

Force in Space
Name: chris
Status: other
Age: 30s
Location: N/A
Country: N/A
Date: 1999
Question:
with regard to space travel and the speed of light,if a
gun is fired on an aeroplane the bullet travels at 1000mph plus the speed
of the plane 500mph (1500mph relative to an independant
observer)therefore would in not be possible to build a series of space
rockets with around 30 extra "jockey sections". the first ship would
attain a speed of say 18000mph, the next ship would then be released from
its possition on the back of the first ship (remember we are travelling
in a vacuum with no friction )the speed would then increse to
36000mph,this second ship would have around 30 further ships on its back
each one on top of the other to give a cumulative speed increase,by the
time we had got to say, the twentieth jockey in a series of thirty ,this
ship would be carrying a series of 10 further ships to be released.what
are the drawbacks of this suggestion,because the speed in a vacuum would
be cumulative and the build up of aggregate velocity must surely
eventually take it to the speed of light (theoretically)can you please
shoot holes in this assumption and tell me why this idea will not work.am
i wrong?????
Replies:
What you're describing is very similar to actual practice of dividing
rockets into stages. The objective is to maximize the speed of the
final stage, given a fixed amount of fuel. Obviously you are dividing
the energy contained in the fuel among all the stages, so in the end
you'd like to minimize the energy given to all stages except the
payload. One way to do this is to minimize the mass of the throwaway
stages. Usually, minimizing the mass also means minimizing the complexity.
No matter what you do, however, this approach will not get the payload
stage to the speed of light, because as you approach the speed of light
more energy is required to increase the speed.
Tim Mooney
The problem is that the speed of light (in a vacuum) always appears the
same to all observers, whether the different observers are moving relative
to each other or not. Say you have your series of spaceships nested lke
Russian dolls and begin launching them, each traveling at a speed if 18,000
mph relative to the one before it. Even if you launch 30 stages like this,
making the sum of their velocities well over the 186,000 mph speed of
light, the 30th ship will NOT travel faster then the speed of light,
relative to the first ship. In fact, if the first ship then shines a light
down the line, each of the succeeding ships will see the light beam rush
past it at the standard 186,000 mph.
How can this possibly happen? If each stage travels 18,000 mph faster than
the previous stage, shouldn't the nth stage travel (nm)186,000 mph faster
than the mth stage? Well, no. Compared to the first stage, the succeeding
stages see time go more quickly and see distance longer. To explain how
this works, I'll use an answer to an earlier question.
"If you took a step in a train going the speed of light , would you be going
faster than the speed of light ?"
No, the theory of relativity gets around that quite nicely. It is
impossible to get an object with mass (such as a train) to travel at the
speed of light, because that would require infinite energy.
You might then ask if you could walk 3 miles an hour on a train traveling 1
mile an hour less than the speed of light, thereby going 2 miles an hour
faster than the speed of light. The answer here is yes, you could walk 3
miles an hour on a train traveling 1 mile an hour less than the speed of
light, but no, that would not make you go faster than the speed of light.
The reason for this is an effect of relativity known as time dilation.
From the viewpoint of the train, light still travels at 3 x 10^10 cm/s, and
you're only moving at 3 mi/h. So you're traveling MUCH slower than the
speed of light.
Now when we say the train is traveling at speed of light  1 mi/h, we
actually mean that it is traveling that fast relative to some external
observer. From the viewpoint of that external observer, time has slowed
down for you. Let's say one of the cars of the train is 30 m (3 x 10^3 cm)
long. If you shine a light from the back of a car toward the front, the
observer will see that it takes 67 s for it to reach the front of the car.
From the viewpoint of the train car, it only takes 10^7 seconds for the
light to travel the length of the train car. So, time has slowed down for
you by a factor of 67s / 10^7 s = 6.7 x 10^8. So, your velocity, which
seems to you to be 3 mi/h faster than the train's velocity, only seems to
the observer to be (3 mi/h) / (6.7 x 10^8) = 4.5 x 10^9 mi/h faster than
the velocity of the train  still slower than the speed of light, no matter
whose point of view you take.
How does this work? The reason is that one of the fundamental laws of
nature, according to the theory of relativity, is that light always travels
at the same velocity from the viewpoint of ANYTHING, no matter how fast
that anything is traveling relative to anything else. This can make it
appear to observers traveling at high speeds relative to each other that
the OTHER guy is getting shorter, or heavier, or his time is slowing down,
but as far as the other guy is concerned, all those things are happening to
you.
Confusing, but pretty cool.
Richard Barrans Jr., Ph.D.
Chris 
Einstein shot holes in your theory. Following his theory, as speed increases
mass increases until  in theory  at the speed of light your mass is
infinite. As mass increases, the amount of energy required to make the same
change in speed would increase... until... well, how do you find the energy to
accelerate a mass of almost infinite size?
Except for this, your theory works. If Einstein is wrong (could be) your are
in good shape.
You might find reading on the subject of relativity to be interesing to you.
Larry Krengel
From Newtonian physics, we learn that F = ma. Force = mass * acceleration.
We can rewrite this as a = F/m. Or acceleration is Force versus mass. If
there is no force, there is no acceleration. Also, imagine if the mass were
really hugeinfinite, then there would be no acceleration also.
Now, on to your problem. Einstein basically disproved Newtonian physics
with the Law of Relativity. A number of different things are stated, but
some of these are:
As you approach the speed of light, time slows down. This has actually been
proven with experiments and different things. It does happen. You have
probably even heard of this.
Another thing that is less well known, is that, as your velocity increases,
so does your mass.
For most of us, this doesn't affect us. The speed of light is so huge that
usually Newtons laws are accurate enough to suffice. We don't worry about
the change in mass because it is so slight (even for planes traveling faster
than sound) that it is almost imperceptible.
In your case, however, as you attemp to approach the speed of light, the
mass of each rocket would increase to the point where you simply can't
accelerate any part of your ship anymore. With the technologies we use,
this inability to overcome would occur quite soon with regard to the speed
of lighteven at 1/100000 the speed of light the mass increase is so
overwhelming we couldn't accelerate something large enough to hold a man.
Also, as a point of interest, it doesn't matter how you build the ship,
whether you build it in one huge part or 1000 small ones.
Think about this for a bit: it doesn't matter whether you appy a given
force 1 second or half that force for 2 seconds. The final velocity will be
the same. Some equations for this:
F = ma (Force = mass * acceleration)
v = at (Velocity = acceleration * time)
Now, thinking about this a bit more, we can also state that: if an object is
traveling at a given velocity, we know what the sum of the forces acting on
the object were. This is a bit far to go, but looking at what we did
before, it doesn't matter how long the force was applied, so lets assume 1
second in our equations.
v = a * 1, or v = a. Now, F = ma. Rewrite as a = F/m. Now v = F/m, or:
v * m = F
This is the formula for momentum. This is interesting because it means, no
matter how an object got to a given velocity, the force acting on it was the
same. This also means, it doesn't really matter whether the force was
applied in 30 bursts or 1 short burst. None of that matters.
Also, we have to realize, all these equations ignore friction and the law of
relativity. Since you are interested in space flight, friction can safely
be ignored. The law of relativity must be delt with. Fortunately, it
doesn't affect the conclusion in this case. Einsteins equations are very
similar to Newton's. They just adjust the mass so it is relates the
object's mass to the speed of light.
So, it takes exactly the same amount of force to accelate a bullet to 1500
mph whether it is shot out of an airplane standing still or an airplane
flying at 1000 mph. The difference is simply with an airplane flying at 500
mph, most of the force was spent in getting the airplane with all the
bullets and a pilot and all that to 500 mph. But the portion of force
acting on the bullet to get it to 500 mph was the same.
In your case most of the force would be spent accelerating the modules that
never achieved the final velocity.
The reason scientists build rockets in stages in this manner is not because
it is the most efficient way, but because it is the only way we can get the
amount of fuel needed aboard the craft. A rocket engine is mostly fuel, and
when the fuel is burnt, the engine is dead weight, so space craft are built
to dump their dead weight. The less mass, the less force is required.
Anyway, I hope this explains a little why it is so difficult to get to the
speed of light and why it doesn't matter so much how you build your ship as
long as you are applying force to get it to move, it is always going to be
hard to get to the speed of light.
Thanks,
Eric Tolman
Click here to return to the Physics Archives
 
Update: June 2012

