y does a liquid flow accelerates when it passes thru a contraction?

Answer 1

fluid dynamics states that the flow rate must be uniform
if you have a smaller pipe, then the liquid will flow faster

Answer 2

I think you may be describing the venturi effect. The liquid - or air - actually creates a lower pressure as it accelerates through a slight contraction. It's kind of counter-intuitive, but that's what it does in reality.
Fits right in with other scientific facts that just seem intended to confuse an otherwise logical conclusion - for example, hot water freezes faster than cold water. It's these little quirks of science and engineering that make it interesting.

Answer 3

Pressure = force per unit area

the smaller the area (contraction) the higher the pressure thus the flow accelerates

Answer 4

Liquid flow is incompressible, so flow through any part of the pipe is equal - you can't 'pile up' liquid in the system. The volume of liquid flowing through the wide part of the pipe is the same as the volume of liquid flowing through the narrow part. Volume of flow equals speed of flow times cross-sectional area of flow. To get the same volume through a smaller cross-section, the flow has to speed up.

Answer 5

Conservation of energy

The forces are the same.
F = mA
Setting the equations equal
mA = mA
Thus, a change in mass causes a change in acceleration.

That applies to velocity as well.
P = mV
mV = mV (Setting the equations equal)
Here again, the mass changes, so there is a change in Velocity.

Force is the same.
A 1000lb car going 5 miles/hr has the same power as a 5lb object travelling at 1000 miles/hr
F = mA
mA = mA
(1000)5 = 5(1000)