Tensegrity structures are structures where the physical integrity of the structure depends on tensional forces rather than the common compressive forces that maintain the integrity of structures such as buildings. The name was coined by the renowned American architect Buckminster Fuller famous for the invention of the geodesic dome. Most tensegrity structures consist of rigid materials such as rods or blocks attached to each other by cords or cables, where the weight of the rigid materials creates tension on the cables which keeps the rigid materials in place. The apparent distribution in space of the rigid materials of tensegrity structures often seems to defy gravity, and many such structures have been produced for entertainment or as works of art or architecture. However, the principles of tensegrity structures are also found at work in biology such as in the interplay between muscles, bones, and tendons, and even in the cytoskeletons of cells.
In the video below, the folks of the YouTube channel Home Science display a tensegrity structure and show how to build it.
In the video below, Dianna Cowern (Physics Girl) shows the interesting thing that happens when the flame of a candle is exposed to falling.
While I understand the need to make the science accessible and fun in a short video, Dianna describes the effects of freefall on the candle flame to be a consequence of “taking away gravity”, and she also says that, “Falling towards the Earth pulled down by gravity, is just like floating out in space with no gravity at all”. Neither of these things is true.
The gravity that we are talking about, is that produced by the Earth. To “take away gravity” or to “have no gravity”, the Earth would have to disappear! What really happens during freefall that gives the candle flame that shape is that the forces that oppose gravity are diminished or nullified. Because this eliminates buoyancy, hot air can no longer rise. This is counterintuitive, and visualizing it is more complicated and difficult to explain in a video, but if you are interested, I wrote a post about it here.
In this video you see a pilot pouring water into a cup while the plane is doing loops. At one point the water seems to be flowing away from the Earth below him or remains in the glass even while the plane is inverted. What happened to gravity? Aren’t things like water supposed to “fall”?
Gravity thankfully is alive and well. What is happening is that the looping motion of the airplane generates a force directed towards the bottom of the plane. This force, called centrifugal force, is strong enough to counteract the force of gravity and make the water stay in the cup or even “fall up” (from the point of view of the camera) while the plane is upside down during the loop.
You can see my demonstration of this force in the video below.
My demonstration is not perfect in that to spin around the cup my arm has to produce a twisting motion that sloshes the soda around and generates foam, but the principle is the same. In the case of the video of the airplane, the motor propels the plane, and the navigation controls inside the cabin lock it into a circular path which generates the force. In the case of my video, the muscles in my arm propel the cup, and my grip on it locks it into a circular motion which generates the force.
Back in the 16th century it was still believed that light objects would fall slower than heavy objects. The Italian scientists Galileo Galilei allegedly proved this theory to be wrong in a famous experiment where he dropped two balls of equal size but different masses from the leaning tower of Pisa. There is some controversy as to whether Galileo actually performed the experiment, but this experiment has been repeated in modern times in many ways. Two of such notable repetitions are shown below. The first is that carried out by astronaut Dave Scott of the Apollo 15 mission on the moon.
The second was conducted at NASA's massive Space Simulation Chamber in Ohio.