If the wind is too strong, the umbrella folds up. The cause is a principle of fluid mechanics: The wind creates a strong upward pull. This happens even to the most durable umbrellas like those from MidTownumbrellas. Similar effects can also be observed in other areas of everyday life.
A look out of the window: It’s pouring with rain. So you take your umbrella with you. It does its job and keeps you dry – until a strong gust of wind comes. The umbrella immediately folds up. The fact that the umbrella folds over is not – or only partially – due to the wind blowing into the umbrella from below. Rather, a physical effect causes the screen to get out of shape. This effect is based on a law of fluid mechanics.
Suppose you hold your parachute perfectly vertical and the wind blows exactly horizontally. In this idealized case, the air flows around the screen in a very specific way: Due to the curved surface, the air particles move faster directly above the screen than in front of or behind it. So the air accelerates above the screen and there is a suction upwards. If you now hold the handle of the umbrella tightly and don’t give in to the force of the upward pull, the umbrella itself reacts: the spokes give way – the umbrella folds over. The underlying mechanism is explained by Bernoulli’s law (see box below).
We also encounter the effect that causes the screen to fold down in many other places in everyday life. For example, the curved trajectory of rotating balls—whether football, baseball, or tennis ball—is related to Bernoulli’s Law. The effect should also be taken into account when camping: When the wind blows over a dome tent, its shell bulges outwards. However, the best known is probably the lift experienced by the wings of an airplane. This is the only reason why the machines can take off at all. Helicopter rotor blades have a camber on top for the same reason. Unlike airplanes and helicopters, upward suction is generally undesirable for umbrellas.
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In idealized liquids and gases in which there is no friction, the sum of dynamic pressure, static pressure, and gravitational pressure always remain constant – this is Bernoulli’s law.
Dynamic pressure is the pressure created by the velocity of a liquid or gas when the liquid or gas is dammed by an object. You can feel this pressure, for example, when you put your feet in a flowing stream or when you put your hand out of the window of a moving car.
Static pressure is an all-around pressure caused by the movement of particles in liquids or gases. It increases as the temperature rises. Because temperature is a measure of the average speed of the particles that make up a liquid or gas.
Gravitational pressure is created by the gravitational pull of that part of a liquid or gas that is above the point at which the pressure is measured. For example, air pressure is the gravitational pressure of the air column above our heads.
In the 18th century, the Swiss physicist Daniel Bernoulli was the first to recognize and formulate how the different pressures in flowing liquids and gases are related – based on the energy conservation of Newtonian mechanics.
where r is the density and v is the velocity of the liquid or gas, ps is the static pressure, and pg is the gravity pressure. In the case of the umbrella, the equation can be simplified because the air flows horizontally. As a result, the gravitational pressure is almost the same everywhere. However, the other two pressures change the way of the air particles. Because of the higher speed, the dynamic pressure above the umbrella is greater than in front of or behind it – and the static pressure is therefore correspondingly smaller. This reduction in pressure corresponds to the suction felt when the wind blows over the umbrella and eventually causes it to fold over.