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Александр Попов:
30.04.2009, 01:39
Хочу поделиться тем, что в убеждает, а что - нет.
1. Снимок Кагуи не убеждает, потому что на нём нет пространственного масштаба. Вполне возможно, что маштаб просто очень мелок для того, чтобы разглядеть остатки оборудования и остаток лунного модуля. Ведь окружающие горы или холмы смотрятся мелко, а они-то гораздо больше того, что хотим увидеть.
2)Фотографии с луномобилем, который ездит по Луне, не оставляя следов, очень наглядны;
3)Незапылённая опора модуля не так наглядна. Пыль в безвоздушном пространстве не клубится, а улетает прочь "по ветру" от сопла. Вот она и улетела.
А, в целом, материал очень полезный. Желаю его автору успехов.
А.Попов., д.ф.-м.н., автор книги "Американцы на Луне: великий прорыв или космическая афёра?"

 

Y.K.>> Ньютон тут сказал бы, что этим банкам незачем что-то отбрасывать - они движутся навстречу друг другу и общий центр масс стоит на месте

3-62> Но иногда понять сэра Исаака непросто.

3-62> Воробей привел еще мувик. С одной банкой. Видимо он настаивает на подходе "то что не замечаем - за материальное тело не считаем"

3-62> Подмывает спросить - в случае с одной банкой - законы Ньютона не срабатывают, разве? И нету массы воздуха "слева", движущейся навстречу банке "справа" так, что ЦМ остается на месте?

Changes in Speed or Direction

Lift is a force generated by turning a flow. Since a force is a vector quantity (like the velocity), it has both a magnitude and a direction. The direction of the lift force is defined to be perpendicular to the initial flow direction. (The drag is defined to be along the flow direction.) The magnitude depends on several factors concerning the object and the flow.

 

Lift is the force that holds an aircraft in the air. How is lift generated? There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on Web sites. Unfortunately, many of the explanations are misleading and incorrect. Theories on the generation of lift have become a source of great controversy and a topic for heated arguments for many years.

The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas. Notice that we place the names in quotation marks because neither Newton nor Bernoulli ever attempted to explain the aerodynamic lift of an object. The names of these scientists are just labels for two camps.

...

Which camp is correct? How is lift generated?

When a gas flows over an object, or when an object moves through a gas, the molecules of the gas are free to move about the object; they are not closely bound to one another as in a solid. Because the molecules move, there is a velocity associated with the gas. Within the gas, the velocity can have very different values at different places near the object. Bernoulli's equation, which was named for Daniel Bernoulli, relates the pressure in a gas to the local velocity; so as the velocity changes around the object, the pressure changes as well. Adding up (integrating) the pressure variation times the area around the entire body determines the aerodynamic force on the body. The lift is the component of the aerodynamic force which is perpendicular to the original flow direction of the gas. The drag is the component of the aerodynamic force which is parallel to the original flow direction of the gas. Now adding up the velocity variation around the object instead of the pressure variation also determines the aerodynamic force. The integrated velocity variation around the object produces a net turning of the gas flow. From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

What is the argument?

Arguments arise because people mis-apply Bernoulli and Newton's equations and because they over-simplify the description of the problem of aerodynamic lift. The most popular incorrect theory of lift arises from a mis-application of Bernoulli's equation. The theory is known as the "equal transit time" or "longer path" theory which states that wings are designed with the upper surface longer than the lower surface, to generate higher velocities on the upper surface because the molecules of gas on the upper surface have to reach the trailing edge at the same time as the molecules on the lower surface. The theory then invokes Bernoulli's equation to explain lower pressure on the upper surface and higher pressure on the lower surface resulting in a lift force. The error in this theory involves the specification of the velocity on the upper surface. In reality, the velocity on the upper surface of a lifting wing is much higher than the velocity which produces an equal transit time.

...

Newton's laws of motion are statements concerning the conservation of momentum. Bernoulli's equation is derived by considering conservation of energy. So both of these equations are satisfied in the generation of lift; both are correct.

 

Y.K.>> Лучше он сказал бы - он по-прежнему настаивает на своем "ящике в 13 метров высотой" и считает крыло чем-то вроде трубы, из которой вниз бьет воздух?

3-62> Тайна сия - велика есть. Но, надеюсь, выпытаем подноготную у Воробья, чай он не Джек-воробей, неуловимый капитан.