Question

why do sharp knives needless force then Blender knife for cutting

Answer 1

Sharp knives has a very less area of contact and thus apply more pressure on the object and the object cut very easily

Answer 2

Because sharp edge will shear the surface of the object better than a blunt edge. The sharp edge has smaller surface area and imparts greater pressure for the same applied force. It’s practical in life as well. Try it at your home, take a sharp knife from the kitchen and try to cut some vegetable with it and observe the pressure that you applied. Now take a blunt knife and repeat the same. You'd observe that the sharp knife required lesser force to cut the object and the cut was clean while the cut with the blunt knife would be rough and would require more pressure to cut. Now repeat the experiment with a razor blade, it would require a lesser force than the sharp knife.