For Relax Max: Terminal Velocity

Relax Max continued: If I may have two questions, the second is about terminal velocity. If a large object (a passenger airliner, say) reaches terminal velocity before impact, do smaller (or more aerodynamic) objects whose terminal velocity are higher (I assume) "pass" the airliner on the way down? Or does everything hit the ground at the same time?

Good question(s). As you asked, items don't have the same terminal velocity. The maximum speed an item attains in air depends on many different factors. Surface area, fluid factors, and mass are key factors, but other factors include roughness, shape, initial speed, etc. A man falling, for instance, can change his terminal velocity drastically between falling spread eagle and pulling his limbs in and falling headfirst (as skydivers do to move up and down relative to each other). A man can't change it enough not to be going too fast for landing without a parachute, but that's a different post.

Terminal velocity is effectively the speed an object obtains when the force of gravity is canceled by the opposite drag on an object so that it stops accelerating. If gravity were not involved, there'd be no terminal velocity because drag would just work to make things go slower with no counterforce. (Newtonian physics I can explain if you'd like). Initial velocity makes a difference because it adds a factor beyond gravity (and velocity has an effect on drag).

A debris field is determined by multiple factors as well: initial speed, what caused the initial breakup, and how much and what kind of debris is generated. A biplane, for instance, that lost it's rudder might have a very limited debris field, where as Columbia's debris field extended over several states. Explosions (whether combustive or pressure built) send debris forward and backward, extending the debris field. Flat low mass debris will fall slower that compact debris.

So, to answer the last, everything doesn't land at the same time - except in a vacuum as they demonstrated on Apollo 15.

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For Boris: What Would I Do?

Boris asked: If you had to choose your line of work again, would it still be the same, i.e. awesome rocketry with NASA (personal bias showing through)? What would be your second choice?

Good question, Boris. It should be one I can answer in a few short sentences. But I can't.

It's an excellent question because I'm not sure how much I "chose" this one. See, I am a rare beastie, an accidental engineer. I'd never had the slightest interest or inclination to do anything math related. Oh, science and math were fun and I enjoyed it, but only about as much as I enjoyed history and literature. I always intended to be a writer/novelist. The only thing in question was what would I do for a day job while I waited to get my book(s) written and sold. Stephanie likes eating.

I could do math, of course, in fact, it was so easy for me it was boring to me, which is likely one reason I never intended to pursue it. I think I'd always assumed I'd do foreign languages (which I enjoy great deal) or journalism or genetic engineering. I know, weird choices. Except, journalism, in high school, bored me silly. Writing the "truth" didn't hold any interest for me compared to fiction and I wasn't sure how to make money with foreign languages short of teaching. And genetic engineering wasn't offered at any of my immediate college choices.

But I wasn't set or picky. After all, I was going to write. So, since I was bright (and had excellent SAT and ACT scores - types of proficiency exams), I started applying for scholarships, a lot of them, anything. I figured wherever I had scholarships, I'd pursue.

Which is how I ended up with Engineering Physics - because I could get a scholarship from the Engineering department and a scholarship from the physics department. And, once I started in what I soon discovered was the "hardest" major on campus, I was too stubborn to get out. I wasn't going to let it beat me. I found my job with the same sort of muddling. I wanted to live somewhere "warm", didn't particularly want to do defense or quality assurance and definitely not petroleum. That left NASA.

Why did I tell you all that? Because, knowing what I know now and having an opportunity to get any major I wanted and work wherever I wanted, I think I'd do the same thing. I was exposed to all the science and every major type of engineering, and I like what I ended up with better than anything else. It's more real and practical that straight physics, more predictable than biology, smells better than chemistry, yet it's also far more versatile than any straight engineering field. But it also incorporates bits and pieces of all of it.

I used to think I would have been better off working for NASA directly (which I missed because I'd already accepted an offer with a contractor). Now, I'm glad that my background is more diverse (bioengineering, human factors, environmental science, robotics, calibration, safety, EVA, etc. etc.). And I love where I work now, more freedom, more variety.

So, honestly, if I could do it all over again, I probably wouldn't have changed anything. I wonder sometimes about genetic engineering. But I'm not unhappy, even with some of the frustrations I've run into along the way. I believe in what I do and believe I've made a difference.

So, call that I'd choose the same, with genetic engineering as a second choice.

However, I do have some regrets. I wish I'd pursued some singing training. I wish I'd pursued languages more so I was actually fluent. And I wish I'd pursued the writing itself more assiduously.

But, all in all, I'm good.

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For Jeff King: Boiling Over

Jeff King asked: What makes water Boil? I know heat does, but what takes place to make the water react so violently on the surface?

This one's rather easy. Each material has three states: solid, liquid and gaseous. Generally, temperature is the key factor in which state a substance is at any one time: get it cold enough and anything will turn solid, get it warm enough and it will turn gaseous. Since the freeze point and boiling point vary from substance to substance, some are gaseous at "room temperature" (like oxygen), some are solid (like iron) and some are liquid (like water).

When you put heat into a substance, assuming you keep putting heat in indefinitely, it will eventually change state, but it take additional energy to do so. For instance, if you take an ingot of iron and heat it, it will increase in temperature proportional to its thermal properties and the heat put into the system. However, at melting point, it will go through a space where the temperature will not increase though heat continues to enter. This is is called "latent heat" and it's the heat required to change phases. Melting will occur only after this heat has been absorbed at which point the iron will melt and, again, increase temperature.

The same thing happens when one boils water. First the water must be brought to boiling temperature (100 deg C/212 deg F at sea level) and then more heat put into the system before the water changes state from liquid to gas. It is the transition from liquid to gas that causes the violence. Bubbles of gas are formed throughout the water (though likeliest at the bottom where the heat is generally coming in) and, as they are less dense than the liquid, the float to the top and are released at the surface. That surface violence is the result. Boiling water is, in theory, always exactly boiling temperature because all the heat put into it is used for phase change until you run out of water.

Contaminants can change boiling/freezing point (as salt does for water) as can air pressure. In vacuum, water will boil without additional heat. At low pressures, also, the liquid state can be bypassed entirely, a process called sublimation, where the gas is pulled off directly from the solid state.

Hope that answers your question.

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