posted by Yash .
An open knife edge of mass 200g is dropped from hieght 5m on a cardboard. If the knife edge penetrates 2m into the cardboard, the average resistance offered by the cardboard to the knife
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force*2=.2*g*5 (the potentialenergy of the knife at 5 meters)
force= 50*g Newtons
this is pretty lame cardboard.