Category Archives: Science And Society

Really Bad Timing

For my move to Florida, if this article is correct.

Scientists say we are in a period of enhanced hurricane activity that could last for decades, ending a 24-year period of below average activity. They also say the law of averages has caught up with Florida, with a change in atmospheric steering currents turning the state into a hurricane magnet.


Ivan probably won’t be the last storm to have us in its boresight this year.

It makes me start to wonder how big, or how many nukes it would take to disrupt these damned things, or if that’s even feasible (ignoring, of course, the radiation issues)?

[Update a minute or so later]

As if they didn’t have enough to deal with, with a Category 5 hurricane bearing down on them, the Caymans and Jamaica just had a Richter 6 earthquake.

I, of course, blame George Bush.

Rogue Waves

ESA (the European one, not the Elbonian one) has some satellite data that validates sailors’ reports of
ship-killing waves.

Mariners who survived similar encounters have had remarkable stories to tell. In February 1995 the cruiser liner Queen Elizabeth II met a 29-metre high rogue wave during a hurricane in the North Atlantic that Captain Ronald Warwick described as “a great wall of water

Uncertainty, Global Warming, and public policy

Another item in the latest Industrial Physicist is a piece on understanding the uncertainties in global warming models, and the public policy implications of those uncertainties. It’s well worth a read if you care about global warming in particular or science and public policy in general.

One of the hardest things about ensuring that public policy is based on sound science is that sound science inherently involves uncertainties. Politicians like yes or no answers, but science only gives really reliable answers in the very long term, far longer than the relevant political timescales. In order to make policy based on sound science, politicians have to take uncertainty into account, and allow for the possibility that the policies may need to be adjusted as new information becomes available.


Ron Reagan (who wouldn’t have this platform if his last name wasn’t Reagan) just made a speech in which one would never know that embryonic research is perfectly legal in this country. I was also struck by this sophistry this morning listening to NPR, when they talked about “restrictions” on such research under the Bush Administration. I don’t agree with the President’s policy, but this is no more “restricting” such research than not funding artists by the NEA is “censorship.”

The policy is that no federal funds will go to such research, not that it is forbidden. But if they told the truth about that, they probably wouldn’t get the political pull that they hope to, and overthrow the evil Bush administration, that ostensibly forbids research that might have saved Ron’s dad (not).

I Am Quite Disturbed

…at the thought that commenter “Brian” from this post teaches undergraduates.

Scroll down a ways, and be amazed.

[Update a few minutes later]

I should add, that there’s another howler there:

Regarding Newton’s second law of motion, F=ma is just fine for all physics short of things traveling greater than 0.95 the speed of light, or quantum effects.

He’s apparently confused, thinking that I’m referring to Einstein’s Special Relativity version of Newton’s Second Law, in which rest mass is converted to true mass via the factor gamma, which is a function of velocity, or F = dp/dt where p = m*v, or in the Einsteinian version, p = gamma*m*v.

Gamma is a function of velocity. It’s 1/(1-v^2/c^2)^1/2 (or in words, it’s the inverse of the square root of one minus the ratio of velocity squared over the speed of light squared). For low velocities, it’s one divided by the square root of one minus a tiny number, or simply one, so at low velocities, mass equals mass. But for high velocities, you’re starting to divide one by a very tiny number (as the difference between 1 and velocity squared over c squared becomes infinitesimal), so gamma blows up to be a huge number. That’s why mass approaches infinity as its speed approaches that of light.

As I pointed out in the other thread, in the Newtonian case it is simple to take the derivative:

F = dp/dt = d(mv)/dt = m*dv/dt + v*dm/dt. But dv/dt is acceleration, so we get:

F = ma + v*dm/dt.

The Einsteinian case is a much more complicated derivative, because it’s a much more complicated function of velocity. But it’s not relevant, since we’re not talking about near-light speeds. The fact remains that Newton’s Second Law is F = ma + v*dm/dt. The only reason that we always see it as the more simple (and incorrect) F = ma, is that this is a special case in which the mass is constant (the derivative of a constant is zero, and the second term goes away). This is the case for most physics problems, but it certainly isn’t for rocketry, in which the vehicle is ejecting mass (that’s what makes it go).

Anyway, as I said, it’s very disturbing that this person is teaching anyone, let alone undergrads.

[Update at noon Eastern]

Professor Hall, who does teach undergrads as well as grads (and I’m glad of it), expands on his comment via email:

I think Brian’s a bit of a putz in his comments. However, he’s right about F=ma and F=dp/dt. Derivation of the rocket equation is a little tricky to work out, as you say. However, the chain rule does not lead to the correct equation.

In the 2nd law,

F = dp/dt

F is the sum of all applied forces, and p=mv is the linear momentum of the particle of mass m.

If you apply the chain rule to this equation, you get

F = m dv/dt + v dm/dt

as you noted.

However, in order for the chain rule to make any sense here, the two v’s must be the same v. What v is it?

If it’s the velocity of the particle, then this equation can’t apply to a rocket, since it couldn’t lift off the ground. On the ground, v is zero, and initially dv/dt is zero, so F is zero. If F is zero, the linear momentum cannot change, so v remains zero.

If it’s the velocity of the mass leaving the rocket, then initially F = v*dm/dt, which is essentially correct. However, the v in the dv/dt term is clearly not the velocity of the mass leaving the rocket. It’s supposed to be the velocity of the rocket.

The correct derivation of the rocket thrust equation uses a control volume approach, which is essentially a summation of Newton’s 2nd law over a continuum of particles of different velocities (the rocket and the propellant clearly have different velocities).

This leads to the following vector equation for rocket motion

F + ve dm/dt – vehat A (Pe-Pa) = ma

The terms on the left comprise the sum of all external forces acting on the

F includes all the forces such as gravity, drag, ….

The term ve dm/dt is the thrust due to the rocket, where ve is the exhaust velocity and dm/dt is the mass flow rate (negative number, since m is the mass of the rocket, which is decreasing). The vehat A (Pe-Pa) term is the pressure force. Vehat is a unit vector in the direction of ve, Pe is the exhaust pressure, and Pa is the atmospheric pressure.

I’ll just add that a) I’m glad that at least some of my readers and commenters are smarter than me and b) while I didn’t say that the chain rule led to the rocket equation, I did imply it, and that was a mistake, and c) I had known that at one time, but it’s been a long time.

And we are in agreement that Brian is a putz.

Suspending Too Much Disbelief

John Derbyshire, contrarian that he is, didn’t like Spiderman II.

Even comic-book movies must obey certain unities. In the realm of science fiction — and c/b movies are a species, even if a low one, of science fiction — the golden rule is: You can have one highly implausible bit of science. The rest of the science should be sound, or at least should follow logically from the central implausibility. THE TIME MACHINE is a great sci-fi novel because, once you have granted the central, fairly preposterous, premise that time travel is possible, everything else is just basic Darwinism and stellar evolution, as it was understood at the time.

The central notion in SPIDERMAN is that if you get bitten by a spider whose genes have been messed about with in a certain way, you will develop the ability to shoot 100-ft silk threads from your wrists (without, apparently, any loss of body mass). This is preposterous — though not at a sensationally high level, as spider genes can be messed around with in an infinity of ways, and we don’t actually know what would happen if you were bitten by a spider whose genes had been messed around with in way No. 29,485,672.

Having been persuaded to suspend our disbelief with respect to Spidey’s powers, we should not then be asked to swallow any more preposterosities. And we know perfectly well what whould happen if you dumped a fusion reaction into the East River — ka-BOOM.

I haven’t seen the movie yet, but I intend to, and won’t let this curmudgeonly review put me off of it, though I actually agree with the principle. That was one of the things that bothered me about the first movie. Once you tell me he’s been bitten by a radioactive spider, then fine, I’ll buy the superpowers on the part of Spidey. I’ll even accept the notion that, as Derbyshire points out, he doesn’t have to conserve mass.

But Mary Jane has no superpowers, yet she performs a superfeat near the end of the movie, when she falls off the cable that’s being flung around (face it, she wouldn’t have been able to hang on to it that long without her arms being torn off), and then catches the side of the cable car as she falls some distance toward it.

Sorry, just Not.Gonna.Happen. It defies physics and the strength, both muscular and structural, of a normal human body, even one pumped on adrenalin. I enjoyed the movie up to that point, but that bit really bugged me, because there was no good reason for it–it could have been just as exciting while being realistic.

And of course, there’s the other thing that bothered me about the movie–the ending.

Parker was under no obligation to keep Harry in the dark about his father’s end. Just because he was requested to, he didn’t agree to the request, and he did himself and Harry a disservice by allowing Harry to continue to live on in a fantasy world about his father’s true nature, a world that’s likely to cause him to attempt to kill Parker’s alter ego (and hence Parker) in the future.

At a minimum, he should have at least pointed out to Harry that the fact that Spiderman returned his father’s body to his home didn’t mean that Spiderman was the killer. He might not have accepted it, but there would have been no harm in exercising a little logic on him, even if he wanted to spare him the knowledge that his father was a murderer (though again, I think that was no favor).

Also, he’s not protecting MJ by not reciprocating her love. The key is to keep his identity a secret (though not from her). I found it highly unsatisfactory, but apparently it was more important to them to set up some dubious sequel plot than to employ logic, or ethics.

I guess that SF movies will never get made right until they hire me as a script advisor. And listen.

[Update on Tuesday]

For those endlessly or otherwise fascinated by bad movie physics, check out this site (including a review of Spidey I). It says The Core (which I haven’t seen, and probably won’t) takes the prize for the worst movie ever in this regard.

Black Irish?

Archaelogists have discovered evidence that some people in the borderlands of England may be descended from Moors from northern Africa, Roman soldiers brought over to guard Hadrian’s wall. One more interesting ingredient to the mix of Picts, Saxons et al.

Also a little ironic, considering that many of these people are the so-called Scots-Irish who settled much of Appalachia and the American south, with its slavery and anti-miscegany laws.