Imagine
that the figure at left is a balloon
suspended by a cord and filled with
air under pressure. The sphere will remain
motionless because pressure is being exerted
evenly against all side of the balloon.
The
pressure is equal at points A,B,C,D.
Lets
say now that you punch a hole in the balloon at point A. The pressure at
C, D are still equal and opposite. The force at point b is is equal and
opposite to momentum of the air escaping at A. (A) would be the jet. The
is the forward reaction to the jet which is known as thrust which is known
as thrust. Thrust is measured pounds.
