Editing Angle of repose (section)

{{Short description|Steepest angle at which granular materials can be piled before slumping}}
{{for-multi|the Wallace Stegner novel|Angle of Repose|the angle of friction between two solid objects|Friction#Angle of friction}}

[[File:Angleofrepose.png|thumb|Angle of repose of a heap of sand]]
[[File:Sandpile Matemateca 22.webm|thumb|Sandpile from the [[Matemateca]] ([[Institute of Mathematics and Statistics, University of São Paulo|IME-USP]]) collection]]

The '''angle of repose''', or '''critical angle of repose''',<ref>
{{cite journal
 |last1=Mehta |first1=A.
 |last2=Barker |first2=G. C.
 |year=1994
 |title=The dynamics of sand
 |journal=[[Reports on Progress in Physics]]
 |volume=57 |issue= 4|page=383
 |bibcode=1994RPPh...57..383M
 |doi=10.1088/0034-4885/57/4/002
|s2cid=250898376
 }}</ref> of a [[granular material]] is the steepest [[angle]] of descent or [[Strike and dip|dip]]  relative to the horizontal plane on which the material can be piled without slumping. At this angle, the material on the slope face is on the verge of sliding. The angle of repose can range from 0° to 90°. The morphology of the material affects the angle of repose; smooth, rounded [[Sand|sand grains]] cannot be piled as steeply as can rough, [[Shear strength (soil)|interlocking]] sands. The angle of repose can also be affected by additions of [[solvents]]. If a small amount of water is able to bridge the gaps between particles, [[electrostatic attraction]] of the water to mineral surfaces increases the angle of repose, and related quantities such as the [[bearing capacity|soil strength]].

When bulk granular materials are poured onto a horizontal surface, a [[cone (geometry)|conical]] pile forms. The internal angle between the surface of the pile and the horizontal surface is known as the angle of repose and is related to the [[density]], [[surface area]] and shapes of the particles, and the [[coefficient of friction]] of the material. Material with a low angle of repose forms flatter piles than material with a high angle of repose.

The term has a related usage in [[mechanics]], where it refers to the maximum angle at which an object can rest on an [[inclined plane]] without sliding down. This angle is equal to the [[arctangent]] of the [[coefficient of static friction]] ''μ''<sub>s</sub> between the surfaces.