Was Rocco Top Speed, now Cement Filled Yugo Physics Discussion

[16v Jason <jason@scirocco.org>]

At 11:56 AM 9/2/99 , you wrote:
>assuming that they would have to be dropped from diferent heights. they
would 
>both reach their teminal velocity at the same point, because they are the 
>same shape. they cement filled yugo has more kinetic enery that is all. it 
>won't fall any faster than the empty yugo. 
>-slsn



Hey Alan,
	I have to differ with you...

	Let's assume that the cars were both dropped from the same height (i.e.
the top of a building).  They would both accelerate at the same rate, at
first, i.e. somewhere around 9.8 ms^-2.

However, the lighter one has a lower terminal velocity... so it would stop
accelerating at some point (if facing downward probably somewhere around
130mph), but the other one would continue accelerating to a higher velocity.

Why?  Well, it's basically the same concept as dropping a piece of paper.
The paper (and we're assuming it stays flat and doesn't swing back and
forth etc) reaches its terminal velocity really quickly... it's got a huge
surface area, and not enough force to overcome it.  Remember that the force
downward is equal to the weight of the object times the acceleration from
gravity.  So, we'll be metric about it:  Let's say that the paper weighs
0.001 kg.  The force yanking it downward is therefore 0.001 x 9.81, which
is 0.00981 Newtons.  So, air resistance (which is a force in the UP
direction) reaches 0.00981 Newtons, the paper reaches its terminal velocity.

Now let's assume that you put a weight on top of the paper.  Remember, the
paper is stiff, so it's not going to bow and flatten out under wind
resistance. (These are MY rules, *I* make them up... but seriously, we're
doing this in comparison to a metal car which wouldn't flex)  The weight of
the paper and the weight combined is 1.00 kg.  So, the force pushing the
paper down is now 9.81 Newtons.  Since the weight is on top of the paper,
it's not changing the surface area of the paper (as seen by the air), and
has minimal consequences on the aerodynamics of the falling paper.  The
terminal velocity of the paper will be much higher-- it won't occur until
the wind resistance is 9.81 Newtons.

The same can be applied to the 2000lb and 10,000lb Yugos.  Think of it this
way:  You have two  Yugos that weigh 0lb at the entrance of an Autobahn.
They have the same wind resistance and aerodynamic properties.  One of them
has a propulsion system (engine or whatever) that is able to give 2000lb of
thrust, and one that can give 10,000lb of thrust.  Since the cars weigh 0,
the only thing the engine has to do is fight wind resistance.  At first,
the cars will leave the mark with identical acceleration.  However, as wind
resistance builds, the 2000lb thrust car will start to lag behind the
10,000lb car and will reach its terminal velocity while the 10,000lb car is
still accelerating to a much higher terminal velocity.

True, it you dropped both cars off a 50-foot roof, they'll probably land
around the same time... but if the distance were greater, the cement car
would slam down first.


So, now that I've done my best to remember College Physics, who's going to
point out my methological flaws and start one of those fun arguments with
me? :)  J/K, but if I'm wrong, someone let me know....

Jason




----------
1987 Scirocco 16v
1989 Mercedes 190E Sport Euro


http://members.aol.com/rocco16v  

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