Question

Why is it easier to cut with a sharp knife than a blunt?

Answer 1

Hint: The cutting power of the knife depends on the pressure exerted by the knife on the object to be cut. Pressure is a scalar quantity and depends on the force applied and area on which force is applied.

Complete step by step answer
The reason for it being easy to cut with a sharp knife than a blunt is as follows. When we cut an object, the knife exerts pressure on the surface of the object. The pressure exerted by the knife is related to the force that we exert as follows:
 $ P = \dfrac{F}{A} $
Here $ F $ is the force that we exert and $ A $ is the area on which the force is exerted. Now, for a sharp knife, the surface area of the contact of the knife with the surface is smaller as compared to a blunt knife.
Since the denominator in the above equation, that area, is smaller, the pressure exerted by a sharp knife will also be higher. As a result, a sharp knife will be easier to cut an object with.

Note
Since knives are sharpened at a certain point, the technique of cutting also determines the ease of using a knife. Pushing the area of the knife that will maintain the least surface area of contact with the object being cut will make it the easiest to cut the object. Regardless, a sharper knife will also be easier to use as compared to a blunt knife. However, sharp blades usually erode with time and become blunt so they should be sharpened every once in a while to maintain their low surface area of contact.