The engineering science concerned with the relation between the forces acting on fluids (liquids and gases) and their motions, and with the forces caused by fluids on their surroundings. A fluid subjected to unbalanced forces or stresses continues in motion as long as unbalanced forces are applied. For example, in the pouring of water from a pitcher the water velocity is very high over the lip, moderately high approaching the lip, and very low near the bottom of the pitcher. The unbalanced force is gravity, that is, the weight of the tilted water particles near the surface. The flow continues as long as water is available and the pitcher remains tilted.