A) \[8N\]
B) \[0\,N\]
C) \[2\times {{4}^{2}}N\]
D) \[\frac{1}{2}N\]
Correct Answer: B
Solution :
The resistive force offered by the wood to the blade should be able to balance the weight of the knife and should be able to retard the kinetic energy gained by the knife in falling through height\[h\]. Potential energy in the knife when it is at a height \[h=mgh\] This is also the kinetic energy with which the knife strikes the wooden floor. If the blade penetrates distance S into the wood, then by work energy theorem kinetic energy lost \[=F\times S\]or\[F=Mg\frac{h}{s}\] Thus, total retarding force\[=Mg+Mg\frac{h}{s}\]
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