[Music] what will happen if we create a strong air flow between two ballls suspended on threads it seems that the flow should push the balls apart we switch on the blower and surprisingly the balls get attracted by one another they are rotating which means the air flow is getting through between them why isn't it pushing them apart let's carry out another experiment we attach a funnel to a hose and switch on the blower then we put a rubber ball into the funnel instead of bouncing out of the funnel it gets sucked into it we turn the funnel upside down but the ball isn't falling out of it so why does the airflow hold the ball in the funnel instead of pushing it out a similar experiment can be conducted with running water if we put a tennis bow next to it the bowl sticks to the water and hangs there on an inclined thread it looks like a fast flow of gas or liquid possesses counterintuitive properties which need to be explained to do so let's consider water flowing through a pipe of variable cross-section when the cross-sectional area of the pipe decreases the speed of water flow increases according to Newton's second law in order to increase the speed some Force must be applied to water thus the pressure in The Wider part of the pipe must be higher than that in the narrower one therefore when the pipe gets narrower the pressure in it will not rise as it may seem at First Sight but fall this puzzling paradx is called berul principle in honor of its Discoverer a Swiss physicist Daniel B newly we made this narrow neck pipe from two plastic glasses when we blow it the air pressure at the exit will be equal to the atmospheric one this means that inside the narrow neck it will be lower than the atmospheric one to check this let us insert a tube into the wall of the neck connecting it to a pressure sensor when we blow the pipe the sensor shows the decrease in pressure by 5 kilop Pascal which is 50 cm of water column let us put the lower end of the tube into the vessel with colored water and blow the pipe again we can see that the water makes a 30 cm rise to increase the pressure even greater we'll connect the pipe to the blower which provides a far higher speed of the air flow we switch on the blower and what we get is a perfect water sprayer now let us attach a plastic bottle to the pipe and switch on the blower again that's incredible a decrease in the internal pressure causes the bottle to be crumpled by the outer atmospheric pressure let us consider water flowing inside the cross-section pipe the volume of water V enters the pipe from the left hand and leaves the pipe on its Right End the difference of pressures at the ends of the pipe does mechanical work which equals P1 minus P2 multiplied V when there are no losses all this work goes to increase the kinetic energy of water mass is the product of volume multiplied by density in this case the volume decreases so we can rewrite the formula in the following way the value of row v² divided by two is called the velocity head of a fluid according to Bern's principle the sum of pressure and velocity head in an ideal fluid remains constant throughout the whole pipe this equation allows to measure the speed and volume consumption of a gas gas or fluid flowing through the pipe to do this a special device is inserted into the pipe it is called Venturi tube by the name of its inventor a differential pressure sensor is attached to the tube it measures the difference of pressure between the narrow and the wide sections of the pipe the speed of flow calculation is based on this difference let us switch on the blower the device shows that 12 L of air get through the pipe every second this means that if the section of the pipe is 3 square cm the speed of air within it is 40 m/s industrial Ventures look much more substantial they can measure the consumption of gas flowing through the pipeline with minor pressure losses [Music]