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Space Physics:
Physics in Everyday Objects and Activities

Moving Around in Space

If you are free-floating in the middle of space with no gravitational forces nearby, is it possible for you to turn around by moving any body parts (like hitting yourself in the shoulder or swinging your arms)?

Remember that in order to move your body, you must be acted on by an external force (a body at rest remains at rest, according to Newton's Law). If you are stationary in space with nothing around you to push off on, you cannot move yourself by pushing on yourself. But you can twist your body around its center of gravity - you just can't move your center of gravity. On Earth, we can walk forward because we push on the Earth and it pushes back on us.

So the answer to your question is yes, you can turn yourself around, but you can't move from where you are.

Dr. Louis Barbier
(February 2003)


Detecting Motion in Space

Can motion be detected in space? If you're in a box in space and it's spinning, would you be able to tell that it's moving? What if the box moved in a straight line?

This question is at the heart of Einstein's Theory of Relativity. Einstein must have asked himself similar questions.

First, it is impossible for an observer in a closed box (nothing coming in from outside) to determine the motion of the box if it is moving at constant speed in a straight line. The laws of physics are the same in all such boxes moving at any speed. This constitutes the foundation of Einstein's Special Theory of Relativity. Consequences of this include the constancy of the speed of light.

A spinning box is more difficult. The speed is not constant and there are consequences of acceleration.

If the closed box is not spinning but is simply accelerating uniformly (the speed is changing at a constant rate in a straight line), an observer in the box would be unable to determine whether the box is accelerating or whether it is subject to a uniform gravitation (such as on the surface of the Earth, neglecting the rotation of the Earth). This is the basis of Einstein's General Theory of Relativity.

A spinning box is more complicated. It is accelerating, but the acceleration is different at different places in the box. The result is not simple and I am unable to answer it here. I believe that the spinning motion could be detected by experiments inside the box.

You might want to read the PBS Nova web page.

Dr. Randy Jokipii
(February 2003)


Inflating a Balloon in Space

Is it possible to blow up a balloon in space? I would like to know more about the gas laws in space.

Yes, it is easy to blow up a balloon in space. The way you inflate a balloon is to put a larger pressure inside the balloon than outside, which is easy to do when the outside pressure is near zero. The balloon will continue to expand until one of two things happens.

1. The force due to the outside pressure (nearly zero in space), plus the stress force of the balloon material (the force with which the balloon pushes in on the gas inside), equals the pressure force of the inside gas.
   
   or
   
   
2. You exceed the stress capacity of the material and the balloon pops.

So all you need is a material that is strong enough to hold all the internal pressure. Space capsules and spacesuits are essentially balloons of gas. They don't expand to the extent a rubber balloon does, but all of the forces (and some expansion) are present.

Dr. Eric Christian

Yo-Yo in Space

How would a yo-yo work in space? Would it come back to you, or not? I think it would come back -- doesn't everything have gravity?

You are raising two questions. First, you ponder whether everything in space has gravity. Second, you ask whether a yo-yo would work in space.

Let's first tackle gravity. It is correct that everything that has mass also has gravity, i.e. any two objects with mass will attract each other. However, for two small objects, such as a yo-yo and a human (small compared to the Earth), this force is so tiny that we have a hard time measuring it with even our most sensitive instruments. You and the yo-yo will not attract each other, at least not enough that you could see the effect. If you are out in a spaceship away from large masses, like planets or moons (actually much farther away than most of our satellites), you feel no acceleration from gravity. Because of the spaceship and all the masses in it there IS still gravity in space, but the force is so weak there, you don't feel its effects; it's called microgravity.

One last note: the astronauts in the Shuttle and on the Space Station don't feel that gravity presses them against the floor either, although they are still close to the surface of the Earth. They and their space vehicle are constantly falling while on their orbit around Earth.

Now the yo-yo: In general, when playing with a yo-yo, the drum of the yo-yo wants to stay right where it is. This is because its mass also gives it "inertia", i.e. the ability to resist any change in its motion or lack of motion. You need a force to make the drum change its motion: On Earth gravity makes the drum roll down the string. To hasten its decent you could throw the yo-yo down. As you hold the yo-yo you can see it unwind and wind up again. If you pull on the string each time when the yo-yo reaches the bottom, the drum rotates faster and faster as the string winds up and then unwinds. As the drum winds down to the end of the string, it continues to rotate because it also has inertia for rotation (in other words, you need force to either make it rotate or to make it stop). It continues to rotate while winding up the string again, back to your hand, using the energy it stored from the gravity on the trip down.

In space gravity doesn't help you to move the yo-yo. The only way to get it away from you hand is to throw it and then pull the string taut, when the drum reaches the end. Then the yo-yo will unwind and wind up as before. However, it may be a little tricky to keep the string taut at all times. You must use force to change its direction each time. Beware! Don't pull too strongly! The yo-yo will smack into your hand with full speed, because there is no gravity to slow it down.

A yo-yo was actually flown on the Shuttle Atlantis to show off in classrooms and as an experiment to test the effects of microgravity.

Dr. Eberhard Moebius
(March 2004)


Sound in Space

Why can't you talk in space? Is it because space is a vacuum?

Sound is caused by pressure waves in air (or something else, sound can go through water, for example). If there is no material (called a vacuum), there is no way for sound to travel, so you can't talk. Inside a space capsule or space station there is air, so the astronauts can talk there.

Dr. Eric Christian
(September 2001)


Tuning Fork in Space

Since sound cannot travel in the vacuum of space, what would happen if you had something that normally produces sound, like a tuning fork, in space? What would happen to the energy?

For a tuning fork to vibrate, it must be struck. On Earth, these vibrations compress surrounding air molecules to produce a sound wave that we can hear. If an astronaut in space were to strike a tuning fork, it would vibrate, and sound waves would occur within the tuning fork itself. However, with no air molecules around, it would not produce a sound that the astronaut could hear. The energy from these vibrations would heat the tuning fork (due to internal friction) and eventually be radiated away.

Dr. Nick Sterling
(March 2007)


Signal Flare in Space

Will a standard signal flare work on the Moon, just like it would work under water?

There are two ways you can get things to burn in a vacuum. In one way, you bring along a source of oxygen (liquid oxygen or LOX tanks are used in many rockets). The other way is to have enough loosely-bound oxygen in the chemical that you're burning for it to be "self-oxidizing". This is the way the solid rocket booster on the Space Shuttle does it, and also, I presume, how a flare works. I'm not sure of the chemical composition of a signal flare, but if it can burn underwater, it is probably self-oxidizing.

Dr. Eric Christian

Sparks in Space

Does an electrical spark need air to happen? Can a spark occur in a vacuum like space? These questions arose from an argument about whether an electrical arc between two separate pieces of metal (such as within a light switch) would be considered "fire," as fire needs fuel to burn.

This is indeed a very interesting question with several important facets. Because the question contains some additional background, let me start a little more generally.

A spark in a switch is not considered "fire" in the sense that something is burning. A spark is very similar to a lightning flash, i.e. a channel, which contains plasma. Plasma is matter in a gaseous state whose atoms have lost at least one of their electrons. Therefore, positively-charged ions (the remnants of these atoms) and negatively-charged electrons fill this plasma instead of neutral atoms. These electrically-charged particles provide for an electric current through the spark (or lightning) channel. The light is emitted by atoms which either just regained their electron or by atoms which have been excited by a rushing electron that has bumped into them.

Having said this, it appears that gas is still needed between the two metal plates to provide the atoms that can be turned into plasma by the electric voltage across the plates. Such a spark is then started by a few free electrons, which are always around, for example, created by cosmic rays. The electrons are accelerated by the electric voltage, bump into atoms in their way, and then kick out more electrons (and so on).

Now you may wonder whether a real vacuum will then suppress the generation of sparks. The matter of fact is even a vacuum can never be a perfect suppressor for a spark. If the voltage is turned up high enough, electrons can literally be extracted out of the metal plate on the negative side. The surface, which may look perfectly smooth at first sight, is pretty rough and peppered with sharp spikes, when viewed under a microscope. Here the electrons are extracted. They fly over to the positive plate and reach it with a very high energy. Metal surfaces always have some gas attached that never disappears completely. The electrons bombard this gas, kick out electrons, and now the remaining positive ions are attracted to the other side. Under the ensuing bombardment of both surfaces the thin layer of gas on the metal is released, which then provides for the spark material. So, even in vacuum you get a spark between two metal plates.

Dr. Eberhard Moebius
(May 2003)


Will a Simple Magnet Work in Space?

Will a simple refrigerator magnet work in space?

Yes, the magnetism force is independent of gravity or atmosphere.

Dr. Eric Christian

Will a Compass Work in Space?

Will a normal camping-type compass work in space?

If you are within the magnetic field of the Earth (which extends about one fourth the way to the Moon) or other magnetized body, yes. Outside of this, the magnetic field away from the Sun and some of the other planets is probably too small to move the needle on a regular compass.

Dr. Eric Christian

Scale and Balance

If two people of equal weight, one on a scale, the other on a balance, go into space on a space ship, what will happen to the scale and balance?

Assuming that you mean zero G (no gravity influence) when you say "go into space", and not when the rocket is firing, the answer to your question is that everything would float: the scale, the balance, the balance weight (if there is one), both people, everything. The scale would read zero, since there isn't a gravitational weight on it. The balance would be balanced, whether or not there is a weight on the other side, because the weight of the person on the balance would be zero.

Dr. Eric Christian

Catalysts

How do catalysts affect situations? In order to have fire you need to have oxygen, and the more oxygen present, the greater the fire? What does a catalyst actually do? If a fire requires oxygen to burn, then how are astronauts able to produce a flame in space? There is no oxygen beyond the atmosphere, correct?

These are really questions for chemists, but I'll try and give them a shot. A catalyst is a material that speeds up a chemical reaction without being used up in the reaction. A fire is rapid oxidation, but the oxygen doesn't really count as a catalyst since it is an integral part of the reaction.

Astronauts can produce flames in space one of several ways:


1. They can produce them in their capsule where there is oxygen (there were several shuttle experiments that checked the effect of zero-gravity on burning candles).
2. Some propulsion flames bring their own oxidizer (like a tank of liquid oxygen).
3. Some compounds have enough oxygen in weak bonds that they are self-oxidizing, and burn even in a vacuum (solid rocket fuel is like this).

Dr. Eric Christian