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ENTERTAINING EXPERIENCES AT HOME
Modeling propellers. Physical experiments
Entertaining experiences at home / Physics experiments for children Everyone is familiar with the propeller. Which of you did not make entertaining turntables that flew up, spun with your palms! And for those who are not familiar with such a turntable, it is easy to make. You need to take a small narrow plank 10 centimeters long and 2,5 centimeters thick, mark its center and cut two thin planes from the center with a sharp knife at a slight angle to each other (see figure). Round the ends of the planes, and you will get a model of a real aircraft propeller. By the way, before propellers for airplanes were also made of wood.
Now it remains to drill a hole in the center and insert an axis into it, having previously lubricated its tip with glue - a thin round stick about 15 centimeters long. When everything is ready, go out into the open, hold the propeller axis between your palms and, depending on which direction the blades are beveled, quickly turn it. The propeller with the axle will take off. It's like taking off a helicopter. You probably made paper pinwheels. At a square piece of thick paper, each corner is cut in half without reaching the very middle of the square. Then the corners through one are bent to the center and pierced with a pin. The pin is stuck into the stick, and even if you blow on such a turntable, it will begin to rotate rapidly. Even now there are windmills for grinding grain. They have large wings: usually these are four blades, mounted on an axis with some rotation to the plane of rotation. The wind, pressing on the wings, makes them spin. From the axis, the rotation is transmitted to the millstones - on them the grain turns into flour. But the room fan has a different task: he himself is instructed to create the wind. Its three or four blades, rotating, drive air forward, well ventilating the room, and in hot weather creating coolness. The wooden propeller you made is powered by beveled blades. They either screw into the air and move forward, or under the influence of an air stream, wind, rotate the axis on which they sit. And it will probably be strange for you to know that there can be a spinner that does not have beveled blades (it is a completely flat half of a longitudinally, along the axis, cut cylinder) and which spins beautifully in a strong air stream. But such a spinner cannot replace a propeller, and a fan cannot be made from it either. She can only turn herself. Such a turntable can be glued from thick paper. Make a blank first, and then glue it. You should get half of the cylinder, as if the cylinder was cut along the axis: one side is round, the other is flat. To glue such a half-cylinder, it is best to use a template, some kind of round stick. The length of the half-cylinder is 10-12 centimeters, the diameter of the round part is 3 centimeters. Make a hole in the center of it. Another option is to make a half-cylinder from a round stick of the same dimensions. Your main task: to very accurately cut a piece of such a stick into two halves strictly along the axis. Treat one half well with a fine emery cloth. In the very center, drill a through hole with a diameter of 2-3 millimeters. Make a wooden handle and drive a small cut-off nail into one of its ends. From a plastic tube, make a washer 1 centimeter high. Put a washer on the nail driven into the handle, and then the half-cylinder with the flat side out. After checking with your hand that the half-cylinder rotates freely, point a working fan at it. With one hand, hold our device with the flat side of the turntable towards the fan, and with the other hand, twist it on the nail. The spinner will now turn itself, blown by the air of the fan. Stop it and turn it the other way. She obediently will rotate in the other direction. It is very important that the hole is drilled exactly in the center and that the spinner is symmetrical about its longitudinal axis, the axis of the former cylinder.
What is the secret of its rotation? After all, it does not have beveled blades, like a propeller. The secret lies in the fact that the main air flow from the fan and the additional lateral flow that is obtained during rotation (at the very beginning it appeared when we turned the turntable by hand), adding up, swirl the air in such a way that this vortex causes our turntable to rotate. The force of friction and the force of air flows are balanced, and the spinner, having gained a certain speed, rotates evenly. Don't be surprised if you find that the fan spins on its own even if you haven't turned it. This is explained by the fact that, no matter how hard you try, some violations of symmetry were obtained during its manufacture. Air turbulences are formed faster, causing the turntable to rotate. Author: Rabiza F.V.
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