Stick & Slip

Friction is not one force with one value. It is a force that changes to match whatever you do to it — right up until the moment it cannot.

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Push 0.0 N
Guided investigation — ages 14–18 physics

Friction, measured rather than assumed

Every task asks for a written prediction first. Friction is the topic where intuition is most often wrong, so the value is in recording what you believed before you checked.

1The push

The force that changes to match you

Mode: The Push  ·  20 min

Predict

A 5 kg block sits on a table with μs = 0.60. You push it horizontally with 10 N and it does not move. State the size of the friction force, and explain your reasoning.

  1. Set m = 5.0 kg, μs = 0.60, μk = 0.40. Raise the push slowly from zero. State what friction does at every value before the block moves.
  2. Find the largest push the block will take without moving. Compare it with μsN.
  3. Watch the graph as you cross that point. Describe its shape in words — what happens to friction at the instant the block breaks away?
  4. Explain why a block that has just started sliding accelerates, even though you have not increased your push.
  5. Select PTFE / steel, then rubber / dry concrete. Determine the push needed to move the block in each case, and comment on the range.
SurfacesμsN / Nfmax / NμsN / NAgree?
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2Depends

What friction ignores

Modes: Area & Weight, The Slope  ·  20 min

Predict

Two questions, both answered before you touch anything. (a) A brick lying flat, then stood on its narrow edge — which takes more force to start sliding? (b) A 1 kg block and a 20 kg block on the same ramp — which slips at a smaller angle?

  1. In Area & Weight, change the area ratio across its full range. State what happens to fmax and explain why, using pressure.
  2. Now change the mass. Deduce the relationship between fmax and N.
  3. In The Slope, raise the angle until one block slips. Record the angle for each block and comment on your prediction.
  4. Take tan of the critical angle and compare with μs. Derive the relationship by setting mg sin θ = μsmg cos θ, showing where the mass goes.
  5. Outline how you would measure μs between two real materials in this room using only a ruler and a protractor.
3Direction & cost

Which way, and what it costs

Modes: Which Way?, The Cost  ·  25 min

Predict

A car accelerates away from traffic lights. In which direction does the road push on its driving wheels? Then: a box sits unsecured in a van that pulls away sharply. Which way does friction act on the box?

  1. Work through every scenario in Which Way?, committing to an answer before revealing each. Record which ones you got wrong — those are the ones worth revisiting.
  2. Explain why “friction opposes motion” fails for the driving wheels of a car, and state a better sentence.
  3. In The Cost, launch at 8 m s⁻¹ and record the stopping distance. Now launch at 16 m s⁻¹. Compare the two distances.
  4. Show that d = v²/2μkg by equating kinetic energy to the work done against friction, and confirm the mass cancels.
  5. A car travelling at 30 km h⁻¹ stops in 5 m. Determine its stopping distance at 60 km h⁻¹, and discuss what this means for speed limits outside schools.
Colour codeApplied force blue, friction rust, normal teal, weight slate, heat amber — on the diagram, in the readouts, in every mode. Nothing changes colour when it changes context.
The modelThe Coulomb model: fs ≤ μsN and fk = μkN, independent of contact area and of sliding speed. Good for rigid surfaces at ordinary loads; it is a model, not a law of nature.
What is computedEvery force is recalculated each frame from the current state and integrated directly — the block is not following a scripted path. Coefficients for real material pairs are typical published values and vary widely with surface condition.
UnitsSI throughout, with g = 9.81 m s⁻². Forces in newtons, energies in joules, angles in degrees.