In this article, the strong equivalence principle is analyzed by the gravitational mechanism of etheric pressure, and the conclusion is drawn that an object at rest in a uniform gravitational field of a certain strength is equivalent to moving in a straight line in a uniform etheric space with a certain speed (not a certain acceleration). According to this gravitational mechanism, gravitational field is a scalar (energy) field, and gravity is caused by the asymmetric density distribution of space energy, resulting in the asymmetric etheric pressure on the object at rest in space. The experimental fact of equivalence between gravity fields and velocity (rather than acceleration) in terms of time dilation has proven the equivalence between uniform gravity fields and inertial systems with uniform motion. It further deduces that gravity fields are equivalent to velocity in terms of the increase in the inertial mass effect of objects, and gravity fields are equivalent to velocity in terms of the contraction effect of the electron orbit radius of atoms. The Ether (energy) field with a gradient of field strength generated by a fluid whose flow velocity decreases perpendicular to its motion is equivalent to the gravitational field with a gradient of field strength. Using these modified and extended equivalence principles, the author explains Newton's bucket experiment, the Casimir effect, and explains what reference system the flow rate of fluid in Bernoulli equation is relative to, and why the higher the flow rate is relative to the reference system, the lower the pressure.