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Question-66621




Question Number 66621 by lalitchand last updated on 17/Aug/19
Commented by lalitchand last updated on 17/Aug/19
8 iii
8iii
Commented by mr W last updated on 18/Aug/19
answer given in book for Q 8(ii) is wrong.    Q 8(iii) in book is not clear. if weight  of 1 kg (stone) is 10 N in water, then  the density of water must be zero,  no matter what density the stone has.  the answer in book is 10g/cc, but no  stone in this world can have such a  large density.    ⇒book is bad
answergiveninbookforQ8(ii)iswrong.Q8(iii)inbookisnotclear.ifweightof1kg(stone)is10Ninwater,thenthedensityofwatermustbezero,nomatterwhatdensitythestonehas.theanswerinbookis10g/cc,butnostoneinthisworldcanhavesuchalargedensity.bookisbad
Commented by mr W last updated on 18/Aug/19
question is wrong, answer is wrong.  answer to question 8(ii) is also wrong.
questioniswrong,answeriswrong.answertoquestion8(ii)isalsowrong.
Commented by lalitchand last updated on 18/Aug/19
some one says answer is 4g/cm^3
someonesaysansweris4g/cm3
Commented by lalitchand last updated on 18/Aug/19
That means question is wrong
Thatmeansquestioniswrong
Commented by mr W last updated on 18/Aug/19
a correct question 8(iii) could be:  if the weight of 2 kg stone is 10 N in  water, what is the density of the stone?    the answer were 2 g/cm^3 .    solution:  let V=volume of stone, density is ρ,  mass is m.  V=(m/ρ)  uptrust given by water is ρ_W gV  weight of stone in water is then  G_W =mg−ρ_W gV=mg(1−(ρ_W /ρ))  with m=2 kg, G_W =10 N, ρ_W =1 g/cm^3   2×10(1−(1/ρ))=10  ⇒1−(1/ρ)=(1/2)  ⇒ρ=2 g/cm^3
acorrectquestion8(iii)couldbe:iftheweightof2kgstoneis10Ninwater,whatisthedensityofthestone?theanswerwere2g/cm3.solution:letV=volumeofstone,densityisρ,massism.V=mρuptrustgivenbywaterisρWgVweightofstoneinwateristhenGW=mgρWgV=mg(1ρWρ)withm=2kg,GW=10N,ρW=1g/cm32×10(11ρ)=1011ρ=12ρ=2g/cm3
Commented by mr W last updated on 18/Aug/19
you can also see that it is impossible  that 1 kg stone has a weight of 10 N  in water, because that would request  1×10(1−(1/ρ))=10  but this is not possible. therefore  the question in the book is wrong.
youcanalsoseethatitisimpossiblethat1kgstonehasaweightof10Ninwater,becausethatwouldrequest1×10(11ρ)=10butthisisnotpossible.thereforethequestioninthebookiswrong.

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