Another Larry Brasfield Self-Aggrandizing Put-Down Anecdote

It's not "changing the rules". It's pointing out that the assertion, "A rocket car starts at rest, accellerating at a constant rate because its thrust is constant. It is burning fuel at a constant rate to produce that constant thrust." is impossible, so the question is meaningless.

If the rocket car is experiencing constant thrust, it will accellerate at a constant rate IF ITS MASS IS NOT CHANGING.

If it is burning fuel at a constant rate, and the thrust is constant, then the accelleration would increase because the mass is decreasing.

I thought this got done with weeks ago.

Thanks, Rich

Well, since it was not part of the original post, it feels like changing the rules to me.

I'll have to check google for that 'cause it's not in my seb headers (I only pulled 100) I went ahead and did the total differential with a changing mass (i think I left that out of the accel/velocity part) and integrated that and ended up with a sum of 2 energies for the total kinetic energy. I think that's what JT suggested I do (write it wrt time)

One of the terms looks like it contains impulse and I forget what you multiply impulse by to get E (I could have checked the dimensions) but it looks normal.

Even if you skip the changing acceleration, the whole thing that sticks out as bs to me is that LB thinks that linearly changing mass and quad changing energy is wrong. That makes no sense. It's like saying that linearly changing length should yield linearly changing volume when we all know that volume will change cubically.

You can't blame Fred and John for launching through the roof with that guy. (not that you did)

--
Best Regards,
Mike

[Irrelevant and scurrilous subject revised.]

...

You are going beyond the evidence, pal. The antecdote, relating to an event in 1967, bears on my thinking at that time. To get from there to "LB thinks" at this time, would required an extremely limited view of my educatability and surmises about intervening processes that go way beyond the evidence. That is another subject of course, and a silly one, so I do not mean to get you going into it. (I won't follow at any rate.) Perhaps, if you are willing to grant me the provocatory status you did earlier, you can see that what you call "bs" is just what you suspected then.

I asked this before: How could it be a puzzle without seeming to be "wrong" on its face?

I am puzzled as to why my puzzle puzzles you. As I've said, it was proffered for fun. You seem to have had fun with it. What is the problem?

I suppose you meant "linearly scaling an object in 3 dimensions" since what you actually stated is more sensible with that substitution. So I am responding to the statement you intended, not the one you made.

With that adjustment, I agree with you.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

It wasn't wrong on it's face, but you asked him, "How can this be?" It was a puzzle to you at the time.

It was your confusion that puzzled me.

You said, "I would be interested in your take on this." My take was that I'd not have puzzled over it. IIRC kinetic E came before conservation of momentum in the class and it was the latter subject where change in fuel mass came up and then only in "The Physics Problem Solver." I still didn't puzzle over it.

So change the tense of "thinks" in:

and know that it was as much bs then as it is now.

Maybe I had fun with the total differential, but that depends on what you consider fun. I'd call it interesting. Learning something new is more fun.

--
Best Regards,
Mike

...

[Old quote from a Brasfield post left for context.]

Yes. I was just a twerp, not yet sophisticated enough to look at all aspects of a problem.

If you like, I was a confused child. In addition to the distractions of the opposite sex, the usual teenager troubles, and a few others, I made the very silly mistake of not even thinking about the exhaust at the time. My teacher's presentation of E=m v^2/2 was very good, and as I tried to incorporate that into my meager other understandings, I failed and so came to him, after class, to get that sorted out. He gave it a try, but his effort did not succeed. [1]

Unless you were (or are) a most unusual young person, you can surely identify with not being able to figure stuff out oftentimes.

[1. As I recall that incident, I realize that he may well have approached the problem by trying to pull me out of my assumption with questions I was unable to grasp. We had many other after- school sessions where he often used the Socratic method with me, and he was most often able to get my questions resolved. He was (or is) at least an intelligent person, if not brighter than I knew at the time. So, I may have unintentionally libeled him by forwarding my punk's impression at the time ("could not resolve it") and presenting it as fact. The most I can truthfully say is that he did not resolve it for me, then. I thought about it for some time afterward, thinking about the role of the exhaust, and he may well have put that finally pertinent thought into my head. ]

Well, "Take" is subject to several interpretations.

Perhaps you refer to college courses. My anecdote relates to a high school physics course for sophmores. Maybe it was a school too podunk to cover momentum before kinetic energy, (or I've forgotten), or maybe I was just too dense or distracted to get it. Whichever it is, you should not be taking my confusion compared to your clarity of perception to indicate much. Stick to the evidence and do not project what you "know" into the gaps. Then it should either become clear or, better yet, accurately and appropriately unclear.

[Active8's point and Brasfield's recast of it cut.]

Well, I can't be changing people's posts all the time just so I can agree with them. That would be much like getting stuck in front of a mirror, too enthralled to move on.

I'm not able to puzzle that out. Sorry.

I consider fun for you fun. It's not much more complicated than that, really. Was that a grudging admission? I hope so.

Yes and yes. You've made it a little more interesting.

Really? If so, likewise.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

Actually, the puzzle is that the power is constant (a constant rate of fuel burn) but the rocket's kinetic energy is increasing quadratically with time, since the velocity is increasing linearly (if you ignore changes in acceleration due to the changing mass of the rocket). Just looking at those two facts, it's a puzzle, because it seems to contradict conservation of energy. The answer is to also consider the ke of the burnt fuel.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

There you go, the statement of the problem is contradictory... sucker bait.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Why is that a puzzle? It starts out at zero efficiency and, just before it runs out of fuel, it's worked its way up to rotten efficiency; that violates no physical laws.

John

And it's dragging on the ground as well. Sno-o-o-o-o-o-ort ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

The static power and thereby force of the engine is constant if the throttle is fixed. If the rocket is tied down, no static energy is transfered from the fuel to the kinetic energy (KE) of the rocket. The kinetic power increases linearly with velocity, assuming constant acceleration. Since P = F*v = F*a*t, twice the power is being converted to KE at the end of 10 sec as compared to 5 sec. The distance travelled is three times as long from 5 sec to 10 secs as compared to 0 sec to 5 sec (½a*t²). If the force is constant, and the distance is three times as long, it should be no suprise that that 3 times the energy is accumulated from 5 sec to 10 sec as compared from 0 sec to 5 sec. Ratch

Actually, that doesn't help at all. We all know it's OK, and that rockets don't violate conservation of energy. The puzzle is why do these two ways of looking at the energy at a given time give different answers?

I think it's a fair puzzle. It points out that conservation of energy is harder than it sounds initially, and that you have to be careful when adding up the energy. It's no deeper than that.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

What "answers"? An observation that a phenomenon has a curve says nothing about the quantitative issues. The original "puzzle" was a sloppy, qualitative look at the way a rocket functions, and created a bogus paradox by assuming that a squared-something curve somehow has to increase enormously without limit and punch a hole in the sky or something.

If you don't do the math, you can't complain about "different answers." The only different answers are the right one, and all the rest.

John

I'm a confused adult. Biased media confuses me on the issues and facts. Only researching legislative proceedings and executive orders would that, I think. Differing views on history and philosophy leave me out there, sometimes.

F the exhaust. If a constant burn causes a constant force ->

constant a... v = at and energy just happens to be proportional to v^2.

What did Socrates do? I don't recall that.

The mv conservation approach has it's uses but that doesn't make it the only way to solve the problem. I had a physics exam with an elevator problem in multiple parts. I tried working it from F = ma instead of U + K = E . F'd it all up.

And F came before E, too. They didn't really teach me about working backwards, though at the time, I'd have worked an amplifier backwards.

I had advanced Physics (that's with calculus) in HS. Then I moved to a podunk school with a space-brain teacher showing pictures from his telescope and figuring how fast to hit a tree with a snow ball to get it to melt. He let me study a Schaum's - taught myself a bit. A Chinese student who'd already passed calculus taught me calculus with the help of MD public TV vids. I learned calc better than physics.

It was the other way.

The question didn't even come up in my mind back then. My clarity now is partly intuitive. I'm not going to question .5*m*v^2 at low velocities.

Well, I had no indication that you didn't still think there was a conundrum. I meant that you can now change the tense and it doesn't change anything else. If my words aren't totally clear and you think you know what I am trying to say, I'm not sure that what you hear is what I mean. IOW, I'm not going to nitpick my own words to death tonight. None of this means a damned thing anyway.

Newtonian Physics hasn't changed.

No, it wasn't fun, just interesting to look at it as a TD rather than a momentum thing since in physics I, we didn't know partial derivatives and therefore couldn't play with them.

I generally harbor no malice towards others. When I do, I try to get rid of it. Negative stuff just eats you from within. I've only changed my sig block for Burridge when I was messing with his thick head, which he asked for, BTW.

If anybody find an obit on him over there, let us know.

--
Best Regards,
Mike

You put that another way in your other post but what does E*3 have to do with v^2 ?

--
Best Regards,
Mike

ke

converted to

long,

5

Sorry, I thought I made it clear. E = W = 1/2 m*v^2. Assume m = 2, a = 1 . At t = 5, v = a*t = 5, W = 1/2 * 2*5^2 = 25, s = 1/2 a*t^2 = 12.5 . At t =

10, v = a*t = 10, W = 1/2 * 2*10^2 = 100, s = 1/2 a*t^2 = 50 . Distance traveled from 0 to 5 secs = 12.5 . Distance traveled from 5 to 10 secs = 37.5 . Three time the distance traveled in the same time, constant thrust or force makes 3 times the KE accumulated from 5 to 10 sec as compared to 0 to 5 secs. Velocity and kinetic power increase proportionately to time, Distance travelled and KE increase proportionately to time squared. So to answer your question, the v^2 relationship gives a 3*distance during the second equal time interval when compared to the first identical time interval. That causes the KE to increase by 3 times during the second identical time interval as compared to the first time interval. Ratch

How many times... how many different ways... can I say it. There *is no puzzle*. If a linear fuel mass expenditure really gave a constant acceleration or constant power, that's fine. Check your old text. They start by showing that v = at and then a = F/m. Then they show that KE varies quadratically with v. If you can accept that, there's no puzzle.

When one assumes that a linear change of one dimension should produce a linear change in a derived unit it's called flawed logic. At least that's what I called it.

Or use momentums and convert to energy later. With your way, you have to guess at the relation between fuel expenditure and thrust. With conservation of momentum, you have to guess the velocity of the ejected fuel mass.

Now we didn't have partial derivatives as tools in physics I, but here's what I did:

let D be the round D

k = fuel mass expenditure wrt time M = initial mass of rocket

m = M - kt instantaneous mass of rocket

1 KE = --- (M - kt)a^2 t^2 2

expand that DE 1 2

-- = - v Dm 2

DE 2

-- = mat - kat Dv

1 2 2 DE = - v dm + mat dv - kat dv 2

dE 1 2 dm dv 2 dv

-- = - v -- + mv -- - kat -- dt 2 dt dt dt dE 1 2 2

-- = - kv + mav - kv dt 2

1 2 2 dE = - kv dt + Fv dt = kv dt 2

integrate 1 2 2 E = - kv t + Fvt = kv t 2

1 2 E = - mv + Fx - kxv 2

. . . Total energy = KE + thrust energy - exhaust energy

QED

--
Best Regards,
Mike

We have plenty of evidence that you are NOT well- educated. Much of your sentence structure is malformed and contradictory bs that says nothing. Your knowledge of engineering is also elementary and dysfunctional, now and in the past. That is why you are a punk programmer working for the AR Group in an insignificant role.

That p.o.s. doesn't know. He only brought that up because I first referred to it in a response JF regarding Brasfield's pedophile-like technique of capturing the attention of juvenile punk OPs.

BTW- have you seen the pseudo-intellectual give a coherent explanation of his so-called puzzle...