Another Larry Brasfield Self-Aggrandizing Put-Down Anecdote

Well, your "fact" is not accepted physics today. It is a very good approximation for velocities well below the speed of light, but it cannot be defended as gold plated Truth. If you doubt this, look here:

That said, I will admit to phrasing my doubt of the matter in a provocative way. If that makes me a troll, so be it. But I posted it because I thought it was an interesting puzzler. The physics are fairly simple, (as you have suggested), but only if you consider the right set of objects. The puzzle is one that took me in as a high school student, along with a smart friend and my high school physics teacher who was dedicated and good at what he did. So, I thought it had a good chance of puzzling folks who were following the thread up to then. All in good fun, and not a typical troll.

I'm glad to not be acquainted with the others, then.

Agreed, certainly.

I take his having been stumped only as an indicator that the problem induces a mindset that can make it hard. As you have shown, and as I puzzled out a few years later, and as several other people showed here, it is not that challenging once you get out of that mindset.

There is another point lurking here, worth addressing since you bring it up and a few take it as an article of faith. Showing another person to be smart or not has no bearing on how smart the shower may be.

What my noisy vile shadow has missed, (and you too, it seems), is that I was as puzzled as my teacher. How that scene, of a teacher and student both wondering how to resolve a paradox, can possibly show the student to be putting down the teacher, or to be smarter, is quite mystefying to me. I think that vile interpretation has to be yet another instance of the projection I manage to induce among a (very) few folks here.

Congratulations for most colorful language of the week :)

But beware the Brasfield, there's mendacity hidden in his problem description. Try solving this versus time, not just end point.

I like "does not a smart larry make" also ;-)

...Jim Thompson

The instantaneous kinetic E obeys .5*m*v^2, but the total solution would require a partial diff eq.

Better yet, if you know the mass *and* speed of the ejected fuel, conservation of momentum is the easiest solution.

I've developed a feeling that Brasfield is always trying to set one up, just like that "ldg" turkey with his wanderings about Spice analysis.

Nothing he uttered ever made sense... like he can't read, yet he concluded, "I was curious at that point what you might actually know. Not curious now."

I hope that made him feel good, because he can't even find his own fookin' arse with both hands and a mirror ;-)

...Jim Thompson

Don't do that. As Confucius say, "It is better to get laid than to solve Algebra problem and appear smart-ass to date." ;-)

...Jim Thompson

And WTF? The equation for E is not a claim, it's a fact. The last 3 sentences above are so indicative of flawed logic ( or trolling ) that they defy comment.

If I s*it a pound per day and grow s*it^2 pounds of tomatoes a year for s*it^3 kcal of energy... I'm s*****ng linearly, growing quadratically, and consuming cubically. I s*it you not, larry. You're my favorite turd this week.

M_r = mass of rocket and fuel, pre-burn V_r = velocity of rocket and fuel, pre-burn m_r = mass of rocket, post burn. v_r = velocity of rocket, post burn v_f = velocity of ejected fuel, post burn m_f = mass of ejected fuel, post burn

M_r * V_r = m_r * v_r + m_f * v_f

A stumped physics teacher does not a smart larry make.

Mendacity... good word, but it's flawed logic at best. See my reply to the beginning of the broken thread that not surprizingly, was detected by my script as a reply 'cause the headers and x-refs retained integrity.

My guess would be to set up a partial diff eq. of f( m(t),v(t) )

Oh, I see another flaw I must include.

Rockets travel well below the speed of light, last I checked. So the above is irrelevant to the discussion.

It is, but it looks more like you were trying to make your physics teacher look dumb and you smart by posing a question you couldn't answer. Hey. I stumped the algebra teacher across the street by mentioning "too many variables" in a given prob. She gave up and agreed. If I'd have kept my mouth shut, she might have looked at the prob differently. I posted to the "Twist You Noodle" forum and someone pulled my head out of my ass for me.

I'm not a bit puzzled. The flawed logic is the assumption that a linear decrease in fuel mass is contrary to a quadratic increase in kinetic energy. And that *instantaneous E" or better yet

| E = .5 * m * V^2 | |m=k

variable mass is another story.

And to hint that it breaks E = .5*m*v^2 is incorrect.

And now I see the mandacity that Jim alluded to. E does not vary quadratically with time, it varies quadratically with velocity :P

I didn't miss that. I've seen a few threadss over the past days that indicate that you're trying to prove you're smart. If nothing, I've learned a bit about how not to post. I'll try.

I have to get showered for a date. Maybe I'll look at the seb thread while I dry. You can turn the clock off, because I won't be researching this subject. I'll let you know when I have to run to the books.

Eh, t^2 does still show up, so maybe the other gotcha is in the assumption of a quadratic vs linear relationship. I'll try it on my napkin when she goes to powder her ... whatever.

It was offered only as evidence to refute the assertion: "The equation for E is not a claim, it's a fact" As I recall, that recently became part of the discussion.

You assume facts not in evidence. What leads you to believe this happened in front of a classroom of kids? As a matter of fact, it occured after school during one of many half-hour sessions he and I spent. How could a teenager expect that his teacher would not be able to answer such a question? What evidence supports the allegation that it was anything but a curious kid asking an expert a question that puzzled him?

....

Looking for more of your favorites, were you?

Yes, that is the key. There is an implicit (but incorrect) reliance on the notion of energy conservation, while paying attention to only 2 of the 3 players. The burnt fuel and the rocket are visible, the exhaust is invisible. (If you ever do this on a whiteboard for someone else's puzzlement, be sure to not draw an exhaust plume!)

And way beyond anybody's attention if actually dealt with.

It could hardly be a puzzle without that dilemma.

Under the stated assumptions of the problem, "accellerating at a constant rate", the velocity varies linearly with time. Therefore the kinetic energy varies quadratically with time and velocity. I fail to see how mendacity enters into this.

Let's assume that the amount of fuel burned at the end of the run is insignificant when compared with the total weight of the rocket. Then we don't have to worry about mass differentials during the run. Let's further assume that all the fuel is converted into thrust with no heat, so we don't have to worry about energy diversions.

First I will explain why the rocket is NOT burning fuel at a constant rate to produce constant thrust. And if that is true, it follows that fuel consumed does not increase linearly with time.

Let's assume at takeoff, acceleration is 2 meter/sec² and an initial velocity of zero. Within 1 second and a distance of 1 meter, enough fuel has to be burned to increase the velocity to 2 meter/sec. Now it clears the gantry at a velocity of 10 meters/sec. If the velocity stayed constant, it would go 1 meter in a 1/10 of a sec. But we assume constant acceleration, so it has less than 1/10 of a sec to increase its velocity. It clears the clouds at a velocity of 100 meter/sec. Now it has less than 1/100 of a sec to increase its velocity within 1 meter. As you can see, due to its higher speed, the rocket has less time to increase its velocity with 1 meter, which because of its constant acceleration it must do. So what happens? It has to burn up its fuel faster and faster to create more thrust, so it can beat the deadline of a higher velocity with the 1 meter measuring reference. That is why it takes 3 times the energy to increase a velocity from 50 to

This NG is a type of classroom. Regardles of who was present then, it looked like you wanted to prove something to us. Forget it. You'll have plenty of opportunities to reveal yourself one way or the other.

How about the admission that you were being your typical smart-assed self?

The exhaust *is* the burnt fuel. Same fuel mass (if oxidized from an onboard source), different place and chemical composition.

When I get to it on paper, I doubt there will be a dilemma.

If Jim really meant to say "mendacity", I'll find something. Otherwise, he meant "trick question, and I'll still find something. My reply to myself indicates that I realized the error of my statement while I was in the shower. I like noodle twisters.

My usual funny d*****ad approach served me well enough for a first date. We hit it off well. I thought she'd be a bore or closed minded, but she's more of a free thinker than I expected.

I slipped the napkin thing in undetected, but by the time I got the waitress's pen, I only had time to write what I'd already done in my head. For my next act, I'll teach her to empty ashtrays and clear off the empty sugar pouches as if grabbing clean ash trays from the next table weren't hint enough :( I think she learned not to spill coffee on me, though :)

This Brasfield has to be the most ostentatious pseudo-intellectual on USENET- and gets his ass handed to him all the time. The mental midget jumps from one parameter to another , related by some means unknown to him, and suggests it constitutes a conundrum. This is such puerile garbage- and only a pseudo-intellectual could consider it an event. And do you notice how pseudo-intellectuals just love arguing fundamentals, definitions, and semantics. Brasfield is true USENET garbage.

when the mass is changing (fuel depletion). This seems

Think of it as an engineering approximation. For an atomic powered, ion rocket engine, it can be a very close approximation.

The reason to make the approximation is to keep the math simple. The error is not relevant to the puzzle's apparent dilemma, so the extra math would be a distraction.

I do not understand how acceleration can be constant due to constant thrust when the mass is changing (fuel depletion). This seems to me to be a contradiction.

John

That's a good candidate for the "mendacity hidden in his problem description." that JT warned me about.

when the mass is changing (fuel depletion). This seems

Oh. They had ion engines way back then?