Mechanics
Friction
Push a block along the groundf ≤ μsN, f = μkN once sliding
Friction vs applied force
Friction RISES to match F (static), then plateaus at μkN once sliding
Show forces
Parameters
Mass5 kg
Applied force25 N
Static μs0.4
Kinetic μk0.3
μk is kept ≤ μs, as it always is physically
Calculated
Normal reaction N49.1 N
Max static friction19.6 N
Current friction14.7 N
Statesliding
Acceleration2.06 m/s²
Curriculum
WAECNECOIGCSESATJUPEB
📋 Teacher notes
- •Static friction is NOT fixed — it exactly matches the applied force, up to a maximum of μsN. Push harder within that limit and friction grows to match; nothing moves.
- •Once the applied force exceeds μsN, the block breaks free and KINETIC friction takes over — μk is always a little LESS than μs, which is why things "jerk" into motion.
- •Friction is independent of the contact area and (to a good approximation) of speed — but always proportional to the normal reaction N.
- •N = mg only holds here because the surface is flat and the push is horizontal — on a slope, or with an angled push, N changes.
- •Real applications: brake pads (want HIGH μ), ice skates and ball bearings (want LOW μ), why worn tyres skid more easily.
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