Kinetic friction and constant-speed tests

What you will learn

Dynamic friction; experimental determination and distinction from static threshold.

Specification mapping: B2.3.2

Prerequisites: B2.3.1

Physical principles

Kinetic or dynamic friction acts during sliding and opposes relative surface motion. In the simple dry-friction model Fk = μkN. The coefficient depends on the pair of surfaces and their condition. It is often lower than the limiting static coefficient, but no universal numerical ratio should be assumed.

To measure kinetic friction, pull the object horizontally at constant velocity. Zero acceleration means the applied horizontal force balances kinetic friction, if other horizontal resistances are negligible. Use the steady moving force, not the initial peak that started the object. If speed is increasing, the applied force exceeds total resistance and cannot directly be equated to friction.

Measure mass and normal force conditions, repeat runs, and average a stable portion of the sensor trace. Keep surface condition, pull angle and speed range consistent. Real friction can depend on speed, temperature and wear; state that Fk = μkN is the model being tested. Rolling a trolley measures rolling resistance, not automatically the sliding coefficient for a block.

Scientific diagram and interpretation

Equations and relationships

Fk = μkN

Worked examples and applications

Common misconceptions

The starting peak and steady sliding force measure different coefficients. Constant force does not itself guarantee constant speed.

Examination technique

Explain why constant velocity is required to identify the applied force with resistance.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.