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chapter 9 q. no.59 in second equation there is an external friction force but we apply conservation of momentum how it is possible PLESE HELP

chapter 9 q. no.59 in second equation there is an external friction force but we apply conservation of momentum how it is possible PLESE HELP
 

Grade:12th pass

13 Answers

Neeti
571 Points
8 years ago
which book? Could you attach a photo of your doubt?
ritesh
18 Points
8 years ago
h c verma..........
 
ritesh
18 Points
8 years ago
h c verma..........
 
ritesh
18 Points
8 years ago
h c verma..........
 
Neeti
571 Points
8 years ago
i’m still unable to find it :( could you be a little more specific?
ritesh
18 Points
8 years ago
two blocks 1st is 2kg and 2nd is 4kg separation b/w these 16cm. friction coeff. 0.2.the collision b/w the blocks is perfectly elastic .find separation b/w the two blocks when they come to rest 
Neeti
571 Points
8 years ago
Yeah it says that the collision is perfectly elastic so we’re supposed to conserve momentum but frictional force is there. have you tried the question by applying conservation of momentum?  are you getting the correct answer?
ritesh
18 Points
8 years ago
but mam as we study if there is external force(in these q friction) on the system then there is no conservation of momntum.
ritesh
18 Points
8 years ago
please explain a little bit to get clear concept of conservation of momntum eq.
Neeti
571 Points
8 years ago
Yes Yes i know that. i read a few articles and they state that DURING the collision, (while the collision is happening) , the time interval is very short. the collision happens very quickly so we can assume that \Deltat is very small. 
Now we know that F\Deltat = \Deltap.
since \Deltat is very small, the \Deltap transferred to the earth (due to friction) is negligible and can be ignored.
 
Neeti
571 Points
8 years ago
i hope you got it :) feel free to ask any follow-up question :)
ritesh
18 Points
8 years ago
thank you very very much. i got dT is very small.......thanku....
 
Neeti
571 Points
8 years ago
You are most welcome :) if you have any other doubt, you can always ask. I’d be more than happy to help :)

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