An open knife edge of mass m is dropped from a height h on a wooden floor. If the blade penetrates upto the depth d into the wood, the average resistance offered by the wood to the knife edge is,
$ v^2 - V_0^2 = 2 ad , (v =0 ) and V_0 = \sqrt { 2gh } $ $ \therefore o = ( \sqrt { 2gh} )^2 + 2 (g-a) d \therefore a $