Friction

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Friction is the fifth lecture in the Mechanics section of PH1011. It covers sliding friction in both static and kinetic cases.

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Cause
Friction occurs due to the formation of weak atomic bonds when two bodies are placed close to one another. Surface irregularities ensure that the actual touching surface is lower than the surface area of the objects, meaning that area cannot be used to calculate friction (and that it has little effect on the friction). This binding force must be less than the other forces present for motion to occur. The friction acts in the opposite direction to any applied force, and is dependent on the surface materials, the magnitude of the normal force and whether the object in question is stationary or in motion.

Static Friction
Static friction refers to a motionless body; any force acting upon it is not having enough of an effect to shift the object, and is therefore smaller than the frictional force. The static friction is not constant - it increases as the external force increases in order to ensure no movement until it reaches its maximum value. The maximum value is proportional to the object's coefficient of static friction and the normal force in the relationship fs max= μsn.

Static friction is independent of the objects' masses and surface areas, and the effects of gravity.

Kinetic Friction
Kinetic friction acts when an object is moving. The work done by this force is mostly lost as heat and sound, and it has a lower value than the static friction due to the surface bonds nnot having time to form properly as the object passes. The relationship remains fk= μkn. μk acts independent of mass, surface area and velocity.

Critical Angle
Up to a certain angle, the force of static friction can prevent objects from sliding down a slope. By dividing the weight of a box into its components (Wx = Wsinθ = fs≤ μsn, |Wy| = -Wcosθ = n) and then dividing Wx by Wy, Wsinθ/Wcosθ = tanθ ≤ μs. The critical angle is given as tanθundefinedμs; from observation of this μs can be directly measured.

Summary
Friction occurs due to the formation of atomic bonds along the surfaces in question. It can act in both static and kinetic cases, although will have a differing magnitude in each - it will be equal to the applied force statically, but vary with the material's coefficient alone for kinetic. The critical angle relates to the coefficient of static friction in the relationship tanθundefinedμs.