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2.6.1Force and Translational Dynamics

Kinetic Friction — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 23 of 36 1 quiz

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2.6.1 Kinetic Friction — AP PHYSICS 1 notes: Describe the origin, direction and effects of kinetic friction on a sliding object and show it on free-body diagrams.Contact force — kinetic friction is the force between two surfaces sliding past each other.Direction — parallel to the…

  • Not tied to the applied force
  • Non-conservative
  • Contact area
  • Dependence
  • Block released on a level surface
  • Constant velocity

Section 1 of 8What kinetic friction is

What kinetic friction is
Contact force
  • Contact force kinetic friction is the force between two surfaces sliding past each other.
Direction
  • Direction parallel to the surface and opposite to the relative motion of the surfaces.
  • A box sliding right is pushed left by the floor.
  • By Newton's third law the box pushes the floor right with an equal force.
Not tied to the applied force
  • Not tied to the applied force it opposes the sliding, not necessarily the applied force.

Section 2 of 8Origin and energy

Origin
  • Origin tiny bumps (asperities) on apparently smooth surfaces catch and form temporary bonds as they slide.
Energy
  • Energy work done against kinetic friction becomes thermal energy, so the surfaces warm up.
Non-conservative
  • Non-conservative the mechanical energy removed is not recovered.

Section 3 of 8Rules for kinetic friction

Speed
  • Speed the size stays constant while sliding and does not depend on speed.
Contact area
  • Contact area for ordinary dry surfaces the size does not depend on contact area.
Dependence
  • Dependence it depends on the normal force and on the materials.

Section 4 of 8Effect on motion

Slowing
  • Slowing kinetic friction slows an object or limits its acceleration.
Block released on a level surface
  • Block released on a level surface friction is the only horizontal force, so acceleration is constant and opposite to the velocity.
  • The velocity–time graph is a straight line with negative gradient reaching the time axis.
Stopping
  • Stopping once sliding stops kinetic friction stops.
Constant velocity
  • Constant velocity the net force is zero, so the pulling force equals kinetic friction.
    • (basis of a lab measurement with a force sensor)

Section 5 of 8Free-body diagrams with friction

Free-body diagrams with friction
Block on a horizontal surface
  • Block on a horizontal surface weight down, normal force up, applied horizontal force and kinetic friction opposite to sliding.
  • At constant velocity the horizontal arrows are equal and the vertical arrows are equal.
Incline
  • Incline friction points up the slope when the block slides down and down the slope when it slides up.
Label
  • Label write fk to avoid confusion with static friction.
Third-law partner
  • Third-law partner the partner force acts on the surface, so it is not drawn on the object.

Section 6 of 8More from the specification

More from the specification
Kinetic friction coefficient determination
  • Kinetic friction coefficient determination pulling loads at constant velocity across horizontal surfaces provides an experimental means to measure kinetic friction.
Slipping
  • Start of relative sliding between surfaces.
Threshold
  • The output value at which a switching circuit changes state.
Normal reaction
  • The contact force perpendicular to a surface.
Reaction force
  • Force from a support pushing up on an object.

Section 7 of 8Key terms

TermMeaning
Kinetic frictionForce opposing relative sliding between two surfaces.
Normal forcePerpendicular contact force pressing surfaces together.
Non-conservative forceForce whose work depends on path and dissipates mechanical energy.
Thermal energyEnergy of random molecular motion, increased by friction.
AsperityMicroscopic bump on a surface.

Section 8 of 8Exam technique

1
Saying kinetic friction always opposes the applied force — it opposes sliding.
2
Believing faster sliding means more friction — kinetic friction is constant in this model.
3
Thinking larger contact area increases friction.
4
Forgetting that friction stops once sliding stops.
5
Drawing friction on the wrong object — it acts on the sliding object, along the surface.

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