What is (a) body forces (b) surface forces?

A) Body forces-

Body forces is the forces which acts throughout the volume of the body. It is basically distributed over the volume and mass of the element of the body. In a body force other body exerts a force without being contract.

For example : Gravity forces, electromagnetic forces, centrifugal forces.

B) Surface forces-

Surface forces is the forces which are distributed all over the free surface of the body. Surfaces forces can be further divided into two perpendicular components as: normal forces and shear forces.

For example : Pressure forces and viscous forces.