Re: varying mass systems: rockets included (medium length)

[Joel Rauber <Joel_Rauber@SDSTATE.EDU>]

> Joel Rauber wrote:
>
> > In the rocket in deep space propulsion problem, you deal
> with a somewhat
> > similar situation.  Your system of interest is losing mass
> rather than
> > gaining mass.  We typically utilize conservation of
> momentum to obtain an
> > equation for the motion of rocket that typically appears as follows:
> >
> > (total mass) a_cm = -U_exhaust* |dm/dt|
> >
> > In this situation we don't hestitate to call the added terms
> > {-U_exhaust* |dm/dt|, in the rocket example} "the thrust "
> or maybe "the
> > force of the thrust".
>
> I believe I followed your statements up to this point.
> Please help me to
> understand your equation.  Why is the left side including an
> acceleration
> factor?  Shouldn't it be velocity?

I mean acceleration, both sides of the equation are intended to be
dimensionally the same as a force.

> On the right side is the
> -U_exhaust the
> velocity of the exhaust as it seems to me it should be?
> Perhaps my font
> makes a
> V look like a U.

Yes, - U_exhaust is intended to be the negative of the exhaust velocity,
relative to the rocket.  What is measured when you put the rocket engine on
a fixed cradle and measure the velocity of the exhaust gases, often done in
Utah I believe.

Joel