Forces acting on a flying bird

　　Three kinds of forces act on a flying bird ;
　　　　1. Force receiving from the surrounding air as the reaction of bird's
　　　　　movement
　　　　2. Gravity
　　　　3. Force with temporal change of bird's velocity

　　The force receiving from the surrounding air can be resolved two forces,
　if the bird is not turning to sideway ;
　　　　1. Force in horizontal direction (＝thrust−drag)
　　　　2. Force in vertical direction (＝lift)

　　According to the law of fluid dynamics, the force that the bird receives
　from the surrounding air is equal magnitude and opposed direction
　to the rate in which the momentum of the surrounding air is increased
　per unit time by the bird's movement.

　　The bird's flying is the dynamics problem for the compound system
　which consists of the bird and the surrounding air.

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