# Re: The actual running Archie paper...

```--Original Message-- From: James R. Cunningham <jrccea@bellsouth.net>:
Saturday, May 08, 1999 01:36 AM

>
>
>John V Jackson wrote:
>
>> "...thrust is the only force that exerts work on _Ax_ along its entire
>> take-off run..." ...well, all right, but...
>
>They're using the mathematical description of work.  No vertical movement,
no
>vertical work.  That doesn't mean no energy has been expended.

I was aware of that, though isn't it a "physic"-al description?

>
>> If you've got to push against something that's actually going backwards
>> relative to you, you can't push so hard as you accelerate.  That's why
>> rockets go faster than jets.
>
>John, this last went over my head.  What do rockets and jets push against
thats
>going backwards relative to them, and what does it have to do with their
>relative speed?

Rockets push against their propellant which is initially travelling at their
speed.  Jets push against the air which is travelling backwards relative to
them.  In order to gain some forward impulse a jet has to push the air
backwards in an absolute sense; a rocket can enjoy a forward impulse even
though the exhaust gases might theoretically be moving in the same direction
as the vehicle.

Since work is force times speed, for a given power you can get more impulse
by pushing against something that is stationary compared to something that
is moving in the opposite direction.  Whether pushing against the ground or
against the air, the principle accounts for the progressive decrease in
accelerative force as speed increases.

Of course the vague impression that "rockets are faster than jets" has a lot
to do with rockets being able to progress in space.

JJ

```