Measuring the coefficient of kinetic friction on an inclined plane
PhysicsForces & DynamicsAges 15–16Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Forces & Dynamics
Virtual lab: a block slides from rest down a board inclined at angle α, and a digital timer with two light gates measures the time t taken to cover distance L. Students raise the angle and release the block several times; the data table automatically calculates a = 2L/t² and μ = tanα − a/(g·cosα), and the mean is compared with μ for the chosen pair of materials. A Motion graphs screen rolls a solid ball or a trolley down the incline past 8 light gates (or marks timed with a stopwatch): students plot x–t and v–t graphs, find the acceleration from the v–t gradient and compare it with g·sinα (5/7 of it for a rolling solid ball).
Lesson: Weight and friction; practical: measuring the coefficient of kinetic friction
What it shows
A block sliding down a rough incline obeys a = g(sinα − μcosα), so timing it between two light gates (a = 2L/t²) gives the coefficient of kinetic friction; at small angles static friction holds it at rest. The Motion graphs screen lets a solid ball or a low-friction trolley roll from rest past marks every 0.20 m. Distance against time is a parabola, and the mean velocity of each interval, plotted at its mid-time, lies on a straight line whose gradient is the acceleration. A trolley gives a ≈ g·sinα; a rolling solid ball gives only 5/7 of it, because part of the energy goes into rotation.
How to use
On Friction coefficient, choose the Materials, set the angle with the handle or Initial angle of incline and press Release several times; each trial adds a and μ to the table. On Motion graphs, choose the Body, the Angle α and the Timing, then press Release. Read the times in the table, compare the x–t and v–t graphs, and repeat at other angles to compare a with g·sinα.
Parameters you can change
- Pair of materials Wood – wood (μ ≈ 0.30), Plastic – wood (μ ≈ 0.20), Rubber – wood (μ ≈ 0.45), Steel – oiled steel (μ ≈ 0.12)
- Initial angle of incline 5–60 °
- Distance between the two light gates 0.5–1.5 m
- Show the true coefficient of friction for comparison
- Screen Measuring the friction coefficient, Motion graphs on an incline
- Body on the incline Solid ball (rolls), Low-friction trolley
- Angle of incline (graphs screen) 2–20 °
- Timing method 8 light gates, Marks + stopwatch
Questions to explore
- Why should you repeat the measurement several times and use the mean value of μ?
- When you change the angle α, t and a change; does the calculated μ change too, and why?
- At roughly what angle does the block start to slide, and how is that angle related to the friction coefficient?
