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Table of contents
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Table of contents

0:00
Точка Торричелли и потенциальная энергия
1:12
Минимум в геометрии и принцип Ферма
2:51
Инцентр и теорема о трех силах
4:25
Метод масс и правило рычага
5:45
Еще одна красивая теорема и титры
Video tags
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Video tags

наука
научпоп
математика
образование
3Blue1Brown
wild mathing
геометрия
Савватеев
школа
11 класс
формула
теорема
доказательство
физика
GetAClass
эксперимент
теорема о трех силах
Торричелли
оптика
Vectozavr
Subtitles
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  • ruRussian
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00:00:01
[music]
00:00:04
where to mark a point inside the depicted
00:00:06
triangle so that the sum of the
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distance from it to the vertices is
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minimal we have already solved this problem
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in three ways,
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but today there will be 4 no less surprising
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imagine that the outgoing segments of the
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yellow node are not those that stretch to the
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vertices in the vertices themselves we will do
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holes through which we tie 3 weights of
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equal mass as in the figure, as a result
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the system will come into equilibrium with
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minimal potential energy, this
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means that the vertical threads
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will have the greatest length in total, then the sum of the
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lengths from the node to the vertices will be minimal
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and we have solved the famous Steiner problem,
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note that the forces acting to the knot
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are directed along the threads are equal in magnitude and
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in total they neutralize each other, the
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sum of any two vectors is equal to 3, all of
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this with a minus sign, this is
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only possible if the angle between any two
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vectors is 120 degrees,
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now another difficult creative
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task, two points are marked inside an acute angle
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where on the sides of the angle he will put
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two new points so that the length of the yellow
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polyline is minimal
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[music]
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let’s, as in the background problem, let’s
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mirror the fixed point
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relative to the sides of the angle, then we can
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shift two segments and then
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work with a new polyline having the
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same length, think now where how
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to put the points the
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sum of the lengths of the links of the broken line is not less than the
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length of the segment connecting the end of this
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broken line and how to achieve a minimum when
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the inequality turns into equality
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only when points b and c lie on the
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segment 1 d 1 the problem is solved but what does
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physics have to do with it and you think about a ray of light
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that comes from from one fixed
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point, reflected from each side of the angle,
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it must hit another point, according to the
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principle of the company, a ray of light will guide us along the
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shortest optical path, let's
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carefully look at how exactly the
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depicted angles are equal due to symmetry,
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but these two angles are vertical, which means the
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corners that I am now highlighting are also
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equal a similar story with the angles
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formed by the right ray, therefore
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points b and c need to be marked so that the
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angle of incidence is equal to the angle of reflection
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[music]
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dual, remember the theorem about three forces, with its
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help you can very beautifully prove
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the fact and those related to the competitiveness of three
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straight lines, for example, you know well that
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the bisectors of a triangle intersect at
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one point, how can this be explained from the point of
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view of physics, imagine that a
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triangle is an absolutely rigid body
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that a swan, a cancer, and a pike on a plane are
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trying in vain to move from their place and you need to
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select three vectors directed along the
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bisectors so that the sum is equal to
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zero, but how can you guess this in advance for
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any triangle everything is very simple,
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first we will let pairs of vectors from the
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vertices of the triangle
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if you want the resulting force of
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each of the three pairs to be directed along the
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bisector, then it is logical to choose the forces
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equal in magnitude for each pair, but in
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addition it is desirable that these two
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forces are opposite in direction
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neutralized each other, which means their
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modules should also be equal for other
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pairs; the same thing means, ultimately,
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we take 6 equal in module and now we
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add each pair of vectors according to the parallelogram rule
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and get three cherished vectors on the
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bisectors, but now we know for sure
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what these three non-parallel vectors
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act on an absolutely rigid body
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which, as a result, is in
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equilibrium;
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therefore, do not their actions
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intersect at one point, which is what was
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required to be proven
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[music]
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finally, the modest charm of the mass method,
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which has already become a favorite of many,
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try first to prove
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on your own in any way that the
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medians of a triangle intersect at
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one point and together we will try
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to place unit masses at the vertices, the
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lower two can be regrouped in the
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middle of the segment, not forgetting that
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the mass will be doubled and then we will find the
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center of mass of the entire system using the
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rule of leverage, note that. with mass
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3 divides the median in the ratio of two to one,
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counting from the top, now we will find the center of
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mass in a different way, first
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we regroup these two points, then the
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remaining two, the center of mass is on
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another media, but it also divides it
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in the ratio of two to one, it turns out that the
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center of mass lies both on one segment
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and on the other, it means being the
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only one located at the point of
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intersection of the segments and in a similar
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way ends up on the third media, not
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that is, we not only proved that the
00:05:29
median of the triangle is competitive but also
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found out that the point of intersection
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they are divided in a ratio of two to one
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and from the vertex if you liked this
00:05:39
topic, write in the comments everyone who was a
00:05:42
sponsor but youtube rather go
00:05:44
to boost thanks for the support think and
00:05:48
do critical math
00:05:50
happy
00:05:51
[music]
00:06:06
[music]

Description:

Может ли классическая механика и законы оптики выручить в сложных геометрических задачах? Трудно представить, но бывает и такое! Поддержите канал Wild Mathing! https://boosty.to/wildmathing Мои курсы: https://vk.com/market-135395111 VK: https://vk.com/wildmathing Задачник: https://vk.com/wall-135395111_14984 ЛИТЕРАТУРА Метод масс: https://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=mo&paperid=15&option_lang=rus Точка Торричелли: https://www.mathnet.ru/php/archive.phtml?wshow=paper&jrnid=kvant&paperid=1769&option_lang=rus Статья по теме (С.М. Крачковский): https://mccme.ru/free-books/uchim/book-8.pdf ТРИ ДРУГИХ РЕШЕНИЯ первой задачи из нынешнего видео: https://www.youtube.com/watch?v=_lOH0r0i_Mc СОДЕРЖАНИЕ 0:00 — Точка Торричелли и потенциальная энергия 1:12 — Минимум в геометрии и принцип Ферма 2:51 — Инцентр и теорема о трех силах 4:25 — Метод масс и правило рычага 5:45 — Еще одна красивая теорема и титры БОЛЬШЕ ГЕОМЕТРИИ от Wild Mathing: 1. Удивительные факты с анимацией: https://www.youtube.com/watch?v=UlfNYVFi37U 2. Теоремы XX века: https://www.youtube.com/watch?v=PH7IDlYD7f8 3. Принцип Дирихле в геометрии: https://www.youtube.com/watch?v=PzYFHbsNuKM 4. Гармония четырехугольников (feat. МО): https://www.youtube.com/watch?v=cJWnxrzR2D8

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