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A.2.7 Frictional Forces — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 18 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
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pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Lesson summary

A.2.7 Frictional Forces — IB PHYSICS SL notes: Distinguish static and dynamic friction, use coefficients of friction and solve problems with friction on horizontal surfaces and slopes.Frictional force — it opposes the motion of an object.It can prevent a stationary object from mo…

  • Fluid resistance (drag)
  • Surface friction
  • Dependence
  • Static friction
  • Coefficient of friction
  • Starting and keeping moving

Section 1 of 11What friction does

Frictional force
  • Frictional force it opposes the motion of an object.
  • It can prevent a stationary object from moving or reduce the speed of a moving object.
Direction
  • Direction it acts parallel to the surfaces, opposite to the relative sliding or its tendency.
Energy
  • Energy work done against friction transfers energy by heating, raising the temperature of the objects and surroundings.
Origin
  • Origin imperfections and tiny peaks in the surfaces interlock and rub, so a force is needed to shear them.
Third law
  • Third law the frictional forces on the two bodies are equal and opposite.

Section 2 of 11Fluid resistance and surface friction

Fluid resistance (drag)
  • Fluid resistance (drag) an object moving through a gas or liquid collides with fluid particles, which slows it and heats the object and the fluid.
Surface friction
  • Surface friction it occurs between two bodies in contact and increases their thermal energy.
Dependence
  • Dependence both static and dynamic friction depend on the normal reaction FN.

Section 3 of 11Static and dynamic friction

FeatureStatic frictionDynamic (kinetic) friction
When it actsBody is stationary on the surfaceBody is sliding
EquationFf ≤ μsFNFf = μdFN
ValueMatches the applied force up to a maximumDefinite, roughly constant value
Relative sizeMaximum is largerSmaller, since μd < μs
1Static friction
  • Static friction it increases to match any push until it can no longer hold the objects still, then movement begins.
2Coefficient of friction
  • μ is the ratio of the friction force to the normal force; it has no unit and is typically 0.1 to 1.
  • A larger μ means the surfaces are harder to slide past each other.
3Starting and keeping moving
  • Starting and keeping moving it is harder to start an object moving than to keep it moving.
4Simple model
  • Simple model friction does not depend on contact area or on sliding speed.

Section 4 of 11Friction graph

Before sliding
  • Before sliding friction equals the applied force, a straight line of gradient 1.
At the limit
  • At the limit the maximum static value μsFN is reached.
After sliding begins
  • After sliding begins friction drops to a constant, smaller value μdFN, shown as a horizontal line.

Section 5 of 11Friction on slopes

Block at rest on a slope
  • Block at rest on a slope static friction equals the component of weight down the slope, mg sin θ.
Normal force on a slope
  • FN = mg cos θ.
Limit of staying at rest
  • Limit of staying at rest at the angle where mg sin θ = μsmg cos θ, μs = tan θ.
Sliding down
  • Sliding down the resultant down the slope is mg sin θ − μdmg cos θ, so a = g(sin θ − μd cos θ).
  • The mass cancels.

Section 6 of 11Using friction in dynamics

1
Find the normal reaction FN from the force balance perpendicular to the surface.
2
If the object slides, friction is μdFN opposing the motion.
3
If it does not slide, friction equals whatever balances the applied force, up to μsFN.
4
Apply F = ma along the surface.
  • Work against friction
    • over a distance d it is Ffd = μFNd.

Section 7 of 11Useful and harmful friction

Useful
  • Useful friction is essential for walking, gripping, braking and tyre grip.
  • The contact patch of a rolling tyre is momentarily at rest, so static friction acts.
  • A skidding car relies on weaker dynamic friction, so it stops more slowly; anti-lock brakes avoid this.
Harmful
  • Harmful friction wastes energy and wears machine parts; lubricants, bearings and streamlining reduce it.
Rolling friction
  • Rolling friction it is much smaller than sliding friction.
Typical μ for tyres
  • Typical μ for tyres about 0.8 to 1.0 on dry tarmac, 0.4 on wet roads and below 0.1 on ice.
Maximum cornering speed
  • Maximum cornering speed mv2/r = μmg gives v = √(μgr), independent of mass.

Section 8 of 11Measuring the coefficient

Dynamic coefficient
  • Dynamic coefficient pull a block along a horizontal surface at constant speed with a newton meter;
    • then μd = F/mg.
Graph method
  • Graph method add masses and plot friction against normal force; the gradient is μd.
Tilted plane
  • Tilted plane find the angle at which the block just slides, so μs = tan θ.
Uncertainty
  • Uncertainty it arises from jerky stick–slip motion and surface variations.

Section 9 of 11More from the specification

More from the specification
Frictional force
  • Frictional force acts opposite to motion and has static and dynamic values, with the dynamic force equal to the coefficient times the normal reaction
Circular motion
  • Circular motion velocity changes direction so there is acceleration towards the centre, called centripetal acceleration
Centripetal force
  • Centripetal force magnitude is mass times speed squared divided by radius, provided for example by friction on a turning car
Angular speed
  • Angular speed angular velocity equals linear speed divided by radius, and momentum change in a collision equals impulse
Centripetal force
  • Net force toward the centre, mv2/r.
Angular speed
  • Magnitude of the angular velocity.
Centripetal acceleration
  • Ac = v2/r toward the centre.
Velocity
  • Rate of change of displacement.
Acceleration
  • Rate of change of velocity.
Collision
  • Short interaction with large internal forces.
Linear
  • A graph that is a straight line through the origin, so resistance is constant.
Impulse
  • J = FΔt = Δp.
    What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
Radius
  • Distance from the centre to the circular path.
Tension
  • Force transmitted along a string or rope.
Buoyancy
  • Another name for upthrust.

Section 10 of 11Key terms

TermMeaning
FrictionContact force parallel to surfaces that opposes relative sliding or its tendency
Static frictionFriction on a stationary body, up to a maximum of μsFN
Dynamic frictionFriction on a sliding body, Ff = μdFN
Coefficient of frictionRatio of the frictional force to the normal force; no unit
Normal reactionContact force perpendicular to the surface, FN
Dissipative forceForce whose work is transferred to thermal energy

Section 11 of 11Exam technique

1
Saying friction always opposes motion in the direction of travel — it opposes relative sliding or its tendency.
2
Using μsFN as the friction on a body that is not about to slip — static friction equals the applied force up to that limit.
3
Thinking a larger contact area gives more friction — in the simple model it does not.
4
Assuming friction acts only on moving objects — static friction acts on stationary ones.
5
Taking FN equal to mg on a slope — use mg cos θ.

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