Take a piece of paper, 1.5 cm wide and 12 cm long, and place one end of
it on your bottom lip. Then, with your top lip only slightly open, blow
a strong horizontal stream of air above the paper. You will
notice that the free end of the paper strip will move upward (from below
the chin to the level of the lips). Yet you might have expected the
paper strip to have been pushed downward by the air above it. What is
going on here? To explain the effect, I must refer to the Bernoulli
principal which, to illustrate a point, is given in two ways:
Version #1 (quoted from on-line sources): “Bernoulli’s principle states that for an inviscid flow of a non-conducting fluid, an increase in speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.”
Version #2: Bernoulli’s principle states that a slow-moving fluid exerts more pressure than a fast-moving fluid.
Both descriptions of Bernoulli’s principle say the same thing, but for my purposes the simpler Version #2 is much preferred over Version #1. Simplicity is something all scientists should strive for. Let me quote Albert Einstein in this regard: “Everything should be made as simple as possible, but not simpler.”
But to get back to our paper strip: When blowing a fast stream of air above the paper strip, the air below the paper is moving relatively slowly. Therefore, according to the Bernoulli principle, the pressure is higher below the paper than above it, with the result that the paper is pushed upward.
Perhaps you have noticed that a curtain surrounding a shower will be drawn into the shower when the water is running. This is another example of Bernoulli’s principle. Air, pulled along by the stream of fast-moving water, exerts less pressure than the stationary air outside the curtain. Hence the curtain moves inward from high pressure to low pressure.
z Now consider the shape of the airplane wing called an airfoil. It is flat on the bottom surfacebut curved at the top surface (see Scheme). As the plane moves forward, the air moves backward both under and over the wings. But the air moving over the curved top has a longer pathway to travel than across the flat underside of the wing. Therefore the “top” air must move faster or otherwise severe air disturbances would be created. According to Bernoulli, there is higher pressure from the bottom of the wing, and the plane experiences an upward lift.
Version #1 (quoted from on-line sources): “Bernoulli’s principle states that for an inviscid flow of a non-conducting fluid, an increase in speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid’s potential energy.”
Version #2: Bernoulli’s principle states that a slow-moving fluid exerts more pressure than a fast-moving fluid.
Both descriptions of Bernoulli’s principle say the same thing, but for my purposes the simpler Version #2 is much preferred over Version #1. Simplicity is something all scientists should strive for. Let me quote Albert Einstein in this regard: “Everything should be made as simple as possible, but not simpler.”
But to get back to our paper strip: When blowing a fast stream of air above the paper strip, the air below the paper is moving relatively slowly. Therefore, according to the Bernoulli principle, the pressure is higher below the paper than above it, with the result that the paper is pushed upward.
Perhaps you have noticed that a curtain surrounding a shower will be drawn into the shower when the water is running. This is another example of Bernoulli’s principle. Air, pulled along by the stream of fast-moving water, exerts less pressure than the stationary air outside the curtain. Hence the curtain moves inward from high pressure to low pressure.
z Now consider the shape of the airplane wing called an airfoil. It is flat on the bottom surfacebut curved at the top surface (see Scheme). As the plane moves forward, the air moves backward both under and over the wings. But the air moving over the curved top has a longer pathway to travel than across the flat underside of the wing. Therefore the “top” air must move faster or otherwise severe air disturbances would be created. According to Bernoulli, there is higher pressure from the bottom of the wing, and the plane experiences an upward lift.
Scheme: Schematic of an airplane wing. Airplane is moving from right to left and, relative to it, the air is moving from left to right.
Bernoulli is not the only explanation for the lift on a plane’s wing.
Isaac Newton also enters the picture. Note that the air stream on the
rear part of the wing’s top surface is directed downward. This serves to
push the plane upward because, according to Newton, “every action has
an equal and opposite reaction”. A jet engine operates by the same
principle. Thus the jet ejects its exhaust with a great force in a
backward direction, thereby pushing the airplane forward with an equal
and opposite force.
Question: In baseball, pitchers give the ball a spin that causes the ball to curve to the left or right rather than follow a “straight” route (making the ball much harder to hit). What causes this curvature?
Answer: A thrown ball experiences air-flow in the opposite direction. Spinning drags air along the surface of the ball, adding to the air-speed on one side of the ball, while subtracting from the air-speed on the other side of the ball. Bernoulli postulates that the ball will move toward the side experiencing the great air velocity.
Question: In baseball, pitchers give the ball a spin that causes the ball to curve to the left or right rather than follow a “straight” route (making the ball much harder to hit). What causes this curvature?
Answer: A thrown ball experiences air-flow in the opposite direction. Spinning drags air along the surface of the ball, adding to the air-speed on one side of the ball, while subtracting from the air-speed on the other side of the ball. Bernoulli postulates that the ball will move toward the side experiencing the great air velocity.
Fonte: http://www.ccell11.com/2016/04/t96.html#ING
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