Q. An open knife edge of mass 200 g is dropoed from height 5 m on a cardboard. If the knife edge penetrates distance 2 m into the cardboard, the average resistance offered by the cardboard to the knife edge is :

1. 7N    2. 25N      3. 35N         4. none ofthe above.

plz reply !!

koi to madat koro !

seems Q is easy. But the soln of the book is peculiar.

So I want to chk.

well done anchit !

my only doubt is that u have conserved the energy when the knife goes inside the cardboard assuming all the ke does work but due to friction some energy will be lost as heat energy .what about the account of that energy?

i also thought of the same approach but had the same doubt thats y i didnt post my ans.

Technically speaking your correct,but we wont be able to do anything about that since we don't know the coefficient of friction.So we ignore it.

If not,we should also consider loss of energy due to viscous drag of air,right?

since we dont have pertinent data about this,we ignore it all.