IIT JEE , AIEEE Forum - Physics , Mechanics

priyesh

same confusion i have also got after 20 sec the box becomes massless

then how come ans is comin as 38.8 sec

coz remember no net force can be applied on a masssless body

karthik2007

Amazing sum man. Do post more. Hats off biki!

karthik2007

The thing to be remembered here is that the sledge has some mass. So the mass of the sand will be zero before 20 seconds, and after that, the sledge will move with its own mass.

rhd92781

even if the sledge is considered massless (as assumed in the problem) :
the problem uses change in momentum to calculate force
after 20 s there is no change in momentum so no force applied as we know no force acts on a massless body
but it will have its velocity
so it should be covering the remaining distance with a constant velocity taking a total of 38 sec

madman

wow that was something
the solutioin and the question were brilliant
and yes hats of to both waterdemon and biki

kane

congrats to both waterdemon and biki for solving it.i was just three steps away from the answer myself but being weak in calculus,couldn't find the solution till the end

biki

there should be no confusion............

consider the mass of the box = M

in the equ^n for change in momentum...

( M + 40 - 2t ) v - { M + 40 - 2( t + dt ) } v = Fdt
=> 2vdt = Fdt...gives the same result...

i m modifying my solution....

koooliitian

i think the answer is appx 3.0985sec

waterdemon

Hey Dude,
The question has been already solved and we got the answer.
No prblems Anyways.............

Better luck next time

Cheers!!!!!!!!!!!!!!

laksy

hows tat -2t cming (M + 40 - 2t)v in this eq

waterdemon

Dude,
The sand is leaking out from the sledge at the rate of 2kg/s
as it has been given in th Question.

So the change in mass every sacond will be:
(M+40-2t)
Here M=mass of sledge as assumed by :"BIKI"
40 kg = weight of sand in the sledge.
So totla initial mass=M+40 kg.

And then he has given the conservation of momentum eq:
(M+40-2t)V - M+40-2(t+dt)V = F.dT

Here t+dt as assuming that the mass of sand that has
leaked out in small interval "dt" to be very small.

Hope you find it useful.
Rate if useful.

Cheers!!!!!!!!@@@!!!!!!!!!

laksy

if u dont mind can u just gimme the source..i mean..on net.any link to dis question..
i couldnt find in IIT 92 paper[:o]

yash_pandit

well i got it less than 10 minutes

laksy

Won't velocity also change over the time interval dt? Shouldn't that also be taken into consideration?

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