how to separate two solids of different densities by a circular motion 

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gsenviro | College Teacher | (Level 1) Educator Emeritus

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Two solids of sufficiently different densities can be separated from each other by using a Centrifuge. It is a device in which a mixture can be rotated at high speeds around a fixed axis. The centrifugal forces thus generated will cause separation of solids based on their densities. The solids with higher density will be deposited near the edge of the centrifuge container, while the lighter solids will be concentrated near the center of the container. 

The assumption here is that the particles will be discrete and thus free to move (i.e., will not be sticking to each other). Another method of separating solids of different densities is by agitating them in a container. This can be done by manual shaking or rolling. This will also cause solids separation, with denser particles occupying position under the lighter particles.

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Nolan McShea | Student, Grade 11 | (Level 2) Honors

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centrifuge can be used. A centrifuge is a lab tool that is used to separate substances of different densities, just as you said. How so? With a rapid, circular motion, which is also what you said. 

A centrifuge works like a centrifuge ride, where riders are strapped in around the circumference of a circle, all facing the center of the ride. When the ride starts spinning, even vertically, riders are safe, even if their buckles were to be broken. This is due to the force of the centrifuge, gluing you on the perimeter of the circle. 

An actual centrifuge machine works similarly, in the way that a substance is put in the machine and the machine spins very rapidly. The result is a tube with several levels, or components, of the substance you put in in the tube in the first place. The higher density components end up on lower levels of the tube.

An example of using a centrifuge would be by separating the components of blood:plasma, red blood cells, white blood cells, and platelets (which clot the blood to prevent excess bleeding when a cut occurs). The result of spinning this in a centrifuge is plasma on the top, white blood cells and platelets in the middle, and red blood cells on the bottom.

One more important thing to know would be that when using a centrifuge, you must place identical substances on both sides of the centrifuge (symmetrical), so that when it is in motion, it keeps the centrifuge balanced well and so it can work properly. You must have equal substances on both sides in terms of mass (weight) if the density of the substance you are using is higher or lower than water, or volume if not.

I hope I helped! Good luck!

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kseddy123 | College Teacher | (Level 1) Assistant Educator

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Centrifugation is used to separate materials of two different densities when gravitational force is insufficient for separation.  In industry centrifugal force is used

(i) To separate fine solids from liquids by centrifugal sedimentation

(ii) To seperate immiscible liquids whose density difference is small by centrifugal decantation.

(iii) In the filtration of solids from liquids by centrifugal filtration.

It is advantageous to opt for centrifugation, particularly when filtration becomes a cumbersome operation due to the small size of the particles.