What is Static Friction?

Static friction is a force that resists the movement of two objects against one another when the objects are initially at rest. A simple example is a wooden block sitting on a ramp — a force needs to be applied to make the block slide down the ramp. Another term, kinetic friction, applies to the force that opposes objects that are already moving against one another. The strength of these forces can be calculated and is known as the coefficient of friction. In real-life situations, the coefficient of static friction is nearly always found to be greater than that for kinetic, but in carefully controlled experiments, where the surfaces of the objects have been thoroughly cleaned, the two are generally the same.

Typically, as the force applied to an object on a surface increases, the static friction force will initially increase to match it, so that the object does not move. After a certain point, however, the object will begin moving, and at this point, the friction force will drop, so that less force is required to keep the object moving. For example, the friction force may match the applied force up to 50 newtons — force is measured in newtons (N) — but thereafter, it may drop to 40 N. Therefore, a force of just over 50 N is required to get the object moving, but thereafter, just over 40 N will suffice.

Static friction coefficients can be calculated for any solid material or pair of materials. A coefficient value might, therefore, apply to wood on wood, steel on steel, or steel on wood. One way of calculating the value for a pair of materials is to place a block of one material on a ramp made of the other — for a single material, the block and ramp would be made of the same substance. The slope on the ramp is gradually increased, until the block slides down. The angle at which this happens can then be used to calculate the coefficient of static friction.

The coefficient, when used in formulae and equations, is given the symbol μ — the Greek letter mu. A subscript is usually employed to distinguish the two: μ_s indicates static friction, while μ_k means kinetic friction. For example, the μ_s for steel on steel is 0.74, while the μ_k for this material is 0.57. These values are for typical real-life situations, and may vary a little, depending on circumstances. Since the μ_s value can be affected by surface irregularities, dirt, and traces of other substances, the μ_k value is regarded as more accurate, and is the one usually given when a simple coefficient of friction is required.

A number of factors contribute to static friction, but usually the most important one is the roughness of the surfaces. Even when smoothed down, different materials will vary in terms of the fine details of their surfaces. In practical terms, no surface is completely smooth, but some will have larger irregularities than others. The difference is obvious, in some cases: for example, a silk sheet has a very smooth texture that creates less friction, while a dry asphalt road is coarse, generating more resistance to movement. Other factors include electrostatic attraction and the types of weak chemical bonds that can form between surfaces.

Many people are familiar with static friction, since they encounter it on an almost daily basis; for example, it is at work when someone slides a book across a table. Initially, a small amount of force needs to be exerted to get the book to move, but once it is moving, kinetic friction comes into play, and less effort will be required to move it. The amount of force required can vary according to circumstances. For example, if a book has a library cover on it and it got damp, the wet book will require more force to move, while a brand new paperback book might slide very easily across a dry wooden table with a varnished surface.

Tables of static and kinetic friction coefficients are available for many common materials and combinations thereof. A higher value indicates greater friction, so that more force needs to be applied to cause motion. For example, the μ_s for aluminum on aluminum is 1.05 – 1.35, which is very high, while the value for polytetrafluoroethylene (PTFE) on PTFE is 0.04, which is extremely low and makes it very slippery. It is difficult to push a stopped car into motion because of the intentional friction between the tires and the ground; this allows the driver more control and makes the car less likely to skid.

One example of the application of static friction is in calculating the breaking distance for a car at a given speed and in particular conditions. In normal circumstances, when the tires are turning on the road, static, rather than kinetic, friction, applies. The μ_s for a dry tire on a dry road is about 1.00, whereas the value for a wet tire on a wet road is only 0.2 — this means that the breaking distance will be five times greater in wet conditions. In dry conditions, a car traveling at 31 miles per hour (50 kph) has a braking distance of 33 feet (10 meters), whereas in wet conditions, the braking distance would be 164 feet (50 meters). When the tires are sliding, rather than rolling, along the surface — as might be the case in icy conditions — it is kinetic friction that is important.