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# Speed of flight

Posted on 2013-12-28
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Imagine (Or build) a hovercraft fan fastened to the front of a car facing foward, and a small contraption that will drop table tennis balls into the airstream from the fan. A sensor also attatched to the car measures the speed of the pingpong balls in the airstream when the fan is running to be 40 miles per hour.
Now start the car and drive forwards at 40 miles per hour( against the direction of the fan).
What speed does the sensor measure the balls to be travelling?
What speed does a sensor fastened to the ground measure the speed of the balls to be?
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ozo earned 180 total points
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To a reasonable approximation, a table tennis ball might be moving with the airstream,
And to another reasonable approximation, speeds might be close to additive.
The additive approximation may not be as good if the fan is not strong enough to keep up its speed within an ambient airflow, or as the table tennis ball moves far enough from the fan to be influenced more by the ambient airflow than by the fan, but we would need more details of the setup in order to work out those details.

So, if the the table tennis ball is moving with the airstream, and if the airstream is moving 40mph relative to the car, then the table tennis ball would be moving 40mph relative to the car.
If the the table tennis ball is moving with the airstream, and if the airstream is moving 40mph relative to the car, and the car is moving 40mph relative to the ground in the same direction, then the table tennis ball would be moving  40mph+40mph relative to the ground.
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☠ MASQ ☠ earned 20 total points
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Are you going to drive this imaginary vehicle in a vacuum or are you going to take into account wind resistance due to the forward motion of the car and balls?

Assuming the fan is capable of overcoming any opposing force and the ping pong balls don't just fly backwards over the windshield the sensor on the car will only see the movement of the ball so that speed is dependent on the mass of the ball and the forces acting on it.

From the ground it's that speed plus the speed of the car.

If it's still in print try:
Mr Tompkins in Paperback by George Gamov
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My feeling is that driving the car forward will stall the forward airflow relative to the car, unsure what will happen to the fan. So the balls could drop onto the car before being whisked away behind the car.
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ozo earned 180 total points
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I would expect that turbulence within the airstream from a 40 mph fan would be greater than the difference between the speed of something as light as a tennis ball and the speed of the airstream, so within that precision, it seems reasonable to approximate the tennis ball as moving with the airstream.
I would also expect that, if the fan maintains its rotation rate, the speed of the airflow relative to the fan could stay close to the same when it is very close to the fan, but by the time your distance from the fan is comparable to the dimensions of the fan, the airflow would be closer to the speed of the ambient air.
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I was hoping this might be analogous to the question about how fast the light from a car's headlight is travelling when the car is moving, hence the title.
I thought it might be a measurable experiment to show that simple addition isn't the answer.
Perhaps I could use a leaf blower instead of the open fan and measure the airspeed at the nozzle?
Would this still have different speeds depending on the distance from the nozzle?
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ozo earned 180 total points
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Air or cars or table tennis balls are not  analogous to light.
The speed of light is the same for all observers.

Nor is the addition of speeds in your example simple, since it could be a turbulent combination of the speeds of the fan, the car, and the ambient air.
But for the speed of any component of that complex flow, measurements of that speed with respect to any observer with a relative speed in the neighborhood of 40mph would obey a Galilean transformation to a very high accuracy.
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BillDL earned 40 total points
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This sounds like a homework assignment ;-)
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Ozo, you lost me with Galilean, is that an easy reference or something I would need to spend time on?
Thanks for the replies, I thought it might lead to a way of explaining light's speed limit, but apparantly it won't.

Bill, not homework, just sonething I wondered while I was cleaning my teeth the other day (onr of those times you can let your mind wander).
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aburr earned 120 total points
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ozo has the right ideas
If you are going to think of velocity additions you have to simplify the problem to remove the complications like turbulence, fan strength, air resistance etc
you want to attach a ball gun to the car that will shoot balls at 40 MPH to the rear when the cat is at rest.
v wrt car = -40 mph            (wrt = with respect to, forward is +, rear is -)
v wrt the ground = -40 mph.
now neglecting air resistance and all other complications, let the car drive forward at 40 mph
what is v wrt car?
what is v wrt ground?
v wrt car = 40 mph
v wrt ground = 0    (40 + (-40))

The above is good if all velocities are small wrt light
Now light is a different ball game.
If the speeds are comparable to the speed of light the addition equation is more complicated. You are now dealing with special relativity.
SR is profound, interesting, and not too complicated but takes a little study.
Suffice it to be said now that the SR has been tested extensivly and is always proven correct.
Three are people who say it is wrong but THEY are wrong and provably so.
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Aburr, I'm not trying to disprove anything, just trying to find an easier way to understand it.
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ozo earned 180 total points
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One way to think about Lorentz transformations as opposed to Galilean transformations
is by analogy with the way measurements of the front to back extent of an object
and the left to right extent of an object can both change when the object is rotated
or when  you rotate the directions you consider to be front-back and left-right,
while the total of (front-back)^2 + (left-right)^2 remains invariant.

Similarly, when you "rotate" your motion through time, your measurements in the
past-future direction and front-back direction may change, but the total of
(front-back)^2 + (left-right)^2 - (past-future)^2 remains invariant.

Note that in the case of that Lorentz invariant (which is known as the space-time interval), - is used instead of +
which can make  the coordinate transform look more like a skew than a rotation,
but the principle that it's just a change of perspective remains.
Also note that a light-like interval is always 0.

I've dropped a few jargon terms which can lead you to other explanations if you search for them. (or if you ask more specific questions)
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aburr earned 120 total points
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"Aburr, I'm not trying to disprove anything, just trying to find an easier way to understand it. "

I was not implying that you were trying to prove anything. I was merely suggesting that when you looked at SR (which you must do if you are to know more about the speed of light) that, especially on the internet, you will meet those who do not believe SR is in accord with reality and emphasizing that SR has been well tested experimentally.
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Thanks for comments. My sttempt at an analogy has obviously failed.
I was thinking that the surrounding air would stall the forward motion if the fan, which as was stated is capable of accelerating a ping pong ball to 40 miles per hour in still air, so if driven forward would still only accelerate the balls to 40 mph relative to the surrounding air, not to the fan.
I watched an air show several years ago with a plane called an Optica. It was a windy day, about 30mph, the plane had a stall speed of about 20 mph and the pilot was able to fly the prop plane backwards in front of us with the prop driving him into the wind. It was very impressive.
I thought light may behave in a similar way, unable to go any faster no matter what was the relative movement of its source.
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*of the fan
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aburr earned 120 total points
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The question of why light cannot go any faster than c is an interesting one which does not have a simple answer (or any answer). It has been shown that any observer in an inertial frame will get the same answer as any other.
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Horn E. Towed earned 80 total points
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air speed @ fan = 40 mph
air speed @ fan+ 12" = < 40 mph [not factoring in the increasing effect of the headwind]
etc.

My super computer is tied up trying to make a cup of tea so the answer to your questions is indeterminate at this time.

hth
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You need to factor in the headwind, that's the whole point. Headwind created by moving the car through air. The speed generated by the fan when the car is at rest is given. I don't think the fan will be able ti propel the balls any faster with respect to a stationary observer.
I know this is not the same mathematics as light, but it is an often asked question.'if I ride my bike then isn't the light coming out of the lamp going faster?'.
I was/am looking for a way to demonstrate that adding a bit of movement to the source does not necessarily increase tbe speed of whatever is being projected.
(unlike a javelin thrown from a moving platform).
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Horn E. Towed earned 80 total points
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>'if I ride my bike then isn't the light coming out of the lamp going faster?'.<

No, but the frequency is shifted. [Doppler Effect]

>I was/am looking for a way to demonstrate that adding a bit of movement to the source does not necessarily increase tbe speed of whatever is being projected.
(unlike a javelin thrown from a moving platform). <

Well......
"unlike a javelin..."  is supposed to be something different from the preceding "that adding a bit of movement to the source does not necessarily increase tbe speed of whatever is being projected." ??

Both of those equate to the same phenomenon, I believe.
The increase in the speed of the {object being projected} is only measurable in relation to whatever the platform is moving in relation to [i.e. external to the moving platform]
As far as the moving platform is concerned there is no difference [ignoring the effects of wind and such]
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The fan experiment is designed to factor in wind resistance, the javelin description was not. If you can throw a javelin 50 yards from a standing position, you will be able to throw it further if you are moving, that's why they run as they throw them.
The ping pong balls are dependant only on airspeed so moving against the flow if air will have a noticeable affect.
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Horn E. Towed earned 80 total points
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True, but it's essentially the same concept; just more variables.

That's why I need my super computer.
It's still hung up with the tea thing.
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aburr earned 120 total points
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"""unlike a javelin..."  is supposed to be something different from the preceding "that adding a bit of movement to the source does not necessarily increase tbe speed of whatever is being projected."""
-
I think that IS his point. There will be an interaction of the air with the ball. That point was considered with light. It was posited that there was a space filling eather which would interact with the light. Lots of experiments looked for it but found nothing.
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bugger!!  You all just gave my super computer a nervous breakdown.   : |
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Horn E. Towed earned 80 total points
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There's an infinite difference between the velocity of light and the velocity of any material object.
I'm not certain how this came to be about the velocity of light.

Back to the original questions.

Based upon the given criteria:
The velocity of the ball, as measured on the vehicle & ignoring air friction, will be 40 mph.

The velocity of the ball, as measured from the ground & ignoring air friction, will be 40 mph + 40 mph = 80 mph

[as ozo pretty much stated]
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ozo earned 180 total points
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An analogy can be made between the speed of light and the speed of sound in a medium.
It is a poor analogy for various reasons (cf. luminiferous æther) but can be useful for some limited purposes.
e.g. http://en.wikipedia.org/wiki/Sonic_black_hole http://rsta.royalsocietypublishing.org/content/366/1877/2905.long
Are there particular aspects of the nature of light speed that you wish to elucidate?
The main things to understand are that the speed of electromagnetic radiation in free space is the same for all observers, and that the laws of physics are the same in any inertial frame of reference.
From that (and a few other assumptions like that space is homogeneous and isotropic) you can derive all of Special Relativity (and much of General Relativity) just by working out the implications of those postulates in various situations.
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aburr earned 120 total points
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"The velocity of the ball, as measured from the ground & ignoring air friction, will be 40 mph + 40 mph = 80 mph"
-
NO    NO   NO

The car is going forward, the ball is going backward
You have, as I said,
v of ball with respect to the ground is 40 + (-40) or 0.    That is zero
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_ earned 40 total points
ID: 39744296
How can the ball go backward, if it is mounted on the front of the car?

>> I thought light may behave in a similar way, unable to go any faster no matter what was the relative movement of its source.

It can't. But it can compress and stretch (shorter and longer wave lengths).

at least that's what my super computer just said, but I think it's still a little twitchy
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aburr earned 120 total points
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"How can the ball go backward, if it is mounted on the front of the car?"
The ball is not mounted but the fan is.
You are probably correct. I may have misinterpreted the question and turned the fan around.
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ozo earned 180 total points
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> turned the fan around.
And demonstrated a failure of the analogy with light.
Even if the parameters worked out to keep the speed of the table tennis balls invariant in one direction, it wouldn't be invariant in the other direction.
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>> The ball is not mounted but the fan is.

picky picky picky.     ; )

>> ...it wouldn't be invariant in the other direction.

whoops.  the computer just crashed, again.   : (
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I think it was aburr that suggested using guns and cats and shooting rearwsrds. Not at all what I am describing.
There is no gun, just a moving airstream generated by a fan which itself is being driven forwards.
Stick it in a wind tunnel if you like.
The big fan driving the wind tunnel is much more powerful( moves more air) than tbe small hovercraft fan. If both are spinning at a rate that would drive still air at 40mph and both running at once, the draft from the little fan will not push air against the main fan at 40 mph as this would be a difference if 80 which the small fan is not capable of.
Maybe the existance of aether would help my anlogy.
I am only trying to answer the moving light source question, but trying to find a simpler way than using higher mathematics, dimensions or transforms.
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_ earned 40 total points
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Oh, that simple. The answer to "Why", is always "Because".

because - I said so
because - it just is/does
because - that's the way ghod made it

etc...
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ozo earned 180 total points
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Even if the little fan is not turning at all when placed in a wind tunnel, there would be a small region around the fan where the air would not be moving at 40mph but would be relatively still.  Within this region, the little fan may be able to move some air forward.  How much is difficult to determine without additional details.  Far from the little fan of course, the big fan would dominate.  But ambient air would dominate far from the fan regardless of whether it was in a wind tunnel.
However, the relevance of any of this to light speed is questionable.

What 'why?' question are you asking, and on what basis would you want an answer?
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The why is almost the first question anyone asks when told nothing can travel faster than light and they say 'Why' doesn't the light from my bicycle lamp go faster when I am moving.
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BillDL earned 40 total points
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Robin, you obviously missed the fascinating lecture by Professor Brian Cox OBE on BB2 some time before Christmas from the Royal Institute.  In front of an audience of celebrities and other intrigued people, to coincide with the 50th anniversary of the UK TV series Doctor Who, he integrated such rules of Physics with simple practical demonstrations to theorise that time travel (albeit by the most miniscule increments) is possible.  I think the 1 hour show was named "The Science of Doctor Who".

I doubt it will be available as a replay video on the BBC2 website, but maybe someone you know has recorded it at home or put it up on YouTube.

And, before you ask, I only partially understood most parts of it.
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The more of this thread I read the less I understand about the question(s) it supposedly poses.

Light, schmight!
Whose balls even get close to c ?

I'm sticking with my original post that answered the original 2 questions.
:-Þ
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ozo earned 180 total points
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In what terms would you want an answer to 'Why doesn't the light from my bicycle lamp go faster when I am moving?' to be framed?
We've already explained that the speed of light is constant for all observers,
which is the most widely accepted reason.
We could say that the speed of light is constant for all observers because of effects like time dilation, length contraction and relativity of simultaneity ,
all of which are consequences of the fact that velocity transforms are Lorentzian  rather than Galilean.
But most people probably tend to think of the constant speed of light as the reason that velocity transforms are Lorentzian rather than the other way around.
We could say that it's because photons are massless, and all massless particles move at the same speed.
We could say that it's because light is an electromagnetic wave, and all electromagnetic waves propagate according to Maxwell's equations,
and when you work those out, you can conclude that the speed of light depends only on
the permittivity of free space and the permeability of free space.
I'm not sure how one might answer a question of 'Why do electromagnetic waves propagate according to Maxwell's equations?'
Would you want to explain Maxwell's equations in term of the deeper laws of Quantum ElectroDynamics?
Some think that String Theory might provide an even deeper explanation than Quantum ElectroDynamics,
but working out the consequences of String Theory is so complicated that no one can figure out what it predicts well enough to test it.
Others may think that Loop Quantum Gravity or the Holographic Principle is a better fundamental basis of reality, but there remains much debate over such ideas.
If anyone tries to tell you that it's because the light is being affected by wind, whether of air currents from a fan or of luminiferous æther,
I would consider such an explanation to be misleading at best.
If what you really mean is 'Why do we believe that the light from my bicycle lamp doesn't go faster when I am moving?'
then we could list all the experiments demonstrating that the speed of light is constant for all observers, starting from the Michelson–Morley experiment.
Another question might be 'Why would I expect the light from my bicycle lamp to go faster when I am moving?'
You might know the answer to that better than we would, but I would suspect that it may have something do with experiences doing things like dropping table tennis balls from bicycles, which occured at such slow relative velocities
that Galilean transformations are a good enough approximation to reality that you never noticed the difference.
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tliotta earned 20 total points
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AFAIK, there is no useful answer to any "Why?" question. The speed of light (as far as we can tell) is a "fundamental". Every "Why?" question eventually decomposes to some "fundamental" that is at the limit of our knowledge.

"Chocolate or vanilla?"
"Chocolate."
"Why?"
"Because I like it."
"Yes, but why?"
"Because it tastes better."
"Yes, but why?"
"Because I..."
"Yes, but..."

Questions of "Why?" always get answered with some substitution statement that simply tries to say it in a different way, in the hope that the new statement is accepted. But if pushed deeper, it always reaches a "fundamental" point that (currently) goes no deeper.

At times in history, we've pushed boundaries that move the "fundamental" points to a new level of understanding. At this time in history, the observed speed of light simply 'is'. We don't know "Why?"  We can only present alternative ways of saying it, e.g., Maxwell's equations. But everything we say is just a substitute way of saying the same thing.

The only answer we have right now is "Nobody knows."

That's the answer that motivates the geniuses of future generations to move the boundaries again.

Tom
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ID: 39746407
Why?
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Because.

Get over it.
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ID: 39746576
How long do I have to implement that?
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ID: 39746768
Until 2014
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kewl... 366 days should be enough.
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This didn't go the way I'd hoped at all. Thanks to those who attempted to understand the question. I was hoping to keep the mysteries of speed of light out of it and keep it to ping pong balls being blown by a fan, but it seems even that becomes confusing.
I just clicked most of the boxes hoping that everyone would get some in varying quantities.
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Thank you much.    : )
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Thank you Robin
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Any analogy can be misleading if you try to interpret it too literally,
and Minkowski geometry can seem unintuitive if you're not used to it,
but this analogy for the cosmic speed limit may be both reasonably easy to understand and not too terribly misleading:
http://youtu.be/IM630Z8lho8
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I don't know about the rest of you but MY velocities add up exactly.
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