Using the dependence of pressure on speed in moving gases and liquids in technology
In a moving gas or liquid the pressure is lower where the speed is higher. If you blow between two vertically held sheets of paper, the air between them speeds up and its pressure drops, so the outer pressure pushes the sheets together. An aircraft wing is shaped so that the air passes faster over its upper side; the pressure above is lower than below, and the pressure difference gives an upward lift force. Air around a spinning ball or cylinder moves faster on one side than on the other, and the pressure difference deflects it sideways – the Magnus effect (a curving kick in football). For a hole at depth h below the surface in an open vessel, Bernoulli’s equation gives υ = √(2gh) (Torricelli’s formula): the speed equals that of a body falling freely through h. The same law underlies the spray bottle, the carburettor and the Venturi tube.
Only in class with the teacher: hold two sheets of paper 3–4 cm apart, blow gently between them and explain why they come together. Do not do it near an open window or other people.