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 2 years ago
a block of mass 1kg is placed on a plank of mass 2kg, length of plank = 2m. Coefficient of friction between plank and block is 0.5 and the round on which the plank is kept is smooth.
A constant force F=30N is applied on the plank; the time after which the block separates from the plank =?
 2 years ago
a block of mass 1kg is placed on a plank of mass 2kg, length of plank = 2m. Coefficient of friction between plank and block is 0.5 and the round on which the plank is kept is smooth. A constant force F=30N is applied on the plank; the time after which the block separates from the plank =?

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shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0dw:1356509438621:dw

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0concepts of physics?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0nthin i thot it was from hc verma well i think i missed smthin fk=5N? why wud they seperate?

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0pseudo force i'd say..

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0and i dont know from which book this ques came from,,it was asked in a test..

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0yup and wat's the acceleration? 10*1

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0shouldnt it be 30 fk = 30  5 ?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0i dont think fk counts initially they both move as a single body

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0i see,,i didnt consider that.. anyways, how does it matter? the question will begin only when the block starts moving..

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0i mean the solution will begin* also, fk isnt high enough relative to the force, so shouldnt it start moving at t=0 only ?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0opps sorry u r right nd u almost got it

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0well i'll show you my attempt..just that i didnt get the right ans..

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0find the independant acc's of both? and find the replative acc and use that goddamn formula

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0wait, can you confirm to me if the normal reaction acting on the block is 1*10N or (1+2)*10 N ?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0wat's block 1? anyways i think i understand ur confusion well i guess it is 10 but u jus need to find fk....and draw fbd with the horizontal forces

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0allow me to try accn of block w.r.t to ground = 30 5 /1 = 25 accn of plank w.r.t to ground = 30  5 /2 = 12.5 relative accn = 37.5 is that right?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0idk...gime sm time i feel giddy

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0i still find something fishy in my solution.. hmm :

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0how do you say force on block 1 is 10 N? shouldnt the pseudo force be 30 N ?

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0as far as i kno pseudo is ma

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0well,,i'd argue there,,i maybe wrong,, anyways, that still doesnt yield the right ans, so got to be some flaw there..

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0ohh u from fiitjee?

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0am dropping my year for jee and ISI and CMI etc, living here in a remote place, just a small tuition like coaching here, no fitjee,resonance,akash here ,nearby i mean..

A.Avinash_Goutham
 2 years ago
Best ResponseYou've already chosen the best response.0ohh i get it....it's okay......u have a better chance than any1 in here nd ur question....sry it's been long since i worked on these.........so i donno if i can help

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0hmmm ..nevermind,,thanks for trying sir! :)

sauravshakya
 2 years ago
Best ResponseYou've already chosen the best response.0dw:1356521627848:dw

sauravshakya
 2 years ago
Best ResponseYou've already chosen the best response.0So, accleration of the block with respect to the plank = 25/1 = 25ms^2

sauravshakya
 2 years ago
Best ResponseYou've already chosen the best response.0Now, s=ut+1/2 at^2 2=0+1/2 25 t^2 t^2=100 t=10seconds

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0@sauravshakya you missed out on the fact that the plank also moves, so the block wont have to cover 2m completely, if you get what i mean.

Diwakar
 2 years ago
Best ResponseYou've already chosen the best response.1When the block starts to move the maximum friction acts on it i.e.5N Then we draw the free body diagram as shown in the file attached. Note that we work in the frame of the plank because it is easier. Pseudo force is given by "ma". Where m is the mass of the body in concern(i.e. in this case 1kg) and "a" is the acceleration of the frame(i.e. the plank). We get the free body diagrams for plank and block as shown. Eqn of motion for plank is 305=2a giving a=12.5m/s/s Eqn of motion for block is 1*a5=1*A where A is the acc of the 1kg block Inserting the value of "a" gives A=7.5m/s/s Length of plank is 2m Using 2nd eqn of motion we get time to cover 2m =0.73s(approx) ***DO TELL THE CORRECT ANSWER PLEASE**

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0thats where i went wrong, so it was a conceptual problem from my side..thanks a lot @Diwakar yep, thats the correct ans

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0just confirm me this, taking into account the pseudo force means A is the relative accn of block right ?

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0relative to the plank..

Diwakar
 2 years ago
Best ResponseYou've already chosen the best response.1Yes, because we are working in the frame of the plank.

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0i understand this concept a lot better now..thanks..

Diwakar
 2 years ago
Best ResponseYou've already chosen the best response.1acceleration w.r.t. ground can be found out as aA=12.57.5=5

Diwakar
 2 years ago
Best ResponseYou've already chosen the best response.1.where a and A are the magnitudes. we used  sign because they point in opposite directions.

shubhamsrg
 2 years ago
Best ResponseYou've already chosen the best response.0yep..again,,thanks a ton! :)
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