4.1 Definition of friction

Friction is the force that opposes the relative motion, or the tendency to relative motion, between two surfaces in contact. It acts along the surfaces (parallel to them), in the direction opposite to the motion or attempted motion, and it arises only when there is contact and a force trying to make one surface slide, roll or flow over the other. Like all forces it is a vector, measured in newtons (N). Friction is both useful and wasteful: without it we could not walk, grip a burette tap or stop a vehicle, yet it also wears out moving parts and turns useful energy into heat. Example: maintenance staff at a maize milling plant in Kamwosor grease the bearings of the hammer mill every week to cut friction losses in the machine.

Kinds of friction

• Static friction: The friction between surfaces that are not yet moving relative to each other. It adjusts itself to match the applied force, up to a maximum value, and so holds a reagent bottle still on a sloping shelf.
• Limiting friction: The maximum value of static friction, reached just as the body is about to slide. Any larger applied force starts the motion.
• Kinetic (sliding or dynamic) friction: The friction between surfaces that are sliding over each other. It is slightly smaller than limiting friction and stays roughly constant at moderate speeds.
• Rolling friction: The much smaller resistance when a wheel, roller or ball rolls over a surface, caused mainly by slight flattening of the surfaces at the contact. It is why trolleys and ball bearings move easily.
• Fluid friction (viscous drag): The resistance to motion through a liquid or gas, or to the flow of a fluid itself. It depends on speed, shape and viscosity, and explains the terminal velocity of a falling sphere in a viscometer.

Friction as a force

• Direction: Always opposite to the relative motion or attempted motion of the surfaces.
• Magnitude: Static friction can be anything from zero up to the limiting value; kinetic friction F = μR, where μ is the coefficient of kinetic friction and R the normal reaction (see topic 4.5).
• Unit: The newton (N).
• Effect on energy: Work done against friction is turned into heat (and some sound), which is why rubbing hands warms them and brakes get hot.

Static friction against applied force

If a small horizontal pull is applied to a heavy box on the floor and slowly increased, the box at first does not move: static friction rises to equal the pull exactly. When the pull reaches the limiting friction, the box starts to slide, and the friction then drops slightly to the kinetic value, so a smaller force is enough to keep it moving at a steady speed. A graph of friction against applied force therefore rises at 45° (friction equals pull), peaks at the limiting value, then falls a little to a level line.

Friction in the laboratory

• Ground glass joints: Friction holds stoppers and joints together; a thin film of grease reduces it so they can be separated.
• Stopcocks and taps: Burette and separating funnel taps need enough friction to stay in place but not so much that they stick.
• Centrifuges and stirrers: Friction in bearings produces heat and wear, so these parts are lubricated and checked.
• Viscometers: Measure fluid friction (viscosity) of oils, syrups and polymer solutions.