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Posted by stk on Oct-23-2004 17:53:

for you physics nerds out there

doed anyone know how to do this:

problem: a 1350 kg car is moving at 23m/s. the driver brakes to a stop. the brakes cool off to the temperature of the surrounding air, which is nearly constant at 15 celsius, what is the total entropy change?

Posted by Lil-H on Oct-23-2004 18:07:

Hmm, we havnt learned that yet in my physic class:
But I do know that Fres=ma
m: 1230kg
a: negative, because it stops? ~ -23m/s
= 31050 N =31kN thats how much it needs to stop..
Thos might be very wrong, and not even what youre looking for..

If anyone told med physics would be this hard, I wouldn't have take it !

Posted by varun on Oct-23-2004 19:31:

Funny...
what does thermodynamics have to do with kinematics?

Whatever shit you've provided is completely un-related and an answer cannot be provided from the info given.

Posted by RenderedDream on Oct-23-2004 19:33:

quote:
Originally posted by Lil-H
Hmm, we havnt learned that yet in my physic class:
But I do know that Fres=ma
m: 1230kg
a: negative, because it stops? ~ -23m/s
= 31050 N =31kN thats how much it needs to stop..
Thos might be very wrong, and not even what youre looking for..

If anyone told med physics would be this hard, I wouldn't have take it !
�

a is not -23 m/s..that's the speed
to calculate a u would need info about the time to stop or the value of something to calcula Fres
i didn't learn what entropy was so i can't help =/

Posted by Chris d(-_-)b on Oct-23-2004 20:44:

quote:
Originally posted by varun
Funny...
what does thermodynamics have to do with kinematics?

Whatever shit you've provided is completely un-related and an answer cannot be provided from the info given.

Agree!

Posted by M1cro5lave on Oct-23-2004 21:00:

entropy is a quantitative measure of disorder.

The entropy change is defined as the entropy change of the hot body minus the entropy change of the cold body.

Stot = Shot - Scold

I can't see right now how this makes any sense with the stated question...

Posted by Lil-H on Oct-23-2004 21:46:

23m/s is V0(start speed) right? V (end speed)

however, good luck..

Posted by Romain on Oct-23-2004 21:49:

quote:
Originally posted by M1cro5lave
I can't see right now how this makes any sense with the stated question...

neither can i...

Posted by Phil r on Oct-23-2004 22:32:

quote:
Originally posted by Lil-H

If anyone told med physics would be this hard, I wouldn't have take it !

angon, you don't have to be TOO smart to figure out that physics is going to be hard (sorry, no offence etc )

and 2 the question....don't you need to know what the braking material is made of, how fast it heats up, and all sorts of other stuff?

ps - i have no idea, i did electronics at uni and now work in a computer storage company :s

Posted by THE_Chris on Oct-23-2004 23:20:

I'll have another look at this tomorrow if I get the chance. I have 50 problem sets to correct, a lab report to do, and two of my own problem sets to do for Tuesday.

A (very) quick look at this problem gives ->

dE=1/2 mdv^2 <-Energy of the car at the start, assume end energy is 0.

dS= (-1/T1 + 1/T2)dE <- Change in entropy = That relation between temperatures at 1 and 2, multiplied by the energy change.

T2 is given, but T1 is not. Either I need another linking equation or I'm barking up the wrong tree.

Unfortunatly, all I've done through college along these lines is statistical thermodynamics, which is difficult to tie in with real world scenarios like this.

Posted by Ek0nomik on Oct-24-2004 04:48:

Hrm, I'm in AP Physics and we haven't touched Entropy. We might in the future perhaps... Seems like an easy problem though, just get a basic understanding or equation on how to figure out what entropy is and plug in / convert your numbers.

Posted by EriK_V on Oct-24-2004 06:17:

quote:
Originally posted by Ek0nomik
Hrm, I'm in AP Physics and we haven't touched Entropy. We might in the future perhaps... Seems like an easy problem though, just get a basic understanding or equation on how to figure out what entropy is and plug in / convert your numbers.

ahh yes, ap physics b and c. those were the days