DaveC426913 said:
Newton's First Law: Any action generates an equal and opposite reaction. It works in space quite nicely. It's how rockets propel themselves in space.
Dave is partially correct, but he is actually referring to Newton's
third law of motion. However, this way of phrasing the law is somewhat antiquated, and cannot be interpreted literally. This often causes the law to be misinterpreted. What exactly constitutes an
action, and what is the correct
reaction?
For example, I have actually heard people that thought the law could be applied to the stock market: a stock goes down in price (an action), therefore there must be an equal and opposite reaction, therefore the stock must go back up in price an equal amount at a later time. Of course, that is completely incorrect! As you can see this way of stating the law is not actually useful.
A much more intuitive explanation for why you can propel yourself through space by throwing things behind you results from the
law of conservation of linear momentum. This law means that in a closed system (ie, if there are no exterior influences), the total momentum of the system stays constant.
Momentum equation:
P = M*V
(P is momentum, M is mass, V is velocity)
Now, for the closed system we can consider the astronaut who is floating in space. Because space is [mostly] empty, the only particles we need to consider are the atoms that make up the astronaut and whatever gear he is wearing. The fact that the system is
closed means that we will not be considering things like other particles flying in and hitting him from the depths of space, which would obviously screw up our analysis.
Ok, so every particle in the spaceman's body has some mass and velocity. Because the atoms are all held together by magnetism (ie, chemical bonds), we can roughly consider the entire astronaut as 1 big particle with 1 mass and 1 velocity.
If the astronaut takes off his glove and throws it behind him, it now has a different velocity, so we need to consider that as a separate particle. Let's represent the mass of the glove as Mg, and the mass of the astronaut without the glove as Ma. The original velocity of the astronaut is V1, the velocity of the astronaut after throwing the glove is V2, and the velocity of the glove after he throws it is V3.
Initially, we have
P = (Ma + Mg) * V1
The law of conservation of momentum tells us that the total momentum P (which means adding up M*V for every particle) is going to remain constant no matter what those particles in the system do. After throwing the glove, we now have
P = Ma*V2 + Mg*V3
Now we can just use elementary algebra to find out what the velocity of the astronaut has to be:
(Ma + Mg) * V1 = Ma*V2 + Mg*V3
(Ma + Mg) * V1 - Mg*V3 = Ma*V2
V2 = ( (Ma + Mg) * V1 - Mg*V3 ) / Ma
V2 = (Ma + Mg)/Ma * V1 - Mg/Ma * V3
In order to understand the result, we don't actually care how massive the glove or the astronaut is, so let's create some arbitrary
positive constants (a mass cannot be negative):
V2 = C1* V1 - C2* V3
Ok, let's say his initial velocity was going in the positive direction. Then he throws the glove behind him, which means V3 is in the negative direction. Two negatives make a positive, so that means his final velocity V1 is still going forward, but it's faster than it was before.
One of the consequences of the law of conservation of linear momentum is that the center of mass of all the particles will always stay in the same place. Using that logic, you can also understand why he can propel himself by throwing something behind him...if part of the mass is moving away, then he has to move forward in order to keep the center of mass of
everything in the same spot.
I hope that clears up your question
