"Stress is the force per unit area." Is it correct?

The velocity v of sound waves, through a medium may be assumed to depend on density of the medium, d and Modulus of elasticity (E) . Modulus of elasticity is a ratio of stress to strain and stress is force per unit area. Deduce by the method of dimensions the formula for the velocity of sound.

Prove that the parallel conductors of infinite length, carry current in the same direction, attravt each other. Deduce the expressions for the force per unit length experienced by each conductor.

Friction is a self- adjusting force. Is it correct?