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2.6.3Force and Translational Dynamics

Static Friction — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 9 parts Lesson 25 of 36 1 quiz

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2.6.3 Static Friction — AP PHYSICS 1 notes: Explain how static friction acts when surfaces do not slide, how it adjusts, and where it matters in everyday motion.Static friction = friction between two surfaces that are not sliding relative to each other.Role — it prevents motion f…

  • Present until motion starts
  • Everyday cases
  • Finding fs
  • Heavier object
  • Conveyor belt
  • Flat curve

Section 1 of 9What static friction is

1Static friction
Static friction friction between two surfaces that are not sliding relative to each other.
2Role
  • Role it prevents motion from starting or keeps two surfaces moving together without slipping.
3Present until motion starts
  • Present until motion starts once the object moves, kinetic friction takes over.
4Everyday cases
  • Everyday cases a pushed sofa that does not budge;
    • the ground pushing a walker forward;
    • the road pushing a rolling tyre forward.

Section 2 of 9A self-adjusting force

Value
  • Value static friction takes whatever value prevents sliding, up to a maximum.
  • A 5 N push on a resting box gives fs = 5 N; a 12 N push that does not move it gives fs = 12 N.
Direction
  • Direction opposite to the direction the object would slide if friction were absent.
Finding fs
  • Finding fs apply ΣF = ma with zero acceleration along the surface;
    • the coefficient gives fs only on the verge of slipping.
  • A block at rest on an incline has fs = mg sinθ up the slope.
Heavier object
  • Heavier object larger normal force raises the maximum static friction and the force needed to start motion.

Section 3 of 9Static friction causing acceleration

Conveyor belt
  • Conveyor belt a crate speeding up with the belt without slipping has fs = ma forward.
Truck bed
  • Truck bed a box on an accelerating truck is accelerated by static friction.
Flat curve
  • Flat curve static friction toward the centre of the curve supplies the centripetal force.
No relative sliding
  • No relative sliding the surfaces do not slide on each other even though the object moves relative to the ground.

Section 4 of 9Static versus kinetic

Static versus kinetic
Comparison
  • Ff,k < Ff,s,max for a given pair of surfaces.
  • Less force keeps an object moving than starts it moving.
Friction against applied force
  • Friction against applied force rises linearly while stationary, peaks at slipping, then drops to a constant kinetic value.
Sudden slip
  • Sudden slip the lurch when an object starts is kinetic friction taking over from static friction.

Section 5 of 9Rolling without slipping

Contact point
  • Contact point momentarily at rest relative to the ground, so the friction there is static.
Energy
  • Energy static friction at the contact point does not dissipate energy directly.
Spinning or locked wheel
  • Spinning or locked wheel the contact point slides, friction becomes kinetic and grip falls.
    • (reason for anti-lock brakes)

Section 6 of 9Finding the direction: no-friction test

Finding the direction: no-friction test
Test
  • Test imagine a perfectly smooth surface and decide which way the object would slide;
    • friction points the other way.
Block on slope
  • Block on slope would slide down, so friction points up the slope.
Box on accelerating truck
  • Box on accelerating truck would slide backward, so friction points forward.
Coin on turntable
  • Coin on turntable would slide outward, so friction points inward.
Which object
  • Which object draw the force acting on the object being analysed.
    • (friction on you points forward when you walk)

Section 7 of 9More from the specification

1Limiting static friction threshold
  • Limiting static friction threshold measuring the maximum tangential force before sliding occurs provides a reliable means to calculate static friction coefficients.
2Threshold
  • The output value at which a switching circuit changes state.
3Velocity
  • Rate of change of displacement.
4Normal reaction
  • The contact force perpendicular to a surface.
5Reaction force
  • Force from a support pushing up on an object.
6Constant velocity
  • Zero acceleration.

Section 8 of 9Key terms

TermMeaning
Static frictionFriction between surfaces that are not sliding.
Maximum static frictionLargest static friction before slipping begins.
EquilibriumState with zero net force.
Rolling without slippingRolling in which the contact point is instantaneously at rest.
Centripetal forceNet force toward the centre of a circular path.

Section 9 of 9Exam technique

1
Using f = μFN for every static friction problem — it only gives the maximum.
2
Thinking static friction is always weight times a constant.
3
Saying friction is zero for a stationary object — it may balance an applied force.
4
Believing friction always opposes motion — static friction may point along the motion, as when walking.
5
Thinking a rolling wheel has kinetic friction at the contact point.

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