Physics
posted by Zion .
A stream moving with a speed of 0.5 m/s reaches a point where the crosssectional area of the stream decreases to onefourth of the original area. What is the water speed in this narrowed portion of the stream?

In steady state flow, which this is, the product of velocity and cross sectional area is a constant that is equal to the volume flow rate.
If the cross sectional is reduced to 1/4 the former value, the flow velocity must increase by a factor of 4.
A*V = constant is called the "continuity equation"