"Get a Straight Answer" Site Map

Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.

### 114.   Why not use a heat shield going up?

The other day a colleague asked me a question that I could not answer and which keeps intriguing me. "We know that (the shield of) a spacecraft that re-enters the earth's atmosphere heats up spectacularly because it is hit and slowed down by air-molecules. Is there a similar problem when it is going the other way, at take off? It is going through the same amount of air and acceleration looks quite similar to deceleration, only the other way around." The only answer that I could think of is that a good part of the acceleration might take place at higher altitudes where there is less air, and braking is done only by using the atmosphere.

The reason you suggested yourself is pretty much what happens.

Going up, it is the rocket engine which provides acceleration and energy. Because air resistance robs energy and is undesirable, the rocket deliberately rises vertically, to go though the denser atmosphere as quickly as possible. The vehicle gets most of its velocity, and almost all of the kinetic energy, at high altitudes where air density is too low to make a great difference. With the space shuttle "Columbia," even that might not have been enough: by the time it reached twice the velocity of sound, the atmosphere around it was still dense enough to rip a piece of foam insulation off its fuel tank, and it hit the orbiter with great force, breaking the heat shield.

On coming down (with spacecraft which we want to come down undamaged), the atmosphere is the brake absorbing the energy. We need that air resistance, and the heating is a result of absorbed energy! The big concern that energy should not be absorbed too fast, otherwise the heat gets too intense and may melt the heat shield and everything else. That is why the space shuttle comes down at a low angle, trying to stay as long as possible in a layer with the right density: If the shuttle comes in too high, not enough energy is lost, if too low, too much. The density is also important in supporting the shuttle, which--like a kite--needs part of the resistance to help it from coming down too fast.

A final note: heat shields get very hot, but most of the energy, almost all of it, is given to the shock which forms ahead of the heat shield. It thus heats the air, not the re-entering vehicle.

### 117.   Why not use nuclear power for spaceflight?

Why not use nuclear energy to power spaceflight? After all, few pounds of plutonium contain as much energy as thousands of tons of rocket fuel!

Nice idea. However, to fly in space takes rocket thrust, not just energy.

By Newton's laws, the forward momentum given to any rocket is always equal to the backward momentum given to the jet fired backwards. That momentum, in its turn, depends on two factors--how much mass is expelled by the jet, how many tons per second, and the speed with which it is expelled. Nuclear energy can supply the speed, but something must provide the expelled mass.

You might think next that given some source of mass (say, a tank filled with water), plentiful nuclear energy would make it possible to eject it much faster. But how? Rocket engines work by converting heat into directed motion, in a very efficient way, but they already run about as hot as available materials can stand. Nuclear energy could provide more heat, but no rocket engine could stand it.

Early in the space age a serious effort existed to build a nuclear rocket, getting its thrust by heating hydrogen with nuclear fission. A jet of hydrogen, coming from a rocket engine at a certain temperature, is much faster than a jet of burned rocket fuel, coming from a rocket engine at the same temperature. The reason is linked to the fact that hydrogen molecules are much lighter than those of any burned fuel.

However, the rate at which rocket engines used in spaceflight supply energy is enormous--e.g. the shuttle's engines burn a ton of fuel or more each second. The stresses are enormous, and the risk of nuclear material and waste products of fission getting into the atmosphere was too great, and so the project ended.

A visionary proposal of the 1950s proposed a "rocket" cabin with a strong flat plate on the bottom (oil would be sprayed on it for protection), and a trapdoor through which small nuclear bombs could be dropped, detonating some distance away and pushing the craft forward. On paper, it seemed feasible, but an actual nuclear test was deemed hazardous, sure to release contamination. The nuclear test-ban treaty of 1963 ended all efforts in this direction.
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I should add here that a book has recently appeared about "Project Orion", by (what I take as) the son of Freeman Dyson, prime mover in that project: Project Orion: The True Story of the Atomic Spaceship George B. Dyson, George Dyson, David Sobel (Editor)

### 119.   Have any changes been observed on the Moon?

I happened to wonder if anyone has looked at the moon in the last 100 years or so and noticed a crater that 'wasn't there yesterday'. How many new craters have been observed and how big are they? That could kind of say things about safety HERE! We do have frequent meteorites, after all. I have even seen one myself

I once had an office on the same floor as a lady scientist, Winnifred Cameron, who very much wanted to find such changes. She used a special viewing device looking at two pictures of a region on the Moon, taken under similar conditions but at different times, flipping from one to the other and looking to see if anything changed. I don't think anything ever did. She was particularly interested in observations of a Russian named Kozyrev, who claimed to see glows.

As for impacts, it is only possible to see pretty big ones. If meteorite impacts are your interest, read the chapter "The Shoemaker Comets" in "First Light" by Richard Preston. It's a great book. Gene Shoemaker is unfortunately gone from us, killed in a head-on collision while rounding a blind curve in Australia's outback. Those roads are usually completely empty, but you never know fate.

### 120.   Why isn't our atmosphere flung off by the Earth's rotation?

I have wondered for years how the earth keeps our atmosphere. The equator moves at almost 1000 MPH and the atmosphere is fluid. The fact that there isn't any wind (to speak of, at least resulting from the earth's rotation) says that the attraction of gravity is stronger than the centrifugal force trying to throw it off. Do we know if the amount of atmosphere is increasing, decreasing or remaining the same? It just seems to me that there should be a lot of turbulence in the atmosphere/space boundary region, although the 'emptiness' of space probably can't provide any drag on the atmosphere.

Concerning the atmosphere... the centrifugal force on the Earth's equator is just a fraction of 1% of gravity; it makes the Earth slightly oval, but nothing falls off. The effect was found in the 1600s, when pendulum clocks accurate in Europe slowed down near the equator. Jupiter is bigger and rotates faster--so its equatorial flattening is larger, large enough to be evident in photos through the telescope.

If you went around Earth at orbital velocity--one circuit in 90 minutes--the centrifugal force would just balance gravity. Our rotation speed (one circuit in about 24 hours) is nowhere near that.

### 121.   Can kinetic energy be reconverted to work?

and have a question. Is kinetic energy available to do work later?

It depends. Kinetic energy is all too easily converted to heat by friction, and if this is allowed to happen, that energy is rarely recoverable. However, if you can convert it to another form, you can extract at least some useful work from it (there is always some friction loss). Examples:

1. You zoom on your bicycle down a valley and gain kinetic energy. That energy can help you rise again on the upslope on the other side. Rising against gravity is doing work.

2. Your car has to stop at a red traffic light. If it is an ordinary car, you press the brake pedal, which pushes brake pads against wheel disks or drums. Friction converts the kinetic energy into heating of the disks or drums, but you can hardly convert that to work.

If however your car is of the new "hybrid" type with electric motors on the wheels (like the Toyota "Prius" or the Honda hybrid), by braking you connect the motors to the car's batteries. The motors act as generators and charge the batteries, turning your kinetic energy into chemical energy of the battery, which can be reused.

3. The space shuttle in orbit has a tremendous kinetic energy. Upon re-entry, its orbit is nudged into the atmosphere, the shuttle turns so the underside of its wings (covered with heat tiles) faces forward, and air resistance, helped by a shock, converts the energy into heating the air. Not much can be done with that.

If however you link the shuttle to a conducting tether, as was done once (with some problems), the motion of the tether across the Earth's magnetic field lines creates a voltage, which may be tapped, e.g. for charging batteries. See http://www.phy6.org/Education/wtether.html.

4. Accelerators of high-energy particles used to depend on pulsed magnets. I remember visiting one such accelerator around 1960, the "Cosmotron" in Brookhaven, Long Island. Its magnet needed a large electric current, which created a great amount of magnetic energy. It made little sense to waste that energy every time the magnetic field was allowed to decay.

The solution was a large flywheel, connected to the generator providing the current. When the current decreased, the generator acted as a motor (a bit like that of the hybrid car) and spun up a flywheel weighing a few tons. The next cycle, the flywheel provided most of the energy for generating the magnet's current, slowing down again; only a little extra power was needed to make good friction losses. Thus the energy bounced back and forth between magnetic and kinetic.

Someone pointed out to me that the way the flywheel rotated was carefully chosen. In case the flywheel's bearings somehow gave way, its rotation was such that it would roll through the wall of the building and out into the field--not in the opposite direction, which would have brought it into the crowded accelerator hall.

I hope you get the idea by now.

### 123.   Speed of toy car rolling off an inclined ramp

This is my first time doing this. I am eleven years old and I have a science project that I need some help on. My dad built me a ramp for model cars. I want to prove that the speed of a car is determined by the weight of a car. I think that the lighter the car the faster it will go down the ramp. How do I prove this? Is there a formula? Please help me.

You are up against something very fundamental, something I hope you will remember in high school, when you study physics.

Every object has a WEIGHT, the force by which gravity pulls it down. A big stone has more weight than a small one. Weight is one way of measuring the amount of material in the stone, or its "mass." A big stone has much more mass than a small one.

However, if you drop them together, you will find that they fall equally fast!! This is, because each object also resists motion, and the resistance (called "inertia") is ALSO proportional to mass. That is why on a horizontal surface it takes much more force to get a bowling ball rolling than a tennis ball. The motion is horizontal, gravity is not too much involved, but the bowling ball has more mass and therefore much more resistance to being set in motion.

Say the big stone has 10 times the mass of the small one. It also has 10 times the inertia, and that inertia does not allow it to move any faster, even when the Earth pulls it down 10 times as strongly.

Model cars on a ramp (I suppose they are moved by gravity, like soap-box racers) obey the same rules. A heavy car is pulled more strongly, but also has more inertia, so the two should roll at the same speed. Try it! Put two toy cars--big and small--together on a slanting board, and let go. All other things being equal, they should move together.

To make the car roll faster, the only thing you can do is reduce friction (and at very high speed, air resistance). A car with well-oiled axles may move faster. Try it!

### 124.   Acceleration due to gravity

I am a high school physics student. My class was given a bonus assignment for the internet and I have yet to find an answer. I was wondering if you could help. I know that the acceleration due to gravity on the earth is ~ 9.8 m/sec^2, but the class was asked to find a website that listed values of acceleration due to gravity at different locations on the Earth including acceleration due to gravity at our high school, Clarion-Limestone High School in Strattanville, PA. My question is:

What are values of acceleration due to gravity at different locations on Earth and what is the value closest to my school?

If you could help me out because I am really interested in finding out the answer, I would be greatly appreciative.

Have you just tried to ask Google or Yahoo for links concerned with "Acceleration of Free Fall"? I did so and got many leads

I am not really supposed to do your work--but look at http://www.haverford.edu/educ/knight-booklet/accelarator.htm Actually, the interesting questions are not "what is the number" but "why does gravity vary from place to place?" and "how do we know?"

It varies because the Earth rotates, adding a centrifugal force to the forces felt locally. That does two things: it makes the Earth bulge at the equator, so that points there are more distant from the center of Earth. And it adds there a force opposing gravity, so that the acceleration is smaller. Newton proposed that around 1690.

The process was experimentally studied by comparing the time kept by pendulum clocks ("grandfather clocks") at different locations. The period of the pendulum depends on gravity, and a clock which keeps correct time in Pennsylvania will probably run slow at the equator.

### Response

Thank you for the additional information on gravity. I actually did look at Google acceleration due to gravity but I guess I didn't look fully enough.

You actually were a great help for me. You explained the subject better than my teacher. I hope you continue this because physics and other sciences really interest me.

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Author and Curator:   Dr. David P. Stern
Mail to Dr.Stern:   audavstern("at" symbol)erols.com .

Last updated 9-17-2004