Merged DDWFTTW in a “Real” Wind

[Modified]

And you "understand" relative motion?

If you were in the car heading for the stationary truck, would you apply the brakes? If you were in the stationary car, with the truck heading towards you, would you put your foot on the brake?

We are ignoring the effects of the ground in this case. If two cars moving at constant speed or one moving at constant speed and the other sitting still collide, the speed of the ground doesn't matter much (even less if they are on slick ice). If you are applying brakes, then it matters a lot.

 

[CORed]

"They pass with several car widths between them. Let's say each is going 60 mph. At the moment they're adjacent to one another, their relative speed is 120 mph. But their closing speed is 0 mph at that moment."

How does one calculate the relative speed of the vehicles? Measure the change in distance between them over time v = dv/dt. That is a constant 120mph, regardless of distance apart.
A closing speed of zero. I see, at one point, just prior to them being 0m apart, the speed is 120mph, then instantly becomes zero, to become instantly 120mph again. "Closing speed" comes from the movies.
Spork is a manager in the sports broadcasting business.

Humber doesn't understand vectors at all.

 

[humber]

I've too have had no trouble finding moving walkways at airports that were not behind security. Our local airport has several.

I did say as far as I know, and you bit. Let's hear the excuses.

The problem is not finding one, but rather with finding one that moves fast enough. Our best model will run down to something in the range of 4mph and the fastest walkway I have found only moves around 3mph (most are close to 2mph).

The BB works of speed less than that, so what is the problem? It starts below WS. You can make a video to see if the little cart works at all, rather than just concentrating on the target you always want to put out of reach. Better to go with "the belt is treaded" or something like that.

I have thought several times about custom building a device that would work down in the 2mph range (it would just take a custom prop) but I likely won't until I'm old and bored. Self-starting on a straight treadmill is really the only thing the device has never demonstrated (though Ynot's turntable is a good as that to all but the hidiots).
JB

I know that will fail, and you have no justification for a minimum speed at all, and certainly none regarding self-start for the cart you already have.

But now you want to accept Ynot's table, even though you deny his other arguments?
Cherry pick criticisms and trial results, and beg, borrow or steal ideas.

 

[humber]

Humber doesn't understand vectors at all.

Na na na na...na nah.

 

[humber]

I would shift into reverse and try to back up real fast!

Windgrins.

But not put your foot on the brake. Motion is relative, because it can't really be anything else, but that does not mean that the world is a mirror where moving things are the "same as" stationary things.
One of many differences is that you would apply the brake in one case, but not the other, and a prop driving a wheel, is not the same as a wheel driving a prop.

 

[windgrins]

Spork and closing velocity

It took me less time (nano-seconds) to know what you were going to do.

You have invented "closing distance", which works for the single value t= 0
There is no such thing as "closing velocity" in this context.

V = dx/dt is a constant.

The distance, x, isn't.

Your equation works when v = 0, and the cars are then stationary, and perhaps parallel parked. Squaring the velocity -v changes the direction, so you can say they are traveling at 60mph in parallel for all t, or some other whatever that falls out of the churn.

Humber,

If you take a look at this webpage and do the arithmetic, you will see that Spork was correct wrt closing velocity:

http://books.google.com/books?id=Os...cQ6AEwBQ#v=onepage&q=closing velocity&f=false

Regards,

Windgrins

 

[windgrins]

Brakes Redux

But not put your foot on the brake. Motion is relative, because it can't really be anything else, but that does not mean that the world is a mirror where moving things are the "same as" stationary things.
One of many differences is that you would apply the brake in one case, but not the other, and a prop driving a wheel, is not the same as a wheel driving a prop.

See my long winded explanation from above if I was time constrained:

Since I value me more than my car:

With no time constraints:

1. I would shift into reverse and try to back up real fast!


2. If I couldn't shift into reverse due to time constraints, I would put my foot on the brake to mitigate the ensuing motion that the cars steering wheel is about to partake in wrt to the driver. Brakes are better than no brakes here to mitigate the damage to the driver but not to the car. The car will be worse for wear!

What would you do?

Regards,

WindGrins

 

[CORed]

I don't believe you are genuine, Windgrins. Too much time and efforts proselyting, and being an apologist, while tip-toeing.

He agrees with spork. He must be a sock puppet.

Then again, maybe it's reality that spork and everybody but humber agrees with.

 

[CORed]

Gee, Sol, you are a wizz.
First you write an equation around the "relative velocity" ( which is also the differential velocity, with a common mode velocity of the surface of the planet, but we are going to make that zero, aren't we now?), and the same equation, relabeled "closing velocity"

You then write an equation that says the closing velocity becomes zero when the distance, b = 0,
But oddly enough, that equation produces a zero when t = 0 or v = 0.
but also 2V, and some other values dependent upon b or t...etc...etc...
Meaning that if you are right, the relative velocity is zero too, but only if t = 0, which gives you a problem for your first two equations....etc...etc...
Good work.
ETA:
You may get away with that, if the velocity can be expressed in terms of "c".

Whoosh .....

That was the sound of basic vector addition flying over humber's head.

 

[windgrins]

WindGrins the Sporkian Sock Puppet

I don't believe you are genuine, Windgrins. Too much time and efforts proselyting, and being an apologist, while tip-toeing.

Nope, Spork has never coached me in any sense. He doesn't have a clue who I am and all comments are mine alone.

You, are, of course, free to believe whatever you would like!

PS, if it makes you feel better, we thought that you were Spork's Devil's advocate Sock Puppet for a long time also!

Regards,

WindGrins

 

[mender]

I'd move out of the way.

 

[humber]

Humber,
The "closing speed" is used in various areas to discuss the idea of how fast it appears that the relative speed between two objects is changing.

A reference, if you please. There is no closing velocity in this context.
Like Sol and Spork you are confusing a constantly changing distance with a constant speed.

This is a semantics issue and not an issue in understanding.

No, it is black and white.

So even though the cars pass at 120 mph, the instantaneous distance between them is not increasing or decreasing at that point so the "closing speed" is 0mph.

The distance is zero.

On Collision courses the closing speed always is the relative velocity of the two objects.

Try looking that up.

On non-collision courses between two objects, the closing speed is a function of velocity of the objects and the path they are on and are not numerically equal except at infinity so this was why Spork made his comment to highlight this subtlety.

It was nonsense disguised as erudition, and tarted up with talk of dimensions, which makes no sense anyway. One dimensional cars. Might be difficult to measure that distance.

With regards to this issue, I think you missed the point of Spork's video in the car. For a NB balloon, the balloon occupies a space that air would normally have occupied.

So, rises because the denser air replaces it.

The air weighs the exact same as the balloon that is occupying the space.

The balloon is less dense, so has less gravitational force for the same volume.
That's how a ship floats.

This is why it is neutrally buoyant. Once this is understood the issue becomes clear: It's an old Jedi mind trick of one's own brain.

More like Imperial Stormtrooper.

This is why the balloon in the car demonstrated the "torque effects" illustrated.

And after I pointed out that balloons may have lift due to the Magnus effect.
A face-saver.

So, the point was that the NB Balloon is behaving just like the air that would normally occupy the same volume except for the mass property effects shown.

No, even air takes time to accelerate. There are several means of calculating the horizontal velocity of a weather balloon, assuming it can be coaxed to fly horizontally without rising. The simplest, I would expect is to use Rayliegh's drag formula and then use the mass of the balloon to provide the time-constant t. Something like this: Vb = Vw - e-t/tau
The balloon will rise exponentially to Vw (but not reach it) and the time t will be related to the acceleration. That is because the driving force falls as the balloon approaches WS, so the acceleration will fall too.

The balloon nor the air in the car reacted instantaneously but it reacted at the same speed as the medium which I believe to be the speed of sound in the media. But since it reacts at the exact same speed of the media in bulk, for the purposes of the discussion, it reacts immediately-same as the surrounding media. This does not imply infinite force contrary to what you stated. This was the subtlety of Spork's video and I'm sure it was hoped by the presenters that "bright" students watching this would observe all these effects and learn something. I can't make the claim to be "bright" but I sure learned something.

All these "subtle" nuances....sure. A brute force and silly show provided by an office manager.
Those "tests" become forced when mathematical and theoretical skill are absent.

With regards to potential energy, the NB balloon has no potential energy with respect to the ground because it occupies a volume containing the equivalent mass of air as the balloon.

PE = m*g*h. A stationary NB balloon has a buoyancy force that is equal to the force of gravity upon it. For an object on top of a ladder, the ladder provides the opposing force. That object still has PE, and so does the balloon.

This is the subtlety that we didn't want to bludgeon most readers with as it is normally assumed it is understood. When questions are asked, it makes sense to explain-or as the phrase goes "Otherwise we'll be here all day!" Sheesh-we sure exceeded that one so maybe its worthwhile after all being initially concise-lesson learned on my part.

How about you didn't think about it until I mentioned it, and Spork put his foot in it?

Rhett has corrected you regarding the KE. Stand corrected, rather than fall over yourself with contradictory exudes, Windgrins.

 

[CORed]

Yes, but remember we've seen pictures of the cart El Jefe and most of the carteers assured us would never be built. Some great sarcasm by spork as I recall, amidst a choir of hecklers. And yet, the cow which is not a hat we'd seen so often, was right there; proof this was Humber's cart. The one that would never exist. After so much time. Now a repeat? I also smell the fear.

I said it before, but the only thing I smell wrt humber's propellerless cart is BS. Either it will never be shown, or it will turn out to be something as ridiculous as the videos of the three wheeled cart with two wheels on a stationary platform and one on a belt sander that humber posted earlier in this thread.

 

[humber]

Humber,

If you take a look at this webpage and do the arithmetic, you will see that Spork was correct wrt closing velocity:

http://books.google.com/books?id=Os...cQ6AEwBQ#v=onepage&q=closing velocity&f=false

Regards,

Windgrins

No already covered that, and refers to a collision, an so accelerating bodies, which is not at all like two passing cars at constant velocity, as Spork says.
He's wrong. Live with that.

 

[humber]

I said it before, but the only thing I smell wrt humber's propellerless cart is BS. Either it will never be shown, or it will turn out to be something as ridiculous as the videos of the three wheeled cart with two wheels on a stationary platform and one on a belt sander that humber posted earlier in this thread.

Smell the fear.

 

[humber]

Nope, Spork has never coached me in any sense. He doesn't have a clue who I am and all comments are mine alone.

You, are, of course, free to believe whatever you would like!

PS, if it makes you feel better, we thought that you were Spork's Devil's advocate Sock Puppet for a long time also!

Regards,

WindGrins

"we"?

 

[ThinAirDesigns]

Yes, but remember we've seen pictures of the cart El Jefe and most of the carteers assured us would never be built.

Lie.

We've seen no such thing.

JB

 

[humber]

I'd move out of the way.

But not put your foot on the brake.
Silly people that like to chime that "physicists get that truck and car thing all wrong", are forgetting one thing. In making the reply, the physicists give such questioners too much credit, by thinking they are talking physics, and not cartoons.

 

[humber]

Lie.

We've seen no such thing.

JB

How are you coming along with the excuses for not making a video of the cart on of the readily available airport belts?

 

[windgrins]

Definition of "We"

"we"?

"we" being me and other posters who mentioned in either this or other posts.

Windgrins