Depth 1 · 81 s
Sorted, and stiff
Triangles are sorted by their sides - equilateral, isosceles, scalene - and by their angles. And unlike a square of sticks, a triangle cannot wobble.
About this video
Why are bicycle frames, cranes and bridges full of triangles? This animation starts with a bicycle frame, two triangles side by side, and then looks at triangles one property at a time.
First by their sides. An equilateral triangle has all three sides the same length, an isosceles triangle has two, and a scalene triangle has none. In a drawing, sides of the same length carry the same small marks across them, so you can read the kind of triangle from the marks without measuring. Then by their angles: a right-angled triangle has one right angle, like the corner of a page; another kind has one obtuse angle and looks stretched out; and in the third all three angles are acute. A triangle never has more than one right or obtuse angle. The two ways of sorting work together: fold a square piece of paper corner to corner and you get a triangle that is right-angled and isosceles at once.
Last, the reason builders love triangles. Four sticks joined at the corners with pins make a square, and a push makes it lean over into a flatter shape with the same sides. Three sticks make a triangle, and pushing it changes nothing: three sides of fixed lengths fit together in only one way. A board nailed across a gate cuts it into two triangles for the same reason. This video goes with the Triangles lesson of Geometry Level 1 on Knaph.
This video shows depth 1 of Triangles.
Learn TrianglesTranscript
- 0:00Bike frames, cranes, bridges. Why so many triangles?
- 0:06Sort them by their sides. All three equal is equilateral. Two equal is isosceles. None equal is scalene.
- 0:18The small marks tell you. Same marks, same length.
- 0:26Now by their angles. One right angle. One obtuse angle. Or every angle acute.
- 0:36A triangle never has two right angles.
- 0:42Fold a square corner to corner. Right-angled and isosceles, both at once.
- 0:50Four sticks with pins at the corners. Push, and the square leans over.
- 0:58Three sticks. Push as hard as you like. The shape does not change.
- 1:06That is why a gate gets a board from corner to corner.
- 1:14Three sides fit only one way.