Why are strings of different thicknesses and different materials used in a sitar or violin?

Why oils of different viscosity are used in different seasons?

Why do different planets have different escape speeds?

Why do different people eat different types of food?

The ball bearings used in a machine are of different materials, this is because

Two discs of same thickness but of different radii are made of two different materials such that their masses are same. The densities of the materials are in the ratio of 1 : 3. The moments of inertia of these discs about the respective axes passing through their centres and perpendicular to their planes will be in the ratio of

Why are conductors different from insulators ?