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![[Post New]](/templates/default/images/icon_minipost_new.gif) 21 Aug 2007 20:44:46 IST
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A block is kept on the floor of an elevator at rest.
The elevator starts descending with an acceleration of 12 m/s2.
Find the displacement of the block during first 0.2 s after the start.
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God has given you one face, and you
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~William Shakespeare
You were born an original. Don't die a copy.
~John Mason |
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21 Aug 2007 20:48:06 IST
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the acceleration of the block will be equal to g
S=1/2*10*.2*.2=0.2m
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21 Aug 2007 20:49:18 IST
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what about a=12 , where should we use it?
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God has given you one face, and you
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~William Shakespeare
You were born an original. Don't die a copy.
~John Mason |
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its no need it is given so that we can understand that the acceleration is greater than g and so the block will lose contact and fall with acc g
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21 Aug 2007 20:54:55 IST
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depends on reference framed if earth is reference framed then.....s=1/2at2.....a=10+12=22m/s2....as both are in same direction =1/2*22*0.04=4.4m.......rate if correct or some1 correct me
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21 Aug 2007 20:58:44 IST
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i agree with nishant. Jinav is wrong
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FAILURE IS NOT FALLING IN LIFE BUT NOT RISING AGAIN AFTER FALLING!!!!!!
I LIKE WAVES NOT BECAUSE THEY RISE AND FALL..
BUT BECAUSE EVERYTIME THEY FALL THEY RISE AGAIN!!!!!!!
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21 Aug 2007 21:00:01 IST
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okkk........sorry i got my mistake
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21 Aug 2007 21:00:34 IST
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nishant is rite ... but there is a possibility of the block stickin to the ceilin as accn of lift is gr8er than the block .... so i feel vertical dimension lift must also be taken into consideration
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21 Aug 2007 21:09:36 IST
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How does vertical dimension of the lift affect the answer?
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God has given you one face, and you
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~William Shakespeare
You were born an original. Don't die a copy.
~John Mason |
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21 Aug 2007 21:19:29 IST
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in ur q it doesnt but if consider a longer time period , ull see that the lift travels a gr8er dist than the block. the diff in dist maybe gr8er than the vertical dimension of the lift. in such case the blk sticks to the ceiling and will hve the same accn as the lift. i hope u got it
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Cogito, ergo sum
Life laughs at you when you are sad
Life smiles with you when you are happy
But life salutes you when you make others happy
........... Chalie Spencer Chaplin |
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21 Aug 2007 21:21:01 IST
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yeah.....thanx
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God has given you one face, and you
make yourself another.
~William Shakespeare
You were born an original. Don't die a copy.
~John Mason |
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21 Aug 2007 21:23:03 IST
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Yes the verrtical dimension will come into play if the time is larger because the distance travelled by the lift will always be more than that travelled by the block and after a certain period of time, the difference in distance will be equal to the height of the lift. After that, the block will stick to the ceiling and its acceleration will also be equal to that of the lift. Here, the time is less and so height of the elevator can be neglected.
Hope u understand.
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22 Aug 2007 20:47:14 IST
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The block on the floor of the lift looses contact, the moment it falls with acc. more than g. There fore it is now like a body falling freely. distance moved in 0.2 s is given by 1/2 * 9.8 * 0.22 . Here 12 m/s2 is given only to understood that body falls freely. The answer is the same for any value of acc greater than g.
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22 Aug 2007 23:53:44 IST
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jinav had used some related concept only.
when earth is used as a reference frame then the acc. is g downwards.
so s=1/2*10*0.2*0.2=0.2 m downwards
when the lift is taken as reference then the acc of the block will be the deficent acceleration as compared to that of the lift.
that is 8-12=-4 that is 4 m/s^2 upward.
so s=1/2*4*0.2*0.2=0.08 m upwards from lifts base.
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23 Aug 2007 19:57:57 IST
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