Difference between revisions of "Friction"

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===Instructional video links===
 
===Instructional video links===
  
* [http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed08.htm MIT OpenCourseWare lecture on friction by Walter Lewin] (embedded, downloadable and streaming video, with transcript HTML and PDF)
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* [http://oyc.yale.edu/physics/fundamentals-of-physics/content/sessions/lecture04.html Open Yale Courses lecture by Ramamurthi Shankar] that discusses friction and many other topics. (downloadable video, with HTML transcript)
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! Video link !! Segment !! Contextual information !! Transcript link !! Transcript segment
* Khan Academy videos on friction: [http://www.youtube.com/watch?v=KvQ3YDALMeg&feature=youtube_gdata Introduction to friction], [http://www.youtube.com/watch?v=1g6eQqZgc50&feature=youtube_gdata Friction on an inclined plane], [http://www.youtube.com/watch?v=kM6D8FDlYPU&feature=youtube_gdata A more complicated friction/inclined plane problem], [http://www.youtube.com/watch?v=YvacYWgygaA&feature=youtube_gdata Work/Energy problem with friction]
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| [http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed08.htm MIT OCW lecture on friction by Walter Lewin] || full video || Covers most of the standard introductory material on friction along with some experimental demonstrations || same as video link || full transcript
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| [http://oyc.yale.edu/physics/fundamentals-of-physics/content/sessions/lecture04.html Open Yale Courses lecture by Ramamurthi Shankar] || ? || ? || same as video link || ?
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| Khan Academy videos on friction: [http://www.youtube.com/watch?v=KvQ3YDALMeg&feature=youtube_gdata Introduction to friction], [http://www.youtube.com/watch?v=1g6eQqZgc50&feature=youtube_gdata Friction on an inclined plane], [http://www.youtube.com/watch?v=kM6D8FDlYPU&feature=youtube_gdata A more complicated friction/inclined plane problem], [http://www.youtube.com/watch?v=YvacYWgygaA&feature=youtube_gdata Work/Energy problem with friction] || full videos || || unavailable ||
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===Other video links===
 
===Other video links===
  
 
* [http://www.watchknow.org/Category.aspx?CategoryID=1744 WatchKnow videos from friction category]
 
* [http://www.watchknow.org/Category.aspx?CategoryID=1744 WatchKnow videos from friction category]
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===Links to online texts/course notes===
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* [http://cnx.org/content/m14068/latest/ Connexions module on friction]

Revision as of 23:48, 11 May 2010

This article describes a force type.
View a complete list of force types

Definition

Intensional definition

Friction (sometimes called sliding friction to distinguish from rolling friction) is the component of contact force between two bodies with a surface of contact acting in a direction along (or parallel to) the surface of contact, or the tangent plane through the point of contact.

The term friction refers by default to dry friction: friction where the two surfaces of contact are dry and solid. The typical model used for such friction is the Coulomb model of friction.

Tendency-based definition

Friction refers to a force exerted by two bodies on each other, through a surface of contact, that tends to oppose slipping of the two surfaces of contact against each other. Note that static friction opposes the tendency indicated by the direction of net external force along the plane of contact, while kinetic friction opposes the direction of actual motion.

Situational examples

Here, where there are only two surfaces in contact, we denote by \mu_s the limiting coefficient of static friction and by \mu_k the coefficient of kinetic friction. g is the acceleration due to gravity.

Example Kind of friction (static versus kinetic) Direction of friction force on each body Magnitude of friction force Reason
Block sliding along a fixed horizontal plane, no external applied forces Kinetic friction Opposite direction of motion of block on block, along direction of motion of block on the plane \mu_kmg where m is the mass of the block The normal force equals mg
Block sliding along an inclined plane, angle \theta with horizontal, no external applied forces Kinetic friction Opposite direction of motion of block on block, along direction of motion of block on surface \mu_kmg \cos \theta where m is the mass of the block The normal force equals mg \cos \theta

Related forces

Types of friction

  • Static friction: This is friction exerted when the surfaces of contact are not slipping relative to each other. This occurs, for instance, both when the surfaces are stationary, and in the case of rolling. Static friction acts to precisely cancel the net external force along the plane of contact. According to the Coulomb model of friction, the maximum possible value of static friction is \mu_sN where N is the normal force and \mu_s is the limiting coefficient of static friction for the two surfaces.
  • Kinetic friction or dynamic friction: This is friction exerted when the surfaces are slipping relative to each other. Kinetic friction acts in the direction opposite to the direction of slipping. According to the Coulomb model of friction, for small relative velocities, its magnitude is \mu_kN, where N is the normal force and \mu_k is the coefficient of kinetic friction.

Forces between rigid bodies

  • Rolling friction: This is not really friction, but rather, a normal force that effectively plays the role of friction due to local deformations of the surface.
  • Normal force: The component of contact force in the direction perpendicular to the plane of contact.

Other friction-like forces

External links

Instructional video links

Video link Segment Contextual information Transcript link Transcript segment
MIT OCW lecture on friction by Walter Lewin full video Covers most of the standard introductory material on friction along with some experimental demonstrations same as video link full transcript
Open Yale Courses lecture by Ramamurthi Shankar  ?  ? same as video link  ?
Khan Academy videos on friction: Introduction to friction, Friction on an inclined plane, A more complicated friction/inclined plane problem, Work/Energy problem with friction full videos unavailable

Other video links

Links to online texts/course notes