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

[sol invictus]

Yes, it has stopped accelerating, but that does not mean that there is no force.

So you also don't know about Newton's second law.... or maybe no one ever taught you what "=" means.

 

[spork]

This is a moderately advanced topic in E&M, so you're going to have to dig a little.

Or we could just ask humber - eh?

 

[69dodge]

I wonder if we could get a conversation going between humber and doronshadmi...

What do you say, humber? Aren't you interested in finding out what is deeper than primes?

 

[Modified]

When let go, the water canoe accelerates to the same speed as the bulk of the water, because being just like water it has no drag.

What exactly are the requirements of being "just like water"? Is a plastic ball with the same density of water "just like water"?

 

[Subduction Zone]

Thanks again sol invictus, you learn something every day (unless you're humber). The more interesting of the two looked like the paper, but it seems you must pay a fee to get past the abstract.

 

[Clive]

We need to start a contest. When people pose questions like this, we need a place for us to all post our predictions as to what happens in the humberverse. The guy with the closest guess gets bragging rights - and the fear that he might be just a little humberlike.

Okay - I'll go first...




Will you judge me humber?

 

[humber]

So you also don't know about Newton's second law.... or maybe no one ever taught you what "=" means.

I do know what 0 = 0 implies.

However, a body in free fall at terminal velocity has equal but opposing forces upon it. There is no differential force, so there can be no acceleration.
Do you think that the = means that gravity disappears?
Wait, perhaps it could be that gravity itself is creating the opposing force as it acts upon the body to drag it through the air?
I my PC is not accelerating are there no forces upon it? I though it was being accelerated around the sun.

 

[humber]

Or we could just ask humber - eh?

Keep digging. Simple one for you. Name a body the travels at a fixed veloicty w.r.t the ground that does not have force upon it. You failed to do that for the KE question.

 

[sol invictus]

I do know what 0 = 0 implies.

Really? Do tell.

However, a body in free fall at terminal velocity has equal but opposing forces upon it.

"However"?

There is no differential force, so there can be no acceleration.

Tell that to this guy:

It could be argued that an object driven to terminal velocity, say a car against a headwind, is always accelerating. It must be. If is doesn't it will slow.

Oh wait...

Do you think that the = means that gravity disappears?

No.... and?

Wait, perhaps it could be that gravity itself is creating the opposing force as it acts upon the body to drag it through the air?

Yes.... and?

I my PC is not accelerating are there no forces upon it? I though it was being accelerated around the sun.

Yes.... and?

 

[humber]

My bad, you're actually correct here. I never really thought newton's third law was relevant to mutual gravitational attraction, but it seems I was wrong.

So you seem to have a good grasp of the third law, but you've made some startlingly inaccurate statements with respect to the first and second laws.

Lemme probe that mind of yours with the following question.

A rocketship is floating in space, motionless with respect to an observer stationed nearby.

The rocket fires its engines for a few seconds, and the force accelerates the rocket. When the rocket reaches 50 m/s, the engines turn off.

Here's the question for you:

What happens to the motion of the rocket, with respect to the observer, after the engines are turned off? (remember, there is no drag, since we're assuming a perfect vacuum).

You know, Spacediver, I put that 3rd law stuff within brackets, for completeness, so as to avoid having some one point it out as an error...

It continues at 50m/s

Let me ask you one. Please name one or more of the startlingly inaccurate statements regarding the other laws.

The odd thing is that although many may not like my explanations, there is still no evidence of devices going at waterspeed, yet you can see from the equation and text for the balloon , that shape, Cd and fluid density greatly affect the final velocity. And that's evidence against my point, but not against the opposite view?

 

[humber]

Yes.... and?\\\\

You must be slow witted. I told you that remark was hypothetical.

The remainder is your way avoiding saying that you made a mistake.
Your never short of answers when you think otherwise.

 

[spacediver]

It continues at 50m/s

Correct.

Let me ask you one. Please name one or more of the startlingly inaccurate statements regarding the other laws.

You've repeatedly said that once an object reaches terminal velocity (in wind or water), then it requires a force to keep it at maximum velocity, but this isn't really accurate at all.

Yes, the wind continues to exert a force at terminal velocity, but this is balanced by the force due to air resistance. So the forces cancel out, resulting in zero net force on the object, which is why it remains at a constant velocity.

Furthermore, while the air resistance force is at a maximum at terminal velocity, the wind force is at a minimum. Importantly, this maximum air resistance force is equal in magnitude (but opposite in direction) to the minimum wind force.

You don't seem to be understanding this.

If we were to graph the air resistance force over time, it would gradually increase to a maximum level at time T, and the wind force would decrease to that same level at time T.

You can agree to all this without conceding that an object will reach wind or waterspeed.

 

[humber]

What exactly are the requirements of being "just like water"? Is a plastic ball with the same density of water "just like water"?

Same drag, same density. It's hypothetical. The point being why should you make a distinction between this and an canoe or other object? If the water has force driving it at waterspeed, why not upon a real object?

 

[John Freestone]

I miss humb. Also, waiting for humberer to chime in.

humb, humber, humberer, humberest...

Make the boat out of water. It occupies the same mass and volume. Were it it be stopped somehow, say held, then there would be the full force of the water behind it. When let go, the water canoe accelerates to the same speed as the bulk of the water, because being just like water it has no drag.

No, because it has drag. For the eleventeenth time, drag is what accelerates the boat or bit of water dropped in the river.

...then why the fuss over the cart? Why cannot a simple sail not reach windspeed?

Friction.

Quite wrong, really. Firstly the case of 100% windspeed balloon is not possible.

Earlier you told me that they could go faster than the wind. So they can go slower, and faster, but somehow jump past the 100% windspeed.

I asked for examples of any bodies that do so. This meteorological balloon is not one, and you will not find one. That's one thing.

Yes it is, and you got several others.

The other is that you can accept travel without force or work. Now show me objects that do that. Travel without work. Not on this planet. This is the major error.

You just have to be kidding. Boats being used to drift downstream (though you might have to work to hollow out a tree trunk and dump it in the river). Hot air balloons (although work is done to put heat in to them). The Moon was set in orbit something in the order of a billion years ago I think (although we haven't exactly used it as a form of transport for various reasons)... your question is clearly a pitiful bit of trolling, humber. 'Travel' - in the sense of motion, uniform motion - has been explained. If you refute Newton, do so more convincingly than asking for examples of when he's right.

If there is no drag at all, then the force will accelerate the object to V, and that force will hold it there.

If there is no drag, what force will accelerate the object? The force that accelerates the balloon, for the twelvthty time, IS DRAG.

(Remember, that if an object is dragged through air by force, the drag increases, by the square of the velocity, because then relative and absolute velocities are the same.)

Yes, but that is when the driving force is another force, not the drag. The force for a balloon is the drag.

Say you have a sleek car, and blocky car driven in air. For the same horsepower engine, which will reach the higher terminal speed?

Wrong analogy AGAIN. BALLOONS DO NOT HAVE MOTORS DRIVING THEM THROUGH THE AIR TO REACH THEIR FINAL SPEED. THEY ARE DRIVEN BY THE DRAG OF THE AIR AROUND THEM. THAT IS WHY "TERMINAL VELOCITY" IS NOT RELEVANT. THE ISSUE INVOLVES AN OBJECT BEING PUSHED BY A MEDIUM, NOT PUSHED THROUGH A MEDIUM. I WILL NOT EXPLAIN THE DIFFERENCE AGAIN.

In a fluid, it means that if the object is sleek, there will be enough force available from the river, to drive it closer to waterspeed,

NO. WRONG WAY ROUND. THE SLEEKER, THE SLOWER IT WILL APPROACH WATERSPEED, AS I POSTED EARLIER. DRAGGIER BODIES GET DRAGGED BY THE DRAG FORCE DRAGGING THEM DOWNSTREAM FASTER. IT'S A DRAG THING!

but in each case, like the two cars, the force is still required to maintain that velocity.

"The Third Law means that all forces are interactions, and thus that there is no such thing as a unidirectional force. If body A exerts a force on body B, simultaneously, body B exerts a force of the same magnitude body A, both forces acting along the same line."

True. I didn't want to confuse you with that point earlier. So, when I said that there is only one relevant force, I meant one relevant pair of forces, since they always come together. It hardly matters that when I push my shopping trolley it pushes back against me. It doesn't actually matter that when the air accelerates a balloon, the balloon pushes back. It hasn't got its back against a wall, no-one has it on a string, so it gets accelerated by the force, or pair of mutual forces, called the drag, or the fluid friction, or whatever you want to call it. NOTHING ELSE EXISTS TO SLOW IT DOWN. I have asked you to tell me what exists to keep it below airspeed, but so far you have not said. For it to go slower than the air, you have to provide that other, retarding, force (or pair thereof if you prefer). Does the skin of the balloon have little threads of steel woven into it and you're following along with a magnet, causing a mutually attracting pair of magnetic forces? Are spirits willing it to go slower than the air? Is there a pink unicorn in the vicinity?

To reverse the same question - if the balloon maintains steady state at less than windspeed, please explain why the air moving past it (which there must be, right) does not accelerate it?

The skydiver does move the Earth, but only very tiny bit. Don't worry though, you won't feel a thing.

Yes I said a similar thing about 12 hours ago in another form: when throwing a ball, you accelerate the earth laterally.

Allow me to correct the following:

We found sticks, barrels and canoes in water. We found hot air balloons in air and even provided with a eight links to an independent documents describing these. And of course my favorite, the dead fish (in the water, not in the air). If the challenge is to find something that you agree with, that is mission impossible.

I'm indulging in a football chant right now. Eight - nil ... eight - nil...

I wonder if we could get a conversation going between humber and doronshadmi...

What do you say, humber? Aren't you interested in finding out what is deeper than primes?

That's it! Doron is the mathematician for the job! Imagine, the humberverse, but with equations!

 

[mender]

First this:

You are suggesting motion without force.

Then this on the same page!

You may need this:-
http://www.glenbrook.k12.il.us/GBSSCI/PHYS/CLASS/newtlaws/u2l3b.html

From the link that humber provided:

"Forces do not cause motion; forces cause accelerations."

Amazing. Humber, you need to read your links before you use them to "support" your arguments. OF course, this isn't surprising in view of how selective you are in your reading.

By the way, my prop cart is working just fine, exactly the way physics says it would. It is even predictable and repeatable. Any tests you'd like me to do?

Keep digging. Simple one for you. Name a body the travels at a fixed veloicty w.r.t the ground that does not have force upon it. You failed to do that for the KE question.

Your body of evidence, your arguments, your logic, your tests ...

ETA: Oh sorry, I thought you meant force behind them. My bad.

 

[Christian Klippel]

From the link that humber provided:
....
Amazing. Humber, you need to read your links before you use them to "support" your arguments.

Hello mender,

let's see if he can swallow his own medicine.

Humber, do you know what your link means?

The links provided so far are inadequate, because they are simplistic.

The links are usually attempts to bluff, or evade.

Exactly that! You try to evade and bluff us by giving links that you don't even seem to have read.

 

[humber]

Correct.

You've repeatedly said that once an object reaches terminal velocity (in wind or water), then it requires a force to keep it at maximum velocity, but this isn't really accurate at all.
Yes, the wind continues to exert a force at terminal velocity, but this is balanced by the drag force. So the forces cancel out, resulting in zero net force on the object, which is why it remains at a constant velocity.

Yes, that is zero net force, zero differential force. The forces are still there. When a body is dragged through a fluid by an external force, say a sphere through oil, the drag opposing that motion rises with velocity.
When those opposing forces are equal, acceleration stops, and that velocity is the maximum that is available for that applied force.
Drag and applied forces are not independent forces, but drag is the reaction to the applied force. So, that applied force is there, sustaining the velocity, acceleration or not.

Do you disagree with that ? (Downwind is not so different)

Furthermore, while the drag force is at a maximum at terminal velocity, the wind force is at a minimum. Importantly, this maximum drag force is equal in magnitude (but opposite in direction) to the minimum wind force.

Wind is complex. Natural forces are rarely constant force or constant velocity. They are related by a v/f profile ,and the type of device used to harness its energy. There is a load line that relates force to velocity. Power is f*v, so there is only a fixed amount of power available at any given force or velocity. Using purely abstract kinematic forces can often lead to the wrong conclusions.

If you look at Raleigh's equation stripped bare, is shows that the drag force is the square of the velocity, so the v/f curve is parabola, and that means the power demand rises as f* v^2. Where does the river or air get that power? It doesn't. Even downwind, the drag outstrips the available power, well windspeed is reached. Cars have low Cd to reduce the required engine power. That relates directly to the fuel, the source of energy.
Rivers and wind only have some much of either, they cannot perform miracles.

You don't seem to be understanding this.
If we were to graph the drag force over time, it would gradually increase to a maximum level at time T, and the wind force would decrease to that same level at time T.
You can agree to all this without conceding that an object will reach wind or waterspeed.

That is what I know you think, and why, but it is wrong.
As the velocity rises, the drag increases, not linearly, but at least the square of that velocity. Now you can see that if applied force is truly constant, ( it is not itself dependent upon velocity) then the object's velocity will rise until the drag equals and opposes that fixed (constant) force. That point is the terminal velocity for that applied force. The applied force is certainly at a maximum, and below windspeed. To get there, takes a lot of power and more than is available by direct means. Don't think that downwind solves this problem.

If you make available a little more force to gain some more speed, the drag being square law, will rise even more, and so will the power.
Drag is serious business. That's why enormous jet engines must power landspeed vehicles, just to gain a relatively small increase. The power demand just increases so rapidly.

It is a really a mistake to think that downwind does not suffer these problems. Rivers too. That is not too hard to show.

 

[humber]

Hello mender,

Do you think I didn't? I said I did. I looked too.
The balloon does not travel at windspeed, not does it have zero force.
Also, it is not a river, but other wind cases may do, if you can find them, or even other evidence at all may be useful.
The others are relative velocity examples, not real world speeds.

 

[humber]

First this:

Then this on the same page!

From the link that humber provided:

"Forces do not cause motion; forces cause accelerations."

Amazing. Humber, you need to read your links before you use them to "support" your arguments. OF course, this isn't surprising in view of how selective you are in your reading.

No, you should read on. Later, you will see why it is expressed that way.(They are trying to get the reader to thin in terms of forces, rather than velocities, as I have said should be done). You didn't read it, then.
I did read it ,which is why I suggested it. It explains some of the mistakes that lead to common errors, which a lot of direct texts do not address. If you read it all, you will see the approach.

By the way, my prop cart is working just fine, exactly the way physics says it would. It is even predictable and repeatable. Any tests you'd like me to do?

If you would, yes please. Do you have a treadmill? The wind is a problem, but a fan might do?

 

[Dan O.]

humber#1: terminology - is it "vector" or "variable"

To try and make some progress (and attempt to stop the building level of insults being bounced back and forth), I'm going to start something different. For each apparent error we can create a single post with a title that describes the error and in the body quote the part of the post that includes the error followed by definitions and links to help resolve the error. Please try to provide links and the text of what the link provides.​

Velocity is a vector. A time-variant quantity. It cannot be described by one coordinate. Momentum is a vector, so that applies too.

While velocity is indeed a vector and velocity can vary over time, the statement appears to say that a vector is a time variant quantity. A better choice of wording and structure may have avoided the confusion.


Euclidean vector^WP

In elementary mathematics, physics, and engineering, a vector (sometimes called a geometric or spatial vector) is a geometric object that has both a magnitude (or length), direction and sense that is orientation along the given direction.

Variable^WP

A variable is a symbol, usually standing for a number whose value is changing or unknown.