Static friction is the force that opposes the movement of two objects at rest, while kinetic friction opposes already moving objects. The coefficient of friction can be calculated and varies depending on the materials and surface roughness. Factors affecting friction include surface roughness, electrostatic attraction, and weak chemical bonds. Static friction is encountered in daily life, such as when sliding a book across a table. Tables of friction coefficients are available for common materials. Static friction is used to calculate the braking distance of a car, with wet conditions resulting in a longer braking distance due to a lower coefficient of friction.
Static friction is a force that resists the movement of two objects against each other when the objects are initially at rest. A simple example is a block of wood sitting on a ramp: a force must be applied to slide the block down the ramp. Another term, kinetic friction, applies to the force opposing objects that are already moving against each other. The strength of these forces can be calculated and is known as the coefficient of friction. In real-life situations, the static coefficient of friction is almost always found to be greater than the kinetic one, but in carefully controlled experiments, where the surfaces of objects have been thoroughly cleaned, the two are usually the same.
Typically, as the force applied to an object on a surface increases, the static frictional force will initially increase to match it, so that the object does not move. After a certain point, however, the object will begin to move, and at this point, the frictional force will decrease, so that less force is needed to keep the object moving. For example, the frictional force can be the force applied up to 50 newtons – the force is measured in newtons (N) – but then it can decrease to 40N. Therefore, a force of slightly more than 50N is required to obtain the object in motion, but a little more than 40 N will suffice afterwards.
Calculation of the coefficient
Static coefficients of friction can be calculated for any solid material or pair of materials. A coefficient value could therefore apply to wood to wood, steel to steel, or steel to wood. One way to calculate 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 occurs can then be used to calculate the static coefficient of friction.
The coefficient, when used in formulas and equations, is given by the symbol μ – the Greek letter mu. A subscript is usually used to distinguish the two: μs stands for static friction, while μk stands for kinetic friction. For example, the μs for steel on steel is 0.74, while the μk for this material is 0.57. These values refer to typical real-life situations and may vary slightly depending on the circumstances. Since the μs value can be affected by surface irregularities, dirt and traces of other substances, the μk value is considered more accurate, and is usually indicated when a simple coefficient of friction is required.
Factors affecting friction
Many factors contribute to static friction, but usually the most important is the roughness of the surfaces. Even when smooth, different materials vary in the fine detail of their surfaces. In practical terms, no surface is completely smooth, but some will have more irregularities than others. The difference is evident, in some cases: for example, a silk sheet has a very smooth texture that creates less friction, while a dry asphalt road is rough, generating more resistance to movement. Other factors include electrostatic attraction and the types of weak chemical bonds that can form between surfaces.
Examples
Many people are familiar with static friction, having encountered it on an almost daily basis; for example, it’s at work when someone slides a book across a table. Initially, a small amount of force must be exerted to get the book to move, but once it does move, kinetic friction kicks in and less effort will be required to move it. The amount of force required can vary depending on the circumstances. For example, if a book has a library cover and it has gotten wet, the wet book will require more force to move, whereas a brand new paperback might slide very easily on 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 means more friction, so more force must be applied to cause motion. For example, the μs 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. A stationary car is difficult to start because of the intentional friction between the tires and the ground; this gives the driver more control and makes the car less likely to skid.
Calculation of the braking distance
An example of the application of static friction is in calculating the braking distance for a car at a given speed and under particular conditions. Under normal circumstances, when tires spin on the road, static rather than kinetic friction is applied. The μs for a dry tire on a dry road is approximately 1.00, while the value for a wet tire on a wet road is only 0.2: this means that the braking distance will be five times greater in wet conditions. In dry conditions, a car traveling at 31 miles per hour (50 km/h) has a braking distance of 33 feet (10 meters), while in wet conditions the braking distance would be 164 feet (50 meters). When tires are sliding, rather than rolling, along the surface, as might be the case in icy conditions, kinetic friction is important.
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