top speed

[Rob Munach]

Bruce Hoult wrote:




That's not true. Your assumption here is that the limiting factor is the
friction between the tires and the road, and that once the top speed is
reached, attempting to go any faster will spin the tires, and thus more
weight is needed to increase the friction between the tires and the road.

The reality is that the aerodynamics of the car push back on the hard as
hard as the engine can push it forward, and it goes no faster. The tires
usually have nothing to do with it.

If we did fill the cabin with water, not only would the top speed decrease,
but the weight distribution would probably be screwed up, which may make it
more difficult to control at higher speeds.

He was talking about making it heavier and dropping it out of an
airplane, not driving it down the road.

 

[k. ote]

He was talking about making it heavier and dropping it out of an
airplane, not driving it down the road.

Right, of course. Doh.

I'll have to remind myself not to post when I've only had three hours of
sleep the night prior. ..

 

[Carl 1 Lucky Texan]

Assuming the CG was the same, they'd fall at the same speed.

A steel ball, and a hollow steel spehere of the same diameter and
surface finish will hit the ground at the same time.

Carl

 

[k. ote]

Assuming the CG was the same, they'd fall at the same speed.

A steel ball, and a hollow steel spehere of the same diameter and
surface finish will hit the ground at the same time.

Carl

If that were the case, then an infinitely thinly surfaced sphere, provided
it has the same shape and finish of the solid sphere, would fall at the
same rate as well. This is, by inspection, not true.

The density of the material has a lot to do with how much effect aerodynamic
drag has on it. If you have a hollow steel sphere as light as a feather and
a steel sphere the same size but filled with something heavy (like more
steel) the heavier steel ball will accelerate faster because it takes more
force to offset its acceleration.

As it falls, the steel ball that weighs next to nothing will not have the
same capability to displace air around it that the solid steel ball does by
nature of its momentum. Weight does indeed matter unless you're in a
vacuum, which we're not. Well, I'm not anyway. Where are you?

Also, consider bouyancy in the atmosphere. Less dense objects are going to
be more bouyant even if we ignore momentum entirely.

 

[Bruce Hoult]

Assuming the CG was the same, they'd fall at the same speed.

A steel ball, and a hollow steel spehere of the same diameter and
surface finish will hit the ground at the same time.

LOLOL

 

[CompUser]

LOLOL

....but how many angels can dance on the head of a
pin?

 

[Carl 1 Lucky Texan]

OK - please tell me then the difference in arrival times of a metal
spehere of aluminum and a sphere of lead if dropped simultanoeusly from
- say - 100ft. assuming no cross winds and precisely the same surface
finish/COD (drag coefficient).

Carl

 

[Florian Feuser /FFF/]

OK - please tell me then the difference in arrival times of a metal
spehere of aluminum and a sphere of lead if dropped simultanoeusly
from - say - 100ft. assuming no cross winds and precisely the same
surface finish/COD (drag coefficient).

Carl

The aerodynamic draft coefficient derived from shape is identical.
However, with the same projected area and identical drag coefficient, a
body of lesser mass will reach terminal velocity* sooner.

*TV = the state where weight equals draft

or:

[gravity * mass] = [Projected Area * Drag Coefficient * Density/2] *
Velocity^2

florian /...or something like that/

 

[Florian Feuser /FFF/]

The aerodynamic draft coefficient derived from shape is identical.

^^^^^

Drag... Drag, not draft!
What was I thinking?

florian /FFF/

 

[Guest]

OK - please tell me then the difference in arrival times of a metal
spehere of aluminum and a sphere of lead if dropped simultanoeusly from
- say - 100ft. assuming no cross winds and precisely the same surface
finish/COD (drag coefficient).

Having being advised of fundamental physical principles you didn't
understand your response is to ask for the answer to a complex example
problem.

If you are trying to appear less dumb I suggest you work it out for
yourself and tell us the answer.

 

[Carl 1 Lucky Texan]

So, the air is smart enough to know what an object is made of and
whether or not it is hollow?

Good to know - thanx.

Carl

 

[Yousuf Khan]

I hit 140 just the other day. It was easy. I just don't know what my STi's
*top* speed is, as I've never approached it and felt safe about it. Too
much wind around here buffeting me around the road.

Then I guess the question to ask would be do those "wings" at the back
of the STI add any downforce to stabilize it at that speed?

Yousuf Khan

 

[Florian Feuser /FFF/]

So, the air is smart enough to know what an object is made of and
whether or not it is hollow?

Good to know - thanx.

Er... yes - in a way it is.
Newton's principle does apply, but there are other factors at play, too:

There IS no terminal velocity in a vacuum. However, in the atmosphere,
the relation between projected area and mass [you may call it density]
will determine the difference in terminal velocity of two objects of the
same exact shape.

Lets save the Subaru for now and drop from that plane a) a ping pong
ball and b) a ball of lead measuring 4cm across.

What do you think will happen?

<http://www.grc.nasa.gov/WWW/K-12/airplane/falling.html>

florian /FFF/

 

[CompUser]

Then I guess the question to ask would be do those "wings" at the back
of the STI add any downforce to stabilize it at that speed?

Allegedly they do...all I've found was anecdotal
stuff on NASIOC, mostly second hand, from people
"that knew people" that tracked em, with and
without...who reported they made a noticeable and
significant difference.

Haven't seen any "lab-grade" specs on downforce @
speeds, windtunnel imaging or anything
approaching it.

Some individuals have reported driving at high
speeds with the trunk deliberately left
unlatched, and that it's still unlatched after
the drive.

 

[Carl 1 Lucky Texan]

I thought we started with a subaru filled with water or with lead?
Anyway - I can see where IF drag equals weight you have an equilibrium
state. Probably need to figure the Reynolds number in too.

thanx for the link.

Carl

So, the air is smart enough to know what an object is made of and
whether or not it is hollow?

Good to know - thanx.

Er... yes - in a way it is.
Newton's principle does apply, but there are other factors at play, too:

There IS no terminal velocity in a vacuum. However, in the atmosphere,
the relation between projected area and mass [you may call it density]
will determine the difference in terminal velocity of two objects of the
same exact shape.

Lets save the Subaru for now and drop from that plane a) a ping pong
ball and b) a ball of lead measuring 4cm across.

What do you think will happen?

<http://www.grc.nasa.gov/WWW/K-12/airplane/falling.html>

florian /FFF/

 

[Carl 1 Lucky Texan]

next you'll tell me the world is flat and very small rocks can float in
water.

For those of you with Dark Ages understanding of physics - I suggest you
explore the experiments of Galileo.

Carl

 

[Guest]

next you'll tell me the world is flat and very small rocks can float in
water.

For those of you with Dark Ages understanding of physics - I suggest you
explore the experiments of Galileo.

When you have finished digging the hole you are in let us know what size
ladder you need to get out.

 

[Keapon Laffin]

When you have finished digging the hole you are in let us know what size
ladder you need to get out.

f=ma
d==drag force = 1N
drag is the same for both objects, and for simplicity assume it is constant,
1 Newton was used for simplicity as well (any value will give similair
results)
(m1)==mass of object 1 =1kg
(m2)==mass of object 2 =10kg
a==accel due to gravity = 9.8m/s/s
(f1)==net force on object1
(f2)==net force on object2

(f1)=(m1)(a)-d
(f1)=1*9.8-1

(f1)=8.8N

(f2)=(m2)(a)-d
(f2)=10*9.8-1

(f2)=97N

solve f=ma for a
a=f/m

(a1)==accel of object 1
(a2)==accel of object 2

(a1)=(f1)/(m1)
(a1)=8.8/1

(a1)=8.8m/s/s

(a2)=(f2)/(m2)
(a2)=97/10

(a2)=9.7m/s/s

(a1)=(a2)?
8.8m/s/s=9.7m/s/s?
NO

conclusion:
The acceleration in a fluid (air,water,etc) is not the same when the weight
is different but all other factors are the same.

 

[Carl 1 Lucky Texan]

Wow! Thanx Keapon. I stand corrected, that's a fairly big difference.(I
am a victim of public schools). Would 1 Newton be a fair force for an
object falling in the lower atmosphere that had a drag coefficient of
about .4 ?

and - uh - sorry everybody - where's that extension ladder?
Never thought it would be a practical difference.

Carl

 

[Guest]

f=ma
d==drag force = 1N
drag is the same for both objects, and for simplicity assume it is constant,

conclusion:
The acceleration in a fluid (air,water,etc) is not the same when the weight
is different but all other factors are the same.

Don't tell me, tell the guy in the hole.

The acceleration is the same initially (g) until the objects have achieved
enough velocity to create enough drag to make any difference. It is also
the same eventually (zero) when both objects achieve terminal velocity.
Terminal velocity of a lead ball is a little more than double that of a
similar aluminium ball.