The friction increases with increasing speed


Lexicon> Letter R> Friction

Definition: a process in which mechanical energy is converted into heat

English: friction

Categories: Basic Concepts, Physical Basics

Author: Dr. Rüdiger Paschotta

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Original creation: 09/25/2014; last change: 03/14/2020


Frictional forces (Frictional forces) occur, for example when solid bodies move against one another when their surfaces are in contact or when a solid body is pulled through air or water. Friction also occurs when gases or liquids are moved through pipelines. Frictional forces inhibit the macroscopic movement of bodies, whereby mechanical energy is converted into heat. It is an example of dissipation.

The part of science and technology that deals with friction phenomena is called Tribology designated.

Different types of friction

Depending on the situation, there are different types of friction, for example the following:

  • Solid body friction (Coulomb friction) occurs when solid bodies directly rub against one another.
  • Fluid friction is used when the energy losses occur within a fluid, e.g. B. a lubricating oil film.
  • Even with bearings lubricated with oil or grease, especially at low speeds, a Mixed friction occur, d. H. a combination of solid body friction (through the contact of solid parts) and fluid friction. At higher speeds, the proportion of solid body friction disappears, but the fluid friction becomes stronger.
  • Adhesive layer friction occurs when there is a solid lubricant layer between solid friction bodies.
  • Static friction refers to frictional forces that occur as long as there is no relative movement between friction bodies. In order to set a slide in motion, for example, the higher static friction must first be overcome, while the lower sliding friction occurs when the movement is already in place.

Energy losses through friction

In connection with energy, friction processes are often undesirable because they convert a high-quality form of energy (pure exergy) into a lower-value (less usable) form of energy. The stronger the frictional forces z. B. in a moving vehicle, the more drive energy the motor has to continuously supply in order to maintain the speed of the vehicle. Even where the resulting frictional heat can still be used, the conversion of high-quality mechanical energy into heat usually results in an overall increase in energy consumption.

Minimizing frictional forces

For the reasons mentioned, one tries to make frictional forces as weak as possible. For this purpose, one uses, for example, lubricants (such as lubricating oil or grease), which reduce the friction in motors, gears and bearings by forming a thin, lubricious film. The lubricant should have a sufficiently high viscosity (viscosity) to be able to build up a coherent lubricating film; however, the viscosity should also not be too high that the fluid friction becomes too strong.

One problem with internal combustion engines is that the viscosity of lubricating oil decreases sharply with increasing temperature and therefore optimal conditions are only possible in a small temperature range; in particular, the viscosity of the oil is significantly higher than necessary immediately after a cold start, which is one of the reasons for the increased fuel consumption. (Even at an oil temperature of 20 ° C, the friction losses in the engine can be twice as high as at an operating temperature of 90 ° C.) After all, modern lubricating oils (especially certain synthetic oils) have a reduced temperature dependency of the viscosity, which reduces this problem.

The drag of vehicles, i.e. H. The friction in the outside air in moving vehicles can be reduced by minimizing the cross-sectional area and also by optimizing the drag coefficient (cW.Values). The latter value depends on the geometric shape of the outer shell of the vehicle.

Similarly, when a ship is moving, friction is reduced if its displacement (determined by its total mass) is minimized and the shape of the ship's hull is optimized.

Dependence on the speed

Depending on the conditions, the strength of the friction can depend more or less on the speed of a movement:

  • The frictional force due to the rolling friction of a car (on the contact area between tires and road and also in bearings) hardly depends on the speed; see the article on rolling resistance. This means that this contribution to fuel consumption per kilometer driven is practically independent of speed. This contribution is proportional to the vehicle weight, and at low driving speeds (e.g. 30 km / h) this is the largest contribution to the total friction. It can increase if the air pressure in the tires is too low.
  • The situation with air friction is completely different. At typical vehicle speeds, turbulent air currents are created and in this case the frictional force increases roughly in proportion to the square of the speed. This means that the energy losses per kilometer driven increase fourfold when the speed is doubled. (The fuel consumption increases a little less quickly, however, as the efficiency of the engine often increases with higher power.) At a speed of 100 km / h, the friction caused by air resistance is clearly dominant.
  • Between the cases mentioned above, the friction lies in slow movements, where laminar (non-turbulent) flows occur. Here the frictional force is proportional to the speed. This situation can exist, for example, when a ship is moving slowly.

The article on the driving resistance of a vehicle has more details and a numerical example for a car.

Targeted use of friction

In some cases, friction is used in a targeted manner, for example in the brakes of vehicles. However, the disadvantage here is that the kinetic energy originally supplied by a motor is converted into mostly useless heat, which can even have a disruptive effect (for example due to the overheating of the brakes). Therefore, other methods of braking without the use of friction are preferable. This takes place as part of the braking energy recovery (recuperation).

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See also: air resistance, rolling resistance, driving resistance, dissipation, power, recuperation
as well as other articles in the categories basic concepts, physical fundamentals