Re: Potentially painful

From: John Larkin (jjSNIPlarkin_at_highTHISlandPLEASEtechnology.XXX)
Date: 03/09/05 

Date: Tue, 08 Mar 2005 21:57:17 -0800

On Tue, 8 Mar 2005 12:44:56 -0800, "Larry Brasfield"
<donotspam_larry_brasfield@hotmail.com> wrote:

>"Robert Monsen" <rcsurname@comcast.net> wrote in message
> news:uMGdnQ0E54dKlbPfRVn-tw@comcast.com...
>> No, as I recall, kinetic energy is 1/2 * m * v^2.
>
>
>I recall seeing that claim from a high school physics teacher when
>I was a smart-ass twerp. I posed the following puzzle to him:
>A rocket car starts at rest, accellerating at a constant rate
>because its thrust is constant. It is burning fuel at a constant
>rate to produce that constant thrust. The kinetic energy of
>the rocket car is allegedly M * V^2 / 2, so it is increasing
>quadratically versus time. But the fuel consumed increases
>only linearly with time. How can this be?
>
>I would be interested in your take on this. My physics teacher
>could not resolve it, (but, to his credit, that bothered him).

At low velocities, a rocket is a very inefficient source of
propulsion; at near-zero velocity, it's using its usual amount of fuel
but hardly delivering any kinetic energy to the vehicle. As velocity
increases, efficiency improves (or rather becomes less terrible) and
vehicle energy accumulates faster. That trend continues until you run
out of fuel.

In a system that goes from extremely inefficient to only rather
inefficient, it's not hard to shape the efficiency curve into a
quadratic. A cog railway can be nearly 100% efficient, so it will need
increasing amounts of fuel if it accelerates at constant gees, but
anywhere on the path it will be a lot more efficient than a rocket.

That make sense?

John

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