CASTING STONES

Momentum, like calculated risk, is something politicians like to refer to; and as with other words that they use, they have not the faintest inkling of what they are talking about.

But in the nonpolitical world, momentum is a very simple concept: it is the mass of a body (in pounds, say) multiplied by its velocity. Apart from its size, a momentum also has a direction: the direction of the velocity.

Now it follows very quickly from Newton's Laws (which this year celebrate their 300^th birthday, if we take them to be born by the publication of the Principia in 1686) that in a closed system the sum of all momenta is constant, and I can hear hundreds of readers complaining that it's over their heads again.

So let me simplify it very ruthlessly. What it amounts to is that when the man on the cart throws a rock forward (see figure), the cart moves backward: the forward momentum of the rock is canceled by the backward momentum of the cart (plus man). The two add up to zero just as before anything happened. Note the equality of the products: the small mass of the rock times its respectable velocity equals a large cart mass times its very low velocity.

However, this system is not closed: there is an external force, that of friction (itself due to gravity), so the cart will soon come to a standstill. To keep going, the man has to have a pile of rocks on the cart, and that will only give it a bigger mass and slow it down. So instead of throwing a rock, he shoots a bullet: its high velocity will give it a larger momentum, and the cart will compensate by an equally large momentum: it will experience "recoil". But not for long, so the man will have to shoot out of a machinegun to keep going.

But again not for long, because he will run out of bullets (or if he stores very many, the cart will be bogged down by its weight). There is, however, a more efficient way of using the machinegun principle: what gets shot out is not bullets, but molecules of a gas. A compressor (marked C in the second part of the figure) will take in a large volume of gas, compress it and force it through a small opening at a very high velocity. The stream has a high velocity; its mass is small, but then so is that of the fuel naming the compressor, allowing the cart to compensate with a higher velocity. The man has progressed from throwing rocks to the jet engine.

But who needs a compressor? The fuel itself can burn, expanding at high pressure with a high velocity through a nozzle, spewing gas molecules instead of throwing rocks, but still working on the recoil principle; that's the essence of a rocket engine.

And what can heat hydrogen to 2000 degrees C without a single molecule of oxygen, just heating it without burning it?

Ah, now you've got it. Go nukes, go!