Define the following terms: mass-average velocity, diffusion velocity, stationary medium, and moving medium.

Define the following terms used in fluid mechanics and mass transport: 1) Mass-average velocity: Mass-average velocity is the weighted average of individual velocities of particles in a fluid mixture, considering their respective masses. It is calculated using the formula: V_m = (Σ m_i v_i)/(Σ m_i), where m_i is the mass of the particle i, v_i is the velocity of particle i, and n is the total number of particles in the mixture. 2) Diffusion velocity: Diffusion velocity is the velocity of a particle or species in a mixture due to concentration gradients, causing particles to move from high concentration areas to low concentration areas. Fick's law is used to determine the diffusion velocity: V_d = -D (dC/dx), where D is the diffusion coefficient, C is the concentration, and x represents the distance in the direction of diffusion. 3) Stationary medium: A stationary medium is a fluid or mixture of particles that are not in motion and have an average velocity of zero. In a stationary medium, properties like pressure, temperature, and concentration are constant, and the concentration gradients drive the movement of particles or species. 4) Moving medium: A moving medium is a fluid or mixture of particles with a non-zero average velocity. In a moving medium, mass transport occurs due to both advection (bulk movement of particles in the flow direction) and diffusion (movement due to concentration gradients). The combined effect of advection and diffusion determines the overall mass transport in a moving medium.