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Friction
Friction
In physics, friction is the force that opposes the relative motion between 2 surfaces in contact.
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Types
Types
Friction is often categorized as 4 main types based on the objects' states and materials involved: Sliding, static, rolling, and fluid frictions.
Static friction
Static friction
Static friction (or just static) is the force that holds an object (no objects involved; screws, nails, glue) from sliding down at an incline, till the incline becomes greater at a some point.
On a flat surface, static acts between 2 surfaces both at rest rest relative. It stops an object from sliding if a certain force is applied, till it's exceeded (the object starts moving). [B1] I.e., static is equal in magnitude and opposite in direction to sum of all weight and applied force components parallel to POC, if no movement occurs inbetween.
As motion becomes imminent, the force due to static reaches its max value and as the object moves, the static becomes kinetic.
The max force due to static friction's approximate max value is found by: FFf = μSFN.
FFs = max force due to static friction's magnitude, parallel to POC (lb, N)
μS = static force's coefficient for materials in contact (unitless); [B1] a material's texture has a major impact on μS, e.g., A smooth slides easier than a loaded steel file
FN = normal force (lb, N)
[B1] Friction's direction isn't given in the formula as it's unrelated to FN's direction.
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[B1] Fig. 4.2a) has a block's weight FW perpendicular to the POC (floor), so no weight component is parallel to the POC.
The cable pull/applied force (FA or FApplied) acts parallel to the POC, the sole force to slide the block along the POC. The force due to friction (FF or Ffriction)and force due to static friction (FFs) will oppose FA.
Fig. 4.2b) has a hand pushing a block across POC, with parallel FA to POC is opposed by friction.
[B1] Table 4-1 gives a short list of possible useful coefficients.
[B1] The coefficient μS for many pairs of materials in contact are listed in tables in many physics books, like this one. The max force's formula doesn't have a term for POC, as in wide limits, the force due to friction FF is inpendent of POC. E.g., If a number of weights is used to hold open a door, regardless of their position (side by side, piled ontop), only as the area becomes small enough that the forces involved cause 1 material to dig into another will the area independene vanish.
Some lubricants can reduce friction 10x times from its dry value, which have high compressive strength, to support the loads placed on them, but at very low shear strength, meaning motion perpendicular to compressive support is easy. Lubricants provide a layer of incompatibility between like materials that can be incline to stick together and remove or evenly distribute heat friction generates, reducing materials' tendency to seize up. Table 4.1's 'lubricated' values are only roughly the exact lubricant used affect the true performance.
lubricant: substance (oil/grease) to used to minimize friction and allow smooth movement
dry value: friction/strength measured without lubricant
compressive: squeezed/crushed resistance
shear strength: resistance to slide sideways—layers moving past each other
Sliding/Kinetic friction
Sliding/Kinetic friction
Sliding/Kinematic/Kinetic friction (or just kinetic) is the force that opposes or act in the opposite direction an object moves or is being pushed/pulled. It's often weaker than static friction, as to why it's harder to start moving an object than to keep it at rest.
The max force due to sliding friction's max value is: FFk = μkFN.
FFk = max force due to sliding friction's magnitude, parallel to POC (lb, N)
Rolling friction
Rolling friction
Rolling friction is the force acting on rolling objects (wheels, balls) as they roll over a surface.
Fluid friction
Fluid friction
Fluid friction is the force as an object moves through a fluid (either a liquid or gas). It depends on the fluid's viscosity and the object's speed and shape.
See also
See also
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Books
Books
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