Split Thread The validity of classical physics (split from: DWFTTW)

Status
Not open for further replies.
bing bong

humber said:
Ooops! Here is a good example. Oxygen masks are tethered, SD.

If the person jumps within the caboose full of air, then KE will be lost due to the vertical component alone. But what you are telling me is not right. What stops the body from being displaced? The air? The usual friction to the floor being optional? Not at all necessary?
I could go on. Is the displacement such that the air can be considered adiabatic?

What happens to a balloon floating in a car as the driver accelerates?
Click to expand...
Attention all passengers. It's my train, it has carriages and it contains a perfect vacuum. This is why it has been fitted with oxygen masks, simply for your in-flight convenience.

We apologise for the inconvenience, but neither hot-air, helium nor hydrogen balloons are allowed on board (nor flying contraptions of any kind), for reasons of international security, orders of humb.

Would the gentleman (cough) in seat mv2 please stop jumping and using long words like 'adiabatic'. All passengers should remain calm and refrain from playing with the gravimetric shear controls under their seats. We have been directed to make an emergency landing at an alternative railway station and will be arriving shortly. Thank you.
 
CORed said:
The person and the air are all moving with the caboose. There is nothing to dissipate kinetic energy, no force to change the motion. The jumper will land on the mark whether he jumps 6 inches or 6 feet (assuming high enough ceiling. For that matter, even if he bounces off the ceiling, he will still land on the mark.
Click to expand...

humber said:
Wrong, and the extensions make it worse. Try again.
Click to expand...
Oh, please Sir, let me. Is it wrong because of the curvature of the Earth's surface? Is it, Sir?

When a passenger jumps, they jump to a higher altitude, and points at higher altitudes are part of larger great circles surrounding the planet. Since they started off at the same speed as the train, and jump vertically, by the time they land, the train in its smaller orbital route round the Earth will have made greater progress, and they will land behind the chalk line. Hence, the 'extensions' (to the roof?) delay them even more. If they jumped vertically into space to about 50 miles high, you'd see they'd have to accelerate massively to keep up with some moving chalk on the surface. Is that the 'right answer' you're thinking of, humber?

We could use this as a form of transportation, actually. We never notice it, but when we travel by underground, this effect happens, and we arrive just slightly earlier than if we had travelled overland, because going down towards the centre of the earth means we spin faster than the city above. I'm sure that must be it.

Or is it this: Could it be that when we jump up, we gain gravitational kinetic energy, just as a hot-air balloon does from rising (see how much I've learned?), and then we can translate some of that extra kinetic energy into forward motion again as we descend, just as a balloon does to go faster than the wind (or somewhere around there). So we might land in front of the chalk mark. So that's obviously already a transportation principle from early aeronautics, although probably not enough use to help get a train to arrive on time.

It probably depends, in the final analysis, which way we direct these changes in KE, perhaps by waggling our ears.

(Incidentally, I'm not sure about the first point - it could be true. It's hardly an issue for the actual thought experiment posed.)
 
mender said:
This has been bothering me for a while. I'd like to make sure I understand what is being said in this example.

This appears to show that using conservation of momentum to solve the question gives a different answer than using KE. If the KE doesn't tally up exactly and needs a "fudge factor" of over ten percent attributed to an undefined loss, doesn't that say that the use of KE in this example is flawed?
Click to expand...

Try looking at it this way: imagine an opaque bubble surrounding the two masses and traveling with the center of momentum. From the outside of this bubble, you see the system as only a single mass with a single velocity that doesn't change. From the inside, in the reference frame of the bubble you see the internal energy of the system initially as two masses carrying kinetic energy that is totally converted into heat or some other form by the collision.

This example wasn't to show simply that energy is conserved but to show that for any inertial frame of reference, the total energy is conserved and the calculations performed from each frame yields exactly the same result.
 
Subduction Zone said:
Just a sidenote for humber who is fascinated by the action of rolling oranges in his warehouse job. It does not take much of a slope for a ball to keep rolling in one place on a conveyor belt. A dip that is not easily perceivable to the naked eye would do. If the rolling resistance of the ball (or orange) was the same as the component of the force of gravity parallel to the belt then the ball could roll endlessly in one spot. The rolling resistance of a ball is very small and would be a constant at the speed that a conveyor belt is typically run at.

Notice the lame attempt he had in trying to answer the hobo question? Basically his answer is "if I cheat I can figure it out".

humber, I don't think spacediver will mind if I refine the question a bit more, you we will assume are the hobo. You are nude, no GPS, no super handy pocket gyroscope, nothing. You are in the aforementioned boxcar. There are various boxes and balls and other nontechnological items lying about. The evil scientist announces on a loud speaker that you have 10 minutes to find the "front" of the car, or else the car will be filled with poisonous gas. How would you do it? Earlier you said it could be done easily so ten minutes should be plenty of time. I will start to time you at the beginning of your next post.
Click to expand...

I suggested both mechanisms (conveyor belt slightly uphill or slight low spot in conveyor belt) for the oranges stuck on the conveyor belt way back in the original thread. I suggested that the weight of the oranges might cause a slight sag in the belt. Humber responded with something about how grapes could climb up a ramp, and the only mechanism he suggested for the phenomenon was "low resistance", which is certainly an important part of the picture, but not the whole story.

I really think that Humber, fascinated by the phenomenon of oranges staying in one place on the belt, concocted some bizarrely wrong hypothesis to account for it, possibly involving some sort of mystical coupling with "the ground" and "force balance" and when he saw the propeller cart on the treadmill concluded that the same thing was happening with the cart.
 
humber said:
ROFL
Click to expand...
It's good to know spork and I didn't waste our time trying to teach you something.

humber said:
And this is how we know the earth to be banana shaped.

Do you think that despite being part of the earth going around so fast and all that, the air in the caboose may be coupled to the gravitational field, in not quite the same way as the jumper? And that may change again when the friction to the floor is no longer there?
I wonder where the energy expended by the act of jumping goes, and there may be all sorts of moments and so forth. Then some friction perhaps, or even a pressure drop here or there. Stuff like that. And even more.

Sure the treadmill works!






I wouldn't want to disturb your turbulence-free world.
Click to expand...
Damn, it looks like I was wrong earlier with my guess about great circles and the orbital motion of the train/plane/automobile. It's about kinetic energy, gravitational fields (I was onto that earlier, in fact, but got distracted with the drinks orders) and turbulence. I'm going to fail this module, I just know it.:(
 
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place, and if it is true, it's the only one I can find. Too bad humber didn't find it, and instead proposes (that's a grand word for 'suggests in a rambling incoherent fashion') various other impossible effects.

Ignoring all other considerations, gravity, friction, etc. the initial velocity of the 'jumper' is equal to the 'train', and remains so, but that is directly tangential to the earth at the surface. Hence, it will take him the same distance in that direction as the train goes, but the train will go that distance in a circle, rotating away from that tangent. Right?
 
John Freestone said:
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place...
Click to expand...

It is a valid argument. But the logic is ever so slightly twisted. Here's how I mean. The question is whether or not a person jumping in a box car will land in the same place when a person on the ground obviously will. In both cases it's sort of assumed that we're talking about inertial reference frames. But in both cases we're not. To the extent that you can ignore the 2nd and 3rd order effects that make either one NOT an inertial frame, they are identical. But.... the guy standing still on the earth is actually circling the globe at about 1000 mph (assuming he's near the equator). The guy on the train might be going ever so slightly faster or slower (or even heading North or South which would lead to an inclined orbit).

So yes, if we ignore the implied assumption that both are inertial frames there are in fact effects of spinning earth, earth orbiting sun, etc.
 
John Freestone said:
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place, and if it is true, it's the only one I can find. Too bad humber didn't find it, and instead proposes (that's a grand word for 'suggests in a rambling incoherent fashion') various other impossible effects.

Ignoring all other considerations, gravity, friction, etc. the initial velocity of the 'jumper' is equal to the 'train', and remains so, but that is directly tangential to the earth at the surface. Hence, it will take him the same distance in that direction as the train goes, but the train will go that distance in a circle, rotating away from that tangent. Right?
Click to expand...
Yes and no (which is a fancy way of saying that you're on the right track). If we interpret the initial question to be implying that the boxcar has a constant velocity, then it isn't allowed to follow the curvature of the earth, because that would mean that it was accelerating. From a theoretical point of view, you can't have something traveling in a straight line on the surface of the earth because the earth is curved. However, if we assume that the planet is very large compared to the distance covered during the experiment (a very reasonable thing to do), the effect becomes negligible. To think about it another way, you could replace the cube that has four labeled walls with a cube in interstellar space that has six labeled faces, and ask our Klingon hobo which way the cube is moving. In that case it becomes clear that the question is not only unanswerable, but meaningless, because the hobo will just answer, "Moving in relation to what? In relation to humber's navel?"
 
Edited by chillzero: 
Edited for civility
 
Last edited by a moderator:
John Freestone said:
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place, and if it is true, it's the only one I can find. Too bad humber didn't find it, and instead proposes (that's a grand word for 'suggests in a rambling incoherent fashion') various other impossible effects.

Ignoring all other considerations, gravity, friction, etc. the initial velocity of the 'jumper' is equal to the 'train', and remains so, but that is directly tangential to the earth at the surface. Hence, it will take him the same distance in that direction as the train goes, but the train will go that distance in a circle, rotating away from that tangent. Right?
Click to expand...

All right I guess I will have to get my comment in on this too. If you have a very accurate measuring system this can be determined. There is a term for it which I cannot remember right now, but I saw references to it on the Flat Earth Soc Forum. If you want to try to argue with a group of hunbers that is the place to go. I am off to google this topic.
 
John Freestone said:
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place, and if it is true, it's the only one I can find.
Click to expand...

Hello John,

well, yes, it is some kind of valid. But then you also have to consider the circumference of the earth vs. the circumference of that imagined "orbit". Add to that the consideration of the time the person jumping would be in the air. In the end that effect would be incredibly small, so i would say that it just doesn't really matter for that thought-experiment. It might have a noticeable effect if the person is to jump several thousand or million times, but that is not the case since we talk about a single jump.

I would say it is just the classic misdirection trick that humber always applies as soon as he moved himself in a corner and sees no other way to get out without admitting that he has been wrong. After all, physics uses idealized models all the time, just for the purpose to make things easier. If there is reason to assume that some effect would introduce some level of error, quite often that is fixed by simply adding some "error margin" to the final result, where that value is given by previously gained knowledge of the overall effect of that error.

If some experiment turns out to disagree heavily with the values derived that way, then everything is accounted for and calculated in detail, and the resulting "correction factor" is used from there on in similar situations. I mean, really, if someone wants to nitpick on the way such calculations are done and complains about some tiny, marginal error resulting by using a idealized model, we would end up with big, heavy simulations on supercomputers running for months to get the result of even the simplest things. Yes, one could simulate each and every air molecule, simulate tiniest fluctuations in gravity, and so on. It would take many months of simulation time, just to get an result that might be 0.000001% off compared to the idealized model.

If we would do that, science would advance horribly slow, because every simple thing would take just too long to calculate. I'm pretty sure that nobody really wants that.

Take electronics as an example. We calculate circuits and use idealized conditions for the calculations most of the time. Look at an resistor. In the idealized model, it is just resistive. But in reality it has a capacitive and inductive part as well. It's value changes with temperature too. But in 99% of the cases, this can be ignored for all practical purposes. For the remaining 1%, things are calculated the whole way through (and even then there still is some error), and later on we just use the derived error margins in future calculations, just to make them simpler. We don't measure every tiny bit and every single part with the highest possible resolution, we use previously determined approximations. In fact, we have to since even if we know all the tiny details for a given circuit and given parts, it would all change with the second, same circuit due to part tolerances.

I don't see why it should be any different in most of everyday-physics.

Greetings,

Chris
 
John Freestone said:
Actually, I'd appreciate any thoughts about that orbital argument I raised. As far as I can make out, it is a valid objection to landing in the same place, and if it is true, it's the only one I can find. Too bad humber didn't find it, and instead proposes (that's a grand word for 'suggests in a rambling incoherent fashion') various other impossible effects.

Ignoring all other considerations, gravity, friction, etc. the initial velocity of the 'jumper' is equal to the 'train', and remains so, but that is directly tangential to the earth at the surface. Hence, it will take him the same distance in that direction as the train goes, but the train will go that distance in a circle, rotating away from that tangent. Right?
Click to expand...

Well, unless the train is at the equator when the person jumps, there is also the Coriolis effect. The jumper will veer slightly to the right of the motion of the train in the northern hemisphere, slightly to the left in the southern hemisphere. Again, this is an extremely small deviation for something on this scale, though very important if you are dealing with large-scale weather systems, or targeting ballistic missiles.
 
Dan O. said:
Try looking at it this way: imagine an opaque bubble surrounding the two masses and traveling with the center of momentum. From the outside of this bubble, you see the system as only a single mass with a single velocity that doesn't change. From the inside, in the reference frame of the bubble you see the internal energy of the system initially as two masses carrying kinetic energy that is totally converted into heat or some other form by the collision.

This example wasn't to show simply that energy is conserved but to show that for any inertial frame of reference, the total energy is conserved and the calculations performed from each frame yields exactly the same result.
Click to expand...

Actually, since asking this I think I resolved it in my mind. I understood about the single mass idea but was wondering what happened to the KE energy that was "lost". I now realize that it isn't lost but is converted
as originally stated to another form.

It wasn't hard once I tried to come up with a way to reverse the effect of the inelastic collision. Basically I considered a few different ways that the objects would end up traveling at the system velocity, then thought about a way to return the two objects to their original relationship. In one scenario, as the two objects are side by side, one of the objects latches onto the other with a cable attached to a suction cup, and the pair end up rotating about the C of G with the same energy as was "lost" during the collision. If the suction cup is released at the right time, the two objects once again resume their relative speeds and directions, converting the rotational KE back to the original values.

Another scenario involved a more direct hit with a spring between them that gets latched then released but the idea is the same.

Thanks for the help though!
 
CORed said:
Well, unless the train is at the equator when the person jumps, there is also the Coriolis effect. The jumper will veer slightly to the right of the motion of the train in the northern hemisphere, slightly to the left in the southern hemisphere. Again, this is an extremely small deviation for something on this scale, though very important if you are dealing with large-scale weather systems, or targeting ballistic missiles.
Click to expand...

This is still a physics forum. So why don't we calculate the actual effect that the earths rotation and non-spherical shape have on a jump or a tossed ball. If this is significant we could sell the correction factors to the major league sports teams and become instant millionaires. :eek:
 
Dan O. said:
This is still a physics forum. So why don't we calculate the actual effect that the earths rotation and non-spherical shape have on a jump or a tossed ball. If this is significant we could sell the correction factors to the major league sports teams and become instant millionaires. :eek:
Click to expand...

Well, that's exactly the business JB and I are in. Unfortunately we're not instant millionaires. :(
 
Michael C said:
Hold it everyone! We're all wrong: there's this guy called Gary Novak who has determined that Energy has been Misdefined in Physics.

Yep! Kinetic energy isn't mv2/2, it's mv! He's proven it! With Mathematics! And he tells us that "About ninety percent of physics is corrupted by the error."

Of course, he does believe in spirit painting and intelligent design. And Einstein is the cause of the war in Iraq. However, this shouldn't distract us from the terrifying fact: kinetic energy = mv!
Click to expand...

Excellent link. I love some of his other science ideas also. Especially his earth science papers. On a sad note I found that one of my favorite authors and I assume one of yours, Terry Pratchett, has early onset Alzheimer's. But he is soldiering on and has just released a new book. I have a link that has the text a speech he gave to an Alzheimer's association:http://www.terrypratchettbooks.com/

He will be missed, he is the most humorous British author since P.G. Wodehouse. But he is nowhere near being dead yet, and who knows how well he will fight this disease.
 
Last edited:
Status
Not open for further replies.

Back
Top Bottom