Friction Notes Friday, 9/30 and Monday, 10/3.

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Friction Notes Friday, 9/30 and Monday, 10/3

What is friction? Occurs when two objects are pressed together. The contact causes a frictional force between the surfaces of the objects. Image source:http://static1.squarespace.com/static/5672d 22fbfe8739f7af9 1a3c/t/5674e6a5d82d5eb8ca94df84 /1450509943480/?format=1500w Image source: http://www.aplusphysics.com/courses/honors/dynami cs/images/timbo_ pulling_traci_in_a_wagon_hg_wht.gif

What is friction? Is parallel to the surface. Opposes the relative motion of the two surfaces.

What is the difference between Kinetic and Static Friction Kinetic friction – when the two surfaces are moving relative to each other (ex: the wheels of the wagon). Kinetic friction is dependent on Normal force between the surfaces Fn The nature of the surfaces Each surface is unique (driving on gravel vs. driving on a hockey ice). The uniqueness of surfaces will dictate the frictional force. In Physics we use the coefficient of friction to differentiate between surfaces. Coefficient of friction (μk) Frictional force is = to product of the normal force and the coefficient of friction. Expressed as Ff,k = μkFn

What is the difference between Kinetic and Static Friction Static friction – when the two surfaces are at rest relative to each other (ex: cat sitting on top of the book). Static friction is dependent on The forces acting on the object The static friction will have a large magnitude because objects resist change (inertia). Coefficient of friction (μs)

Maximum static friction The maximum force of static friction is dependent on: Normal force between the surfaces (Fn) Nature of the surfaces coefficient of friction (μs). (note this is different than coefficient of μk. Ff,s,max = μsFn 0 ≤ Ff,s, ≤ μsFn Static friction is stronger than kinetic friction μs >μk

accelerating to the right In a truck bed accelerating to the right Sliding Down an Incline Box sliding to right

A 4 kg box slides to the right on a level surface A 4 kg box slides to the right on a level surface. The coefficient of kinetic friction between the box and the surface is 0.50. What is the force of kinetic friction?

A 10 kg box is at rest on a surface which is inclined at a 30° relative to the horizontal. What is the frictional force on the box?