Tom Hutchins of Auckland asks :-
Bernoulli's Theorem predicts that air flowing around objects will exert the lowest static pressure where the air flow velocity is highest, and the highest static pressure where the velocity is least. What common examples demonstrate these forces?
Marlyn Jakub, a physicist at the University of Otago, responded.
Place a small nail (as a centering piece) upright through a piece of light cardboard. Place a cotton reel over the nail and blow hard. You will now be able to lift the cotton reel and the cardboard will follow. Force is pressure multiplied by the area. The Bernoulli-type lift force can exceed all downward forces due to gravity plus the deflected airflow in the small area below the hole.
An upright umbrella develops lift forces during high winds and may blow out or up and away. The air moving over the top surface produces a lower pressure than the slower airflow under the umbrella canopy. Similar conditions explain lift forces for thrown Frisbees and for a house roof during very high winds.
An open door will often slam shut due to a draft moving past one side. The pressure is higher on the side nearest the wall where the air is stationary.
Another example can be seen by blowing air between two sheets of paper. Initially they draw closer together rather than moving apart.
Blow across the end of a straw standing in a liquid. This lowers the pressure inside the straw and raises liquid up the straw. This method for raising fluid in tubes is used in carburettors in older cars, in perfume bottles with squeeze bulbs, and in some squeeze-handle spray containers.
To achieve lift with low drag force, a wing aerofoil uses a small angle of attack to cause air circulation. This slows air below the wing by moving air upward in front of the wing, thus producing faster airflow above the wing.
With spinning balls (eg, golf, tennis, ping-pong, cricket, soccer) the surface friction can set up air flows around the ball, leading to lift or curved trajectories.