23okkk tnx
31 Answers
23sir if there is a question that
[q] The Change in KE of a system is equal to work done on the system by the externalas well as internal force.........
True or false????
62the internal force can do no work on the whole system.. but if you break the system into parts, then there will be work on each part......
and becasue there is no work on the system as a whole, kinetic energy of the system does not change...
23sir so my inference is that internal forces can do work ... on the constituent particles of a system ...but it doesnt change the KE of system
62yes.. it is ...
I dont have HCV..
But I hope you are clear with the whole thing now :)
23we take it as internal energy ......
so KE of a system = KE of COM right??
sir actually i m asking so many questions on thid bcz i m not able to interpret properly wat HCV has given on pg 121 left paragraph ........do u hav HCV ??
or i shud write down wat he has said dere?
62Have you studied ThermoD?
Do we consider the kinetic energy of the gas molecules as internal energy or as kinetic energy?
23sir pls tell how do we define KE of a system ??
it is equal to KE of com of KE of constituent particles ............
1@Nishant Bhaiya : wht abt. a man pushing a box in a moving train...Pls explain
62you should consider charged particles.. there is no reason why no...
Internal forces dont do work on the system...
There is something called the total mechanical energy of the system.. that takes into consideration the energies you are talking about.....
If you go thermodynamically.. then what i am saying makes sense..
1For the simple reason that i don't want to get into complex situations in the midst of class 11 .... just a man pushing a box in a moving train will do for me....
23question :
internal force scannot do work ...true or false???
btw aveek ....wy shud i not consder charged particles ??pls explain without charged particles ..tnx in advance
62Work done is frame dependent..
not because the work done by internal forces is zero.. but because some of the forces dont appear at all.. or because of the explanation by XYZ in post 2
work F.x.costheta
x is frame dependent
23okk sir ...i m still not clear.....can u pls giv on easy example where work done by internal forces is zero???
also.....suppose there is a system.....so how wil u define the KE of system ???? the KE of system = KE of COM or KE of constituent particles?
62this is true..
because the work done by internal forces is zero.. so whether or not you include them it will not matter :)
62if you are standing in a moving train .. then work done by friction in the frame of reference of the train will be zero..
but in the ground frame there will be +ve work....
62In that example, the internal energy of a human being goes in speeding him up...
62See work done is heat supplied + change in internal energy...
If we look at the system of two bodies, then internal force will not do any work.. but the internal energy will change due to the internal force..
if we look at the two bodies seperately then the same force will do work and this will be the work done .. while there will be no change in internal energy....
It is a very subtle difference..
23by the way nishant sir ...usaid that a lot of internal energy is used ....... so pls explain where does it go ??
1Sir cud u plzz make it simpler.i mean mathematical proof...cause i've not yet studied thermodynamics
62Precisely.. read the last line.. it goes to change the internal kinetic energy...
If you read some thermodynamics there is a componetnt.. (Internal Energy) that is what is changing..
And that is what i was saying in post #5
1@ â—‹|~_^|â—‹ : Don't consider charged particles....now answer. I think Nishant Bhaiya is right
62but if you are looking at both the masses as a single system.. then you talk about the velocity fo the center of mass.. and not of individual components....
23y not ?
yes ...the internal forces cancel out ...but not the work done ....
consider a system of 2 oppositely charged particles when placed close to each other, when they are released they will exert force on each oder ...and they will get displaced away from each oder ..so is the work done by this internal [ electrostatic] forces = 0??? how ?
23work done by ( internal + external) forces = change in KE
isnt it true?
62No that is a different reason..
The reason is that most of what we study is applicable in point mass systems or rigid mass systems..
Here a lot of internal energy of the body is being used as well..
That goes to the thermodynamic equation..