ABSTRACT
Newton’s Second Law states that the net force acting on a body in any fixed direction is equal to the rate of increase of momentum of the body in that direction. This chapter aims to derive a relation by which force may be related to the fluid within a given space. It begins by applying Newton’s Second Law to a small element in a stream-tube. The flow is steady and so the stream-tube remains stationary with respect to the fixed coordinate axes. A stream of fluid is formed, which moves over the surface until the boundaries are reached, and the fluid then leaves the surface tangentially. The change of fluid momentum is produced by a fluid-dynamic force additional to this static force. For a pipe-bend with a centre-line not entirely in the horizontal plane the weight of the fluid in the control volume contributes to the force causing the momentum change.
