Angle of repose
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The angle of repose is an engineering property of particulate solids. When bulk particles are poured onto a horizontal surface, a conical pile will form. The angle between the edge of the pile and the horizontal surface is known as the angle of repose and is related to the density, surface area, and coefficient of friction of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose.
This property is sometimes used in the design of equipment for the processing of particulate solids. For example, it may be used to design an appropriate hopper or silo to store the material. It can also be used to size a conveyor belt for transporting the material.
There are numerous methods for measuring angle of repose and each produces slightly different results. Results are also sensitive to the exact methodology of the experimenter. As a result, data from different labs is not always comparible.
An alternative measurement, useful for many of the same purposes, is testing with a Jenike shear cell.
The angle of repose is sometimes used as a synonym for the tipping point. To understand the origin of the term, consider a person who leans back in a chair. If the person tips back beyond a certain angle, then the chair will tip over and the person will fall onto his or her back. That angle is the angle of repose, and it is analogized to apply to any process in which beyond a certain point, the rate at which the process (chemical, sociological, etc.) proceeds increases dramatically. Mathematically, the angle of repose may be seen as an inflection point.
Angle of Repose is also a 1971 novel by Wallace Stegner.