Question

To sharpen the blade of a knife by rubbing it against a surface, which of the following will be most suitable?
(A) Stone
(B) Plastic block
(C) Wooden block
(D) Glass block

Answer 1

Hint : High frictional surfaces are more suitable for sharpening the blades of a knife. Harder surfaces tend to be more will be the friction between the surface and the blade.

Complete step by step answer
To sharpen the blade of a knife one rubs it an angle to the surface of a hard material. This hard material sharpens the blade by ridding the surface of dull, corroded particles and leaves the shinier under layer to be exposed. During this process, a very high frictional force thus heat is produced.
The frictional force that can be generated by a material is proportional to the hardness of the surface. Hence, investigating the hardness of each surface will lead to which is highly frictional, and hence will rid the duller particles of the surface of the blade which implies better suitability for sharpening the blades of the knife.
From knowledge stones are generally considered hard, and also depending on the exact substance, are also hard even by scientific standards.
Plastic blocks are easily deformable and thus cannot provide the force required to remove the dull corroded particles from the surface easily. Besides the blades they could cut them rather than get sharpened.
Wooden blocks – relative to plastic wood may be considered hard, however, in general, or compared to stone wood is not a very hard material.
Glass – though brittle glass is somewhat hard by scientific definitions but however less hard compared to stone
Hence, the correct answer is option A.

Note
In general, hardness is not the only factor affecting friction. Other factors include smoothness / roughness of a surface. For example, sand paper is not considered a hard substance, but can be used in sharpening knives due to its extreme roughness.