# Re: starship-design: Linear EMP motor (name needs work)

```In a message dated 1/13/00 1:40:32 PM Pacific Standard Time, stevev@efn.org
writes:

<< Subj:     starship-design: Linear EMP motor (name needs work)
Date:  1/13/00 1:40:32 PM Pacific Standard Time
From:  stevev@efn.org (Steve VanDevender)
Sender:    owner-starship-design@lists.uoregon.edu
To:    chithree@boo.net (Connor)
CC:    starship-design@lists.uoregon.edu (starship-design@lists.uoregon.edu)

Connor writes:

> So once I was reading about an idea for propulsion, one where the sun
> and the earth were connected by a long aluminum bar. So the idea is,
> the sun emits an electromagnetic pulse for a minute, and it takes
> eight minutes to get to the earth. When it reaches it, the earth is
> pulled foreward in the pulse for a minute, and the whole structure
> moves foreward. This happens again and again, and the whole thing
> gets going pretty fast. By the way, this is hypothetical.
>
> So this isn't really practicle for interstellar travel, because it's
> so big, and I don't think the sun does that :). But I was thinking
> that you could keep the apparatus in a big tube of liquid sodium. The
> speed of light in liquid Na is only 52 ft. Per second, so with a
> fancy computer it would probably only have to be 25 feet long. How
> does this sound to anybody?

I don't get it.  If the Earth pushes or pulls on this rod, then the
compression or extension travels back along the rod at no greater than
the speed of light and pushes or pulls back on the Sun, and there is no
net motion.  Remember conservation of momentum?  There's also no such
thing as a perfectly rigid instantaneously-transmitting rod.  Matter is
held together by electromagnetic interactions, so any tap on one end of
a rod propagates at at most the speed of light along it to the other
end.

:Pardon my colon color form but mail replier has bug  (no >>, > placed
correctly. I : :am attempting to fix.

:You are correct only a shockwave from the tap travels to the other end.
Rigidity is a :hardness measured factor between 0 and 1 with 1 being diamond.
The shock wave :speed varies with the material. To determine the magnitude of
the impact shock :wave use the equation Force of impact = Rigidity times Mass
times Velocity, Use :the mass of the "tapper" and the velocity of the Tapper
on impact. Steel has a :rigidity of very near one so in ballistics with metal
bullets the rigidity 1 is usually not :writen as 1MV= MV with MV undersatood
to be 1MV. In Martial Arts this ballistics :equation allows a 200 pound man
to deliver a 900 pound striking impact force per square :inch. .

Basically, there is no way to induce motion that violates conservation
of momentum; if something moves one way, something else has to move the
other.

:Not nessasarily. Attach a board to the tops of two five gallon buckets open
side :down. Stand on board and move body forward rapidly without moving feet
along a :ine parallel with the bucket centers. You will find you can walk
across the concrete :floor inch by inch with each motion. Conservation of
momentum is maintained but :your conclusion is proven false. Mass times
velocity one way = mass times velocity :the other way in rockets. In particle
accelerators E= 1/2Mass times velocity so :does not have the counter force.
That is why the energy in the rapid motion :translates to forward motion of
the man an buckets.  Rockets and Particle :accelerators have different max
velocities as different equations are used.....

This can get rather subtle, in the case of propulsion that
involves emitting electromagnetic or gravity waves in one direction so
that the drive system goes in the other, but there is always _something_
that gets thrown out the back or pulled on from the front.

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