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2.7Force and Translational Dynamics

Kinetic and Static Friction — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 10 parts Lesson 28 of 36 1 quiz

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Lesson summary

2.7 Kinetic and Static Friction — AP PHYSICS 1 notes: Describe kinetic and static friction between surfaces and use fk = μkFN and fs ≤ μsFN.Friction = the component of the contact force parallel to the surface, opposing relative motion or its tendency.Cause — microscopic imperfec…

  • Independence of area
  • Coefficient μk
  • Self-adjusting
  • Rising applied force
  • Direction can be forward
  • Angled pull

Part 1 of 8What friction is

Friction
Friction the component of the contact force parallel to the surface, opposing relative motion or its tendency.
Cause
  • Cause microscopic imperfections of the two surfaces catch on each other.
    • (electromagnetic interactions between atoms)
Independence of area
  • Independence of area in the ideal model friction does not depend on the area of contact.
Dependence
  • Dependence it depends on the materials and on the normal force pressing the surfaces together.

Part 2 of 8Kinetic friction

Condition
  • Condition acts when two surfaces in contact move relative to each other.
Direction
  • Direction opposite to the motion of each surface relative to the other.
Magnitude
  • Magnitude fk = μkFN, roughly independent of speed.
Effects
  • Effects slows the moving surface and heats the contact surfaces.
Coefficient μk
  • Coefficient μk a ratio of two forces with no unit, set by the pair of materials.
  • Rough surfaces such as brick or concrete have higher μ than ice or plastic.

Part 3 of 8Static friction

Condition
  • Condition acts between surfaces that are not moving relative to each other.
Self-adjusting
  • Self-adjusting its size and direction match what is needed to prevent slipping, up to a maximum.
Limit
  • Limit fs ≤ μsFN, with fs,max = μsFN just before slipping.
Rising applied force
  • Rising applied force static friction rises with it until the maximum is exceeded and the object slips.
Direction can be forward
  • Direction can be forward static friction pushes a walking person or an accelerating car forward.

Part 4 of 8Static versus kinetic

FeatureStaticKinetic
Relative motionNoneSliding
MagnitudeVaries from 0 up to μsFNConstant, μkFN
Coefficientμs (larger)μk (smaller)
DirectionOpposes tendency to slideOpposes sliding
Force-time graphRises with applied forceDrops after slipping, then constant

Part 5 of 8Normal force and friction

Rule
  • Rule friction depends on FN, not directly on weight.
Angled pull
  • Angled pull pulling upward at an angle reduces FN and so reduces friction.
Heavier object
  • Heavier object a larger FN raises friction and the force needed to start motion.
Why μs > μk
  • Why μs > μk less force keeps an object moving than starts it, so Ff,k < Ff,s,max.

Part 6 of 8Incline results

Verge of slipping
  • Verge of slipping mg sinθ = μsmg cosθ, so tanθ = μs.
Constant-velocity slide
  • μk = tanθ.
Normal force on incline
  • FN = mg cosθ.

Part 7 of 8Deciding whether an object slides

1
Draw the free-body diagram and find the normal force.
2
Calculate the maximum static friction μsFN.
3
Find the force component tending to cause sliding.
4
If it is not larger than the maximum, the object stays at rest and fs equals that component.
5
If it is larger, the object slides; use fk = μkFN in ΣF = ma.

Part 8 of 8More from the specification

More from the specification
Coefficient of friction and normal load
  • Coefficient of friction and normal load evaluating the ratio of frictional resistance to normal force provides an experimental means to assess roughness.
Threshold
  • The output value at which a switching circuit changes state.
Normal reaction
  • The contact force perpendicular to a surface.

Key terms

TermMeaning
FrictionForce parallel to the surface opposing relative motion or its tendency.
Static frictionFriction between surfaces at rest relative to each other; fs ≤ μsFN.
Kinetic frictionFriction between sliding surfaces; fk = μkFN.
Coefficient of frictionDimensionless ratio set by the two surfaces.
Maximum static frictionμsFN, reached just before slipping.

Avoid these mistakes

1
Taking static friction as always equal to μsFN — that is only its maximum.
2
Using friction = μmg when FN is not mg — find FN first.
3
Saying a larger contact area gives more friction — not in the ideal model.
4
Using μs for a sliding object — sliding uses μk.

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