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*distance= work
force*2=.2*g*5 (the potentialenergy of the knife at 5 meters)

force= 50*g Newtons
this is pretty lame cardboard.