Question

A needle is require for stitching cloth. We use a knife for cutting fruit. If the tip of the needle or the edge of a knife becomes blunt, then the needle does not penetrate the cloth and the knife does not cut into the fruit. Why this is so?

Answer 1

Here, pressure is inversely proportional to Area

Pressure = ${\huge{\frac{Force}{Area}}}$

Thus, when area of knife increases ( blunt) , pressure on the object decreases. Hence, we are not able to pierce the object with blunt knife.

Answer 2

Because as needle and knife become blunt their area increases and the force along the needle or knife which acts to stichting and cutting fruit will decreases so it may use our more power to do work.