top speed

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. .. :)

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 wrote:

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.

LOLOL

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

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

k. ote wrote:

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/

^^^^^

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

florian /FFF/

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.

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

Florian Feuser /FFF/ wrote:

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

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?

florian /FFF/

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.

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

Florian Feuser /FFF/ wrote:

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

nospam wrote:

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.

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

Keap>

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.