Question

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

• A. $7N$
• B. $25N$
• C. $35N$
• D. $None$

Answer 1

The correct option is A $7N$

We can use work energy theorem ,

Total kinetic energy lost is $K=mg(h+d)$

Work done by the resistance force $F$ is $W=Fd$

By equating , we get $F=\frac{mg(h+d)}{d}$

$\Rightarrow F=\frac{0.2\times 10(2+5)}{2}=7N$

Therefore the correct answer is $A$