Nimitz class aircraft carrier the Schwarzschild radius is about 10-19 meters. General relativity is not relevant to this discussion.

An object's density, shape and even surface texture are important because the object is falling through the atmosphere. Atmospheric drag builds with velocity. At low Reynolds numbers drag is proportional to velocity. Once turbulent flow kicks in the drag force grows with the square of velocity. As a falling object gains speed the drag force will grow to be equal to gravitational force. This velocity where drag force is equal to gravitational force is called the object's terminal velocity stops accelerating at this point, called terminal velocity.
but if there was two objects of similar size and shape, the difference one has twice the density, they would still have the same terminal velocity?

Syzygys
06-07-10, 05:54 AM
but if there was two objects of similar size and shape, the difference one has twice the density, they would still have the same terminal velocity?

Nope. A bowling ball has a bigger TV then a same sized ball made of styrofoam.

"Terminal velocity varies directly with the ratio of weight to drag. More drag means a lower terminal velocity, while increased weight means a higher terminal velocity."

D H
06-07-10, 06:26 AM
but if there was two objects of similar size and shape, the difference one has twice the density, they would still have the same terminal velocity?
Of course not. That has nothing to do with relativity, however (the point you were trying to make in post #27). It has everything to do with atmospheric drag.

iceaura
06-07-10, 09:05 AM
Have you actually tried it? yes.

Arrows are less dense than water, should fall more slowly on that criterion alone. If shot straight up, watched carefully straight down, they will overtake even fairly large raindrops after just a couple of hundred feet of free fall, at most. Mist and the like just floats.

Of course shape matters. But falling things, even liquids, do not automatically assume the best shape or orientation for maximum speed. Leaves from trees don't, for example. Air is complicated stuff to push through.

D H
06-07-10, 09:24 AM
My guess is that the fastest shape is the raindrop shape, a spherical shape with a tail. That has the lowest natural air resistance ratio...
That is a misconception. From http://www.newton.dep.anl.gov/askasci/gen01/gen01429.htm

If the drop is larger like a raindrop in free-fall, it has a domed top and a semi-flattened bottom because as it falls it must push the air out of its way. That "upward" push of the air being displaced causes the falling drop to have a rather flattened bottom.

Contrary to popular misconception, a free-falling raindrop is not shaped like a teardrop -- round on the bottom and pointy on top.

Using the equivalence relationship "1 picture=1000 words," here are 3091 words on the shape of a raindrop.

http://static.photo.net/attachments/bboard/00L/00L9fH-36530184.jpg

http://static.photo.net/attachments/bboard/00L/00L9fF-36529984.jpg

Syzygys
06-07-10, 09:55 AM
>a free-falling raindrop is not shaped like a teardrop

I guess I misspoke. One of the best aerodynamic is the teardrop, at least that is what carmakers are trying to copy when they want to make a small airresistence car...

I guess the rain is too soft and it bends, thus kind of parachuting itself. But making it from hard material, a teardrop would come down pretty much with the fastest speed possible...

See the shape on the top:

http://www.zamslube.com/documents/aptera/aero_1.jpg

Dinosaur
06-09-10, 03:25 PM
It is my guess, that mass has (little) or no effect on the downward force. This is surely true in a vacuum.

However, the retarding drag force would have more effect on a less dense object. The equivalence of inertial mass & gravitational mass results in cancelling of the mass in gravitational equations. This cancelation is not applicable to drag/lift equations of aerodynamics.

sifreak21
06-09-10, 03:58 PM
A recent TV program claimed that the maximum speed of a penny is between 35 and 65 miles per hour (56 to 104 km per hr). I guess that a penny does not fall edge first, or it would reach a higher speed.

I think that a falling human body reaches a maximum speed of about 125 MPH (200 km per hr).

How fast does a bullet fall? I have read about people being killed by bullets fired into the air and hitting a person when they fall.

Would a spherical steel ball bearing fall faster or slower than a bullet? I expect very small steel spheres to fall slower than large ones due to a larger area to mass ratio, but am not sure that medium size ones would fall slower than large ones.

Does anybody know how fast various objects fall?

everything has a terminal velocity.. abullet when fired is instanly WAYYY past that so the milisecond it leaves the barrel "maybe before" the bullet is slowing down

Dywyddyr
06-09-10, 04:24 PM
everything has a terminal velocity.. abullet when fired is instanly WAYYY past that so the milisecond it leaves the barrel "maybe before" the bullet is slowing down
Not quite.
A bullet is still accelerating as leaves the barrel, because it still has gas expanding behind it. (Although, of course, at a much-reduced pressure compared to when it was still in the barrel).

three-brane
06-10-10, 12:42 AM
Of course not. That has nothing to do with relativity, however (the point you were trying to make in post #27). It has everything to do with atmospheric drag.

so like I was saying earlier the density is the determining factor for an objects t.v. atmospheric drag, size, and shape, they effect less dense objects. where as the greater the density the greater the t.v.

Syzygys
06-10-10, 05:15 AM
Not quite.
A bullet is still accelerating as leaves the barrel, because it still has gas expanding behind it. (Although, of course, at a much-reduced pressure compared to when it was still in the barrel).

I always wondered about bullet speed accelerations. I would say it takes several feet before a bullet reaches its maximum speed after leaving the barrel. Why? Because it built momentum in the barrel and it takes a while before the momentum reaches its peak and start to get lost.

I can be wrong on this one but that's how I see it...

Well, how about a thrown rock? Does it reach its highest speed at the time when it is leaving the throwing hand????

James R
06-10-10, 05:33 AM
A bullet reaches its maximum speed before leaving the barrel of the gun. And once it has left the barrel, air resistance is working on it all the time, slowing it down.

A thrown rock has its highest speed at the moment it leaves your hand, for the same reason.

Of course, for the rock, if you're throwing it downwards from a high place (plane, cliff etc.) then it can actually speed up after leaving your hand. The same is not true for a bullet, unless you're on the moon.

Syzygys
06-10-10, 07:49 AM
Actually I have to side with Dyw here and I say the bullet still accelerating at least for a few inches after leaving the barrel. The exploding gas behind it still propels it until the pressure quickly can finally go sideways in the air.

When the bullet is in the gun the speed is zero. At one point in its flight let's say the speed is 500 m/s. But obviously from zero it doesn't go to 500 m/s in 1 nanosecond. I would say it needs time to get to its highest speed and it happens outside of the barrel...

Edit: since this is offtopic, I am going to start a new thread on it...

Dywyddyr
06-10-10, 08:05 AM
A bullet reaches its maximum speed before leaving the barrel of the gun. And once it has left the barrel, air resistance is working on it all the time, slowing it down.
Don't you think the friction in the barrel might be higher than that from air resistance? After all a bullet is larger than the bore size, and while in the barrel has to have the rifling grooves forced into the metal - it's effectively being extruded.

A thrown rock has its highest speed at the moment it leaves your hand, for the same reason.
But when you throw a rock there's no propellant gasses still escaping from the barrel and pushing against the rear of it. (Admittedly any extra push would be a minute increment - it also causes some instability until the spin of the bullet takes "control" and stabilises it).

sifreak21
06-10-10, 10:07 AM
Not quite.
A bullet is still accelerating as leaves the barrel, because it still has gas expanding behind it. (Although, of course, at a much-reduced pressure compared to when it was still in the barrel).

yeah after i posted this i thought about that so id say what like 3-4 feet out of the barrel it startes slowing down

Dywyddyr
06-10-10, 10:13 AM
yeah after i posted this i thought about that so id say what like 3-4 feet out of the barrel it startes slowing down
Far faster than that, I'd say. Any "extra" push from the gasses escaping the barrel will be gone well within a foot.
When I wrote "A bullet is still accelerating as leaves the barrel" I literally meant as it leaves.
I haven't ever found figures (probably because as Phlogistician said in the thread (http://www.sciforums.com/showthread.php?t=102217) dedicated to the question - muzzle flash makes measurement difficult) but I'd guess that deceleration starts before the bullet's travelled much more than its own length.

James R
06-10-10, 11:06 PM
Don't you think the friction in the barrel might be higher than that from air resistance?

Certainly.

But when you throw a rock there's no propellant gasses still escaping from the barrel and pushing against the rear of it.

The maximum speed will be before the bullet leaves the barrel and the propellant gases start to escape.

(I could be wrong.)

Dywyddyr
06-10-10, 11:09 PM
The maximum speed will be before the bullet leaves the barrel and the propellant gases start to escape.
(I could be wrong.)
Take a look at the thread on the subject. :D