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J. Perelman
"Entertaining physics". Book 2.
Chapter 3. Circular motion

"DESTROYED" WEIGHT

"Water is not poured out of the vessel, which rotates, is not translated, even when the vessel is turned upside down, because this prevents rotation" - wrote two thousand years ago Aristotle. The figure shows this spectacular experience, which, no doubt, familiar to many: rotating fast enough bucket with water, as shown in the picture, you reach that water is not poured out even in the part of the path, where the bucket tilted upside down.


Why not poured water from the rotating bucket?

In everyday life it is customary to explain this phenomenon of "centrifugal force", meaning by it the imaginary force, which if applied to the body and causes his desire to retire from the center of rotation. This force does not exist: specified desire is nothing other than the manifestation of inertia, and any movement by inertia is without power. In physics under centrifugal force mean something else, namely the real force with which the rotating body tightens holding his thread or presses on its curved path. This force is not applied to the moving body and an obstacle, preventing him to move rectilinearly to the thread for rails on a curve section of the route, etc.

Turning to the rotation of the bucket, try to understand the reason of this phenomenon without resorting at all to the ambiguous notion of "centrifugal force". Let us ask ourselves: where will sent a jet of water, if the wall of the bucket to make a hole? Don't be gravity, water jet inertia would go on a tangent AK to a circle AB. Gravity also causes the jet to fall and to describe the curve (a parabola AR). When the peripheral speed is high enough, this curve will be located outside of the circle AB. Jet finds in front of us the way in which during rotation of the bucket would move water, if not prevented pressing on her bucket. Now it is clear that the water is not going to move straight down, and therefore not poured from a bucket. It could result from it only in the case if the bucket was drawn to the hole in the direction of its rotation.

Calculate now how fast you need this experience to rotate the bucket so that the water is not poured down. This speed should be such that the centripetal acceleration of the rotating bucket was not less than the acceleration of gravity, then the path is committed to moving water, will lie outside the circle described by the bucket, and the water anywhere from the bucket will not lag behind. The formula to calculate the centripetal acceleration W is as follows;

W=v2/R,

where v is the radial velocity, R is the radius of the circular path. Since the acceleration of gravity on the earth's surface g = 9.8 m/s2, then we have the inequality v2/R = 9,8. If we set R equal to 70 cm,




It is easy to calculate that for such peripheral speed must be done by hand about one and a half revolutions per second. This speed is achievable, and experience possible without effort.

The ability of the liquid to stick to the walls of the vessel in which it rotates around a horizontal axis are in the technique for so-called centrifugal casting. When it is essential that heterogeneous fluid is stratified according to the weight: the heavier component parts are located further from the axis of rotation, the lungs take place closer to the axis. As a consequence, all the gases contained in the molten metal and form the so-called "sinks" in the casting, are allocated from the metal in the interior, the hollow part of the casting. Products made in this way, become dense and free from shells. Centrifugal casting is cheaper than conventional injection molding and does not require sophisticated equipment.

Entertaining physics J. Perelman

 




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