Re: beginner thought experiments


Sue... wrote:
Randy Poe wrote:
Sue... wrote:
Randy Poe wrote:
Sue... wrote:
Space ships melt long before they achieve relativistic speeds.

1. There is no air friction in deep space.

Some of the more math oriented posters might like to know the
type of notation you use which doesn't distinguish 'none' from
an average density of about 1 atom per cubic centimeter.
http://www-ssg.sr.unh.edu/ism/what1.html

And the resulting friction of 1 atom per cm^3?

What would a blast of hydrogen do to your skin at the
speed of sound?

What "blast of hydrogen"? You mean "collision with one
atom per cm^3"?

Well, let's see: Assume I have a cross-section area of 1 m^2.
If I'm moving at 300 m/sec, I sweep out 300 m^3 per second,
and interact on average with about 300*10^6 atoms of hydrogen.
If they all impact me at 300 m/sec and bounce off at 300 m/sec,
they've had a net change of momentum of m*600 m/sec. So they've
changed my momentum by 600*(mass of 300*10^6 atoms) or
about 600*300*10^6*(1.7*10^-27) = 3.1*10^-16 kg m/sec. If my
mass is 75 kg, then every second I'll lose about 4*10^-18 m/sec
of speed. It would take me the life of the universe to slow to 0.

Maybe you can convince NASA to do it that way:
http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

No doubt NASA does it that way when they are
interested in drag due to flow through air, as opposed
to near vacuum, 1 atom per cm^3.

Go back to your first link and look at the estimated
number of atoms per cm^3 in the medium where
your second link applies.

Are you REALLY having this much trouble remembering
we're talking about interstellar space instead of
atmosphere?

- Randy

.

Relevant Pages

  • Re: beginner thought experiments
    ... There is no air friction in deep space. ... an average density of about 1 atom per cubic centimeter. ... If they all impact me at 300 m/sec and bounce off at 300 m/sec, ... If you think the ships give a hoot about my point of vieiw you may ...
    (sci.physics.relativity)
  • Re: beginner thought experiments
    ... an average density of about 1 atom per cubic centimeter. ... If they all impact me at 300 m/sec and bounce off at 300 m/sec, ... the electron cloud in a particle accelerator ... the next gap in one half of the rf cycle. ...
    (sci.physics.relativity)