[Phys-l] rocket car story revisited

[Brian Whatcott <betwys1@sbcglobal.net>]

In an earlier thread....
> > ...
> >  In the rocket-car episode, they announced 
> > that 1 pound of thrust is equivalent to 2 horsepower.  And they meant it!
> > They /repeatedly/ asserted that their 1500 pound-thrust rockets provided
> > 3000 horsepower. 
> >     
> ......
>
> A low-ball estimate considering the likely maximum velocity involved, 
> wouldn't you think?
>
> :-)
>   
Hmmm...I feel uncomfortable - passing on from this small triumph of the 
scientific edifice with just a flip comment.   The relation between 
work, force and speed is so elegantly simple yet so useful, I want it to 
be celebrated more persuasively.

Look at the units! The newton, the joule and the meter are carefully 
selected to
strip relations of unwanted scaling constants.   While it is true that 
previous systems have employed the Slug or the g for a similar purpose, 
this international system chooses as its force unit the newton - and who 
could not recall that it represents the effect of holding a (small) 
apple in the hand?

The joule which may evoke for English students, the family brewery - 
stirs up ideas of warming liquids in a barrel, or for American students, 
the idea of the sweat equity invested in boring a gun-barrel. Together 
with that brave idealistic French attempt to reduce the arbitrariness of 
a length measure to some physical feature - an even fraction of the 
quarter Polar circumference  though Paris, no less!

So here we are with work,  given by the force applied times the 
distance  travelled in the direction of that force.    The joule the 
newton, the meter are easily extended to the rate of doing work. So much 
better to dump the concept of the rate at which a dray can work a mill 
multiplied by some factor, in favor of the watt the newton and the
meter/second.

John D reasonably railed at defining force (as thrust) in terms of power,
though Jack made the reasonable observation that the unspoken third term 
seemed to be a datum point for speed, namely the speed of sound.

Still, this relation between power, thrust and speed deserves more 
attention, it seems to me.    We have several misleading usages for 
power in terms of thrust - I mentioned one: the electric trolling motor. 
YOU know that this is misleading, but it seems that most adults do 
not.   When I ask why this should be so, I realize that the customary 
units have obscured the beautiful simplicity of the relation.
HP = Thrust(lbs) times Speed (MPH) just ain't so!
I need to suppose that nobody has really heard or understood, that
Power(watts) = Thrust (N) times Speed (m/s).

Even so, if this relation were widely understood, what would we make of 
a given power working into a fixed object?   This reminds me of the 
question raised here every year or two: the person pushing against a 
wall expends no energy??

In the case of the tethered boat or airplane, the question is resolved 
more easily.
Though the vehicle is fixed, the medium acted on is moving - so that the 
effective speed
of the fixed vehicle (propeller)  never reaches zero: the boat has a 
stream of water driven by the prop, and likewise, the aircraft has a 
considerable stream of air.
  Hence we never face that bane of scientific models,   It turns out 
that when we think that one factor grows very small, then  another 
factor of a product should grow indefinitely large - here it does not in 
fact.

BrianW