Any movement in nature and technology, which involves the presence of physical contact between solids, is accompanied by the occurrence of friction. In this article, we give examples of the friction force and show in which cases it plays a useful role, and in which it is undesirable.
What types of friction between solids are
In this article, we consider only examples of friction forces that act between solid objects that have physical contact with each other.
One of the important types of friction is rest friction. Based on the name itself, it can be assumed that it appears when one body on the surface of another is at rest. Everyone knows that in order to move some heavy object from the place, it is necessary to apply some external force directed along the contact surface of this object and the surface on which it stands. Counteraction to this force is exerted by the friction force of rest. It acts between the contact surfaces of bodies. Rest friction occurs due to the presence of roughness on the touching surfaces, no matter how smooth they are.
The second type of friction, which we will consider, is sliding friction . It also arises due to the roughness mentioned above, when the bodies begin to move relative to each other by means of sliding. The direction and point of application of the sliding friction force are exactly the same as for rest friction. The only difference between these forces is that the gliding force is always less than the rest force.
The third type of friction, which plays no less a role in technology than the first two, is rolling friction. As its name says, it appears when one body rolls over the surface of another. The cause of rolling friction is the deformation hysteresis, which leads to the "dispersion" of the kinetic energy of the rolling body. In a number of practical cases, this friction force is 10-100 or more times less than the previous types of friction considered.
All types of friction forces are directly proportional to the reaction force of the support with which the latter acts on the body under consideration.
The Harm and Benefits of Rest Friction: Examples
Of all the mentioned types of friction, perhaps the rest friction is the most "harmless". The fact is that in practice it almost always plays a useful role. Its only negative point is that it is more than sliding friction. The latter fact means that for any beginning of the movement it is necessary to make a great effort. For example, to start skiing in the snow, you should first literally “tear” them from the snow surface.
There are many examples of the benefits of static friction. We list them:
- Nails and screws that firmly hold two solid bodies of wood, plastic and metal, perform their functions due to the action of the force in question.
- A person’s walking, car driving on the roads is due to the fact that the friction of rest is greater than the sliding friction. Otherwise, it would be hard for us to move, people and vehicles would slide in one place.
- Any bodies that rest on inclined surfaces are due to the action of rest friction. If the latter were not there, then it would be impossible to put a car on a slope or any household item on a table that has a slight inclination to the horizon on the hand brake.
Sliding friction and its benefits
Unlike rest friction, which mainly plays a positive role in human life, sliding friction, as a rule, is a harmful force. Nevertheless, two examples of useful sliding friction force can be given:
- Since sliding friction leads to heating of the surface of objects (the natural and easiest way to transfer mechanical energy to heat), this effect can be used to increase the temperature of bodies. So, in antiquity, our ancestors fired using sliding friction.
- When the driver wants to stop the vehicle, he presses the brake pedal. In this case, the brake discs slide inside the wheel rim and inhibit its rotation.
Harm of sliding friction
Examples of the action of the sliding friction force are the movement of the cabinet on the floor when we want to rearrange it in the room, the skier and skater sliding, the wheels of the car slip when they lock or when moving on a slippery road, the sliding between the rubbing parts of the mechanisms of various machines.
In all these cases, sliding friction plays a harmful role. The mentioned examples of harm to the sliding friction force are related to the fact that it interferes with the mechanical movement and “eats” a certain fraction of kinetic energy (skis, skates, moving parts of machines). In addition, the transfer of part of the mechanical energy into heat leads to the heating of the rubbing parts. An increase in their temperature leads to a change in the microscopic structure, which violates the properties of materials. Finally, the listed examples of sliding friction force lead to wear of rubbing surfaces, the appearance of unwanted furrows on them, and thinning.
Rolling friction and its harm and benefits
If we fundamentally consider the issue of the benefits of rolling friction, it turns out that it does not exist at all. Indeed, rolling friction always prevents mechanical rotation, it leads to wear of the working parts and to their undesirable heating. Nevertheless, the phenomenon of rolling is widely used in technology (bearings, wheels of vehicles). This is explained by the fact that the rolling friction force is much less than the similar sliding force, which reduces the extent of its harmful effect by orders of magnitude.
Increase and decrease in friction
As we saw in the examples above, the static and sliding friction forces are sometimes useful, and sometimes harmful. In this regard, mankind has long been using methods to change the scale of the action of friction both in the direction of increasing the corresponding force, and in the direction of decreasing it.
Vivid examples of how to increase the friction force are sanding with salt and ice on the roads. As a result of these actions, there is an increase in the roughness of the ice surface and, as a result, an increase in the friction forces of rest and sliding.
Another way to increase the forces under consideration is to use special surfaces. A striking example is the surface of a winter car tire, which is characterized by a deep tread and the presence of metal spikes.
During skiing, as well as during the rotation of bearings of various mechanisms, friction plays a negative role. To reduce it, special greases are used, usually based on fats (wax, lithol).