A vehicle moving through a fluid (such as air or water) is acted on by four forces. Animals moving through fluids experience the same forces. In the case of a bird or airplane, the forces are lift, weight, thrust, and drag. Weight is the force pulling the object to the earth, which is countered by lift to keep the airplane or bird in the air. Likewise, drag is the force resisting the motion of the bird or airplane through the air, while thrust is the force countering the effects of drag to keep the bird or airplane moving. The same statements can be made for submarines and fish, except that there is an additional vertical force in these cases, namely, buoyancy. In this overview, we are interested propulsion, so we will focus on the balance between thrust and drag. To understand how these forces affect vehicle motion, we must first review Newton’s laws of motion…