Re: Potentially painful

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/09/05 

Date: Wed, 9 Mar 2005 04:15:15 -0800

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote
 in message news:sa3t2113mifbj3e38eovdn74qc546dni4i@4ax.com...
> 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?

Yes. I should have known this little puzzle would
not take you folks long to sort out. The key is to
consider that starting condition. All the energy ends
up in the exhaust at the limit of zero rocket speed.
The problem gets a little easier to see if it involves
chucking cannonballs from a railway car. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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