An example of the application of Bernoulli's equation is the Venturi tube. It is used to measure the velocity of a liquid in pipes. The tube is slightly tapered on a short section (see figure). As a result, the pressure in this part is slightly less than the pressure in the wider part of the pipe. With the help of a mercury manometer, we can measure the pressure difference between the two parts of the pipe and calculate the flow velocity.
The heights of the mercury manometer columns (see figure) differ by 7.5 cm. What is the velocity of the water in the pipe? The cross-sectional areas of the pipe are 
and 
the density of mercury is 
and that of water is 
.
Venturi tube
