What are the different uses of friction

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In linguistic usage, friction has a negative connotation: friction disrupts harmony, we want smooth processes. And yet friction is useful in our everyday lives: when making a fire, when steering, even when writing. And since da Vinci, the physics of friction has been a topical research area.

By: Sabine Schmitt & Stefan Gneiting

Status: 02.02.2012

Here you will learn, among other things:

  • know the basic types of friction,
  • that sliding friction and rolling friction forces are smaller than static friction forces;
  • · Why aids such as sledges, rollers and carts with wheels make it easier to transport heavy objects;
  • that friction also has positive effects and is even necessary for many processes.

Gliding unchecked over the snow or being able to move heavy furniture without exertion - who wouldn't want that. But even on the most beautiful snow, sooner or later a toboggan loses speed and when moving around the room, the cartoon character Suse (from the picture gallery above) breaks a sweat. This is due to the physical phenomenon of friction.

The frictional force occurs wherever two bodies touch. It is directed against the direction of movement and therefore has a braking effect. It is the reason that sledges stop at some point and that Suse cannot push her wardrobe through the room so easily. But there are also tricks that can be used to reduce friction: sleds with polished steel runners slide more easily than those with rough wooden runners, and an empty wardrobe is easier to move than a full one.

But would our lives become easier if we eliminated friction everywhere? Definitely not, because friction is useful in many areas: Today we use the friction between the rim and brake shoes to always bring the bike to a stop in time and use shoes with treaded soles to be able to safely walk up a mountain. Friction also does valuable work for us when it comes to grinding, sanding and grinding. And: We couldn't even walk anymore if we eliminated friction altogether.

Even the Stone Age people made use of friction and kindled fires by drilling, i.e. rubbing wood against each other. And long before our era, the Sumerians and Egyptians used sledges to transport the tons of stones that they needed for their monumental structures. So people knew early on how to make the phenomenon of friction their own, or how to overcome the resulting obstacles.

Scientific work to research and quantify the friction phenomena did not exist until much later. At the end of the 15th century, Leonardo da Vinci carried out quantitative studies on friction. He found that the frictional resistance is independent of the size of the contact area and proportional to the weight of the body. And da Vinci thought about how one could design smooth-running machines: in 1495 he described the first ball bearing he developed.

Da Vinci's work was forgotten for a long time. It was not until 1699 that the Frenchman Guillome Amontons presented to the Académie Royale the connections between frictional force, contact area and weight, which had actually been discovered by da Vinci, and brought the results back to the memories of the scientists.

The Swiss mathematician Leonhard Euler also dealt with friction. For example, around 1750 he solved the rope friction problem and introduced the coefficient of friction μ. Especially animated by Amonton's work, Charles Augustin Coulomb dealt with friction problems. Among other things, he investigated the dependence of the frictional force on the time that the materials are at rest. And he realized that the amount of static friction is greater than sliding friction. He published his findings in 1781 in his book "Théorie des machines simples".

Whenever a body moves, a frictional force (or in short: friction) acts on it. The friction is always opposite to the direction of movement and therefore slows the body down. The most important types of friction are: static friction, sliding friction and rolling friction.

Static friction is the force that you have to overcome if you want to set an object in motion; for example a wardrobe on the floor. Because this force increases proportionally with the mass of the body, an empty wardrobe is easier to push than a full one.

As soon as the static friction is overcome, the wardrobe slides more easily on the floor, since only the sliding friction has to be compensated. It also depends on the mass of the moving body. An empty wardrobe is easier to move than a full one. In addition to the weight, a second factor also plays a role: Depending on whether the cabinet is supposed to slide over a rough wooden floor or over smooth tiles, you need more or less force.

If you put a wardrobe on castors in order to eliminate the sliding friction, you still have to use force to overcome the rolling friction. It also depends on the two factors, the mass of the moving body and the material properties of the floor and rollers.

If you compare static friction, sliding friction and rolling friction of a certain object on a given surface, you will find that the static friction is always the strongest and the rolling friction the smallest.

By the way: Not only movements on the ground are slowed down by frictional forces. Air and water also slow down moving bodies - but the frictional forces when swimming or flying are considerably lower than when gliding, running or rolling over a solid surface.

The friction is based on several principle mechanisms.

Adhesion: The force of adhesion is an electromagnetic force that prevails between atoms or molecules of different substances. These are so-called van der Waals forces, which have a very short range. This means that their strength drops rapidly as the distance between the two surfaces increases. When surfaces move relative to one another, the bonds created by the adhesive forces are constantly being loosened and re-formed. The resulting forces counteract the movement. The adhesion does not change the surfaces. Important areas of application for the adhesive forces are adhesives or self-adhesive envelopes.

Plastic deformation: As the name suggests, plastic deformation involves a change in the surface. Plastic deformation converts mechanical kinetic energy into other forms of energy, mainly heat.

Abrasion: Abrasion or furrowing takes place primarily between bodies of different hardness. Thereby, a microscopic penetration of roughness peaks of one body into the other takes place. The movement tears particles out of the body. Abrasion is essentially responsible for the wear and tear of moving components, for example hinges or axles. Sanding wood with sandpaper is essentially based on abrasion.

Energy dissipation: With energy dissipation, mechanical energy is converted into other forms of energy. The processes are complex and the subject of research to this day. With energy dissipation, for example, energy can be dissipated via lattice vibrations. Among other things, heat or structure-borne noise is generated.

Writing with chalk on the blackboard also has something to do with friction. Because the white writing on the dark blackboard is nothing more than the abrasion of the chalk pencil, which occurs because of the high friction between the two materials chalk and blackboard. The decisive friction mechanism here is the furrowing, also called abrasion. The prerequisite for abrasion is that the two materials are of different hardness; In the specific example, the chalk is much softer than the blackboard. During abrasion, the harder body penetrates the softer surface and tears small particles out of it because of the forward movement. The torn chalk particles stick to the board due to the adhesive forces and make the path that the chalk has covered on the board visible.

A squeaky door can be opened again silently if you drip a little oil on the hinge; a piece of wood is easier to saw through if you have put a little soft soap on the saw blade beforehand; and a puddle of oil on the street makes cars skid dangerously.

Lubricants reduce the friction between two materials because they form a film between the sliding surfaces and separate them from one another. But contrary to what the term lubricant suggests, they don't always have to be slippery or oily: Even normal water sometimes serves as a lubricant, for example when ice skating.

Strictly speaking, the blades of the ice skates do not slide on the ice, but on a film of water. That this occurs is a result of friction: the movement of the runners over the ice causes frictional heat that melts the ice. The film of water between the skate blade and the ice acts as a thin lubricating film that significantly reduces friction. On cloudless winter days, when the sun warms the ice additionally and the top layer thaws a little, ice skaters can set real speed records.

Incidentally, until a few years ago it was still believed that the pressure of the runners on the ice would heat the surface so much that the top layer of ice would melt. This claim still haunts some university lectures and can be found in books. But it has now been refuted: pressure can heat the ice selectively, but a 70 kg athlete with 300 mm long and 0.5 mm wide runners only brings a pressure of 4578 kPa onto the ice. The two scientists Jürgen Vollmer from the Max Planck Institute for Dynamics and Self-Organization and Ulrich Vetter from the Institute for Atomic and Nuclear Physics at the University of Göttingen calculated that this is just enough for a warming of 0.2 ° C and is therefore not an explanation suitable for the formation of the water film. "Even at ice temperatures just a few degrees below freezing point, no water film would form," they summarize.

Tribology is a scientific discipline that studies friction processes. The aim is to use the knowledge to optimize movement systems by regulating the effects of friction. Nature often provides these scientists with fascinating role models. A well-known example is the shark skin: a fine pattern of grooves reduces the friction with the surrounding water so much that the sharks can glide through the sea with almost no resistance. The example from nature was used, among other things, in swimming suits by athletes, who actually set speed records with them.

A relatively young research project brought amazing things to light on the sandfish. The sandfish (Scincus albifasciatus) is a 15 centimeter long reptile species that is at home in the Sahara. It can bury itself in the sand in a flash and “swim” in it, because its scaly skin opposes the sand with even less frictional resistance than high-tech materials such as Teflon, nylon, glass or polished steel. That is why the sandfish glides through the sand with almost no resistance. At the same time, the scales are so robust that the pointed grains of sand cannot scratch them. Under the electron microscope, the scientists discovered why this is so: the skin is covered with a fine pattern of tiny swellings that run across the direction of movement of the sandfish and have extremely fine tips. These miniature combs, which are integrated into the sandfish skin, clean the sand grains from adhering fine dust so that the now smooth grains glide over the animal's skin with less friction and almost no abrasion.

The scientists already have a lot of ideas as to where the sandfish effect could be used to reduce friction: wherever liquid lubricants have disadvantages. For example on floors where a sandfish surface could prevent abrasion by sand particles between the base and the shoe sole. The use of liquid lubricants is also problematic in food technology because traces of them get into the food again and again in industrially produced or packaged foods. If it were possible to produce a material with sandfish properties, an effective, maintenance-free dry lubrication could be developed. But it can take a long time before that happens: So far, the laboratory has only succeeded in producing an imitation sandfish with an area of ​​just one square centimeter - for industrial use, however, the artificial sandfish skin would have to be several square meters in size.

List processes from everyday life in which friction plays a major role and explain the influence of friction. Examples can be: light a match, go uphill, ice-skate, drive a car.

Friction can be a major problem when moving or transporting heavy objects. But there are also situations in which friction makes life easier. Give examples of the positive effects of friction when cycling.

Explain why a bicycle chain slides better when you oil it.

Name the three most important frictional forces and order them according to their size.

Which piece of furniture can be moved more easily without tools: a 40 kg chest of drawers with a floor space of 0.7 square meters or a 40 kg cupboard with a floor space of 1 square meter?

Curriculum reference (Bavaria)

secondary schools
PCB
7th grade
7.4 Fundamentals of mechanics
7.4.1 Forces
- different forces, e.g. B. Frictional Force
- Measuring forces

secondary school
physics
8.1 Mechanics of friction
- Static friction force, sliding friction force, rolling friction force
- Presentation of the model
- Relationship between frictional force and contact pressure (normal force); Coefficient of sliding friction
- Significance of frictional forces in the environment, technology and road traffic

high school
nature and technology
7th grade
7.2.1 Forces in nature and technology
- Reference to frictional and magnetic force