Friction is an external factor
Publish: 2021-05-01 20:00:39
1. The surface roughness of the object, the material of the object, the air humidity
2. What is the cause of friction, that is, the essence of friction? As early as the beginning of the 15th century, people began to explore the friction phenomenon
friction occurs between two contact surfaces. We can see the smooth solid surface with the naked eye. When we observe it with a magnifying glass, we will find that the surface is uneven, as if it is full of peaks and valleys. The peak height of metal surface processed by lathe can reach 5 microns; After careful grinding, the peak height can be reced to 0.1 μ M. But this is still very large relative to the atomic scale. Therefore, when two objects contact each other, the real contact is only the peaks and hills on the surface of the object. In these real contact areas, the atoms on the two contact surfaces are very close, and there is a very strong interaction between the atoms. In other areas outside the contact point, the distance between the atoms on the surface of two objects is relatively large, ranging from a few angstroms to tens of angstroms (1 angstrom = 10-6 cm). The interaction between these atoms is relatively weak and can be neglected compared with the real contact area. The sum of the contact areas is much smaller than the apparent area of the object
when the two contact surfaces slide relatively, it is bound to destroy all the original contact points in the form of shear, so that the convex parts of the two contact surfaces collide and proce fracture and wear, thus forming an obstacle to the movement of the object. The shear force needed to destroy the contact point parallel to the contact surface is equal to the friction force
when the surfaces of two objects begin to contact, it is a kind of point contact. Under the action of normal positive pressure. The load of these contact points is very large, which will cause the elastic-plastic deformation of the convex parts in contact, thus increasing the contact area. Hard materials make it difficult to proce this kind of deformation, that is to say, it needs more load to proce the same amount of deformation. Therefore, the real contact area of two surfaces is directly proportional to the applied normal positive pressure and inversely proportional to the hardness of the contact material
friction coefficient μ It is equal to the ratio of friction f to normal normal pressure n, i.e μ＝ f／N friction coefficient μ It has nothing to do with the apparent contact area and sliding speed of two objects. The experimental results show that the friction coefficient of the machined surface is higher than that of the machined surface μ It has nothing to do with the surface roughness of the object; For very rough surfaces, because the convex and concave parts of the contact surface mesh alternately, the friction coefficient will decrease μ enlarge; For very smooth surfaces, especially for particularly clean surfaces, the friction coefficient decreases e to the increase of the real contact area and the bond strength at the contact points μ It will also increase. The smoother the surface, the higher the friction coefficient μ The higher the value is, which is different from common sense.
friction occurs between two contact surfaces. We can see the smooth solid surface with the naked eye. When we observe it with a magnifying glass, we will find that the surface is uneven, as if it is full of peaks and valleys. The peak height of metal surface processed by lathe can reach 5 microns; After careful grinding, the peak height can be reced to 0.1 μ M. But this is still very large relative to the atomic scale. Therefore, when two objects contact each other, the real contact is only the peaks and hills on the surface of the object. In these real contact areas, the atoms on the two contact surfaces are very close, and there is a very strong interaction between the atoms. In other areas outside the contact point, the distance between the atoms on the surface of two objects is relatively large, ranging from a few angstroms to tens of angstroms (1 angstrom = 10-6 cm). The interaction between these atoms is relatively weak and can be neglected compared with the real contact area. The sum of the contact areas is much smaller than the apparent area of the object
when the two contact surfaces slide relatively, it is bound to destroy all the original contact points in the form of shear, so that the convex parts of the two contact surfaces collide and proce fracture and wear, thus forming an obstacle to the movement of the object. The shear force needed to destroy the contact point parallel to the contact surface is equal to the friction force
when the surfaces of two objects begin to contact, it is a kind of point contact. Under the action of normal positive pressure. The load of these contact points is very large, which will cause the elastic-plastic deformation of the convex parts in contact, thus increasing the contact area. Hard materials make it difficult to proce this kind of deformation, that is to say, it needs more load to proce the same amount of deformation. Therefore, the real contact area of two surfaces is directly proportional to the applied normal positive pressure and inversely proportional to the hardness of the contact material
friction coefficient μ It is equal to the ratio of friction f to normal normal pressure n, i.e μ＝ f／N friction coefficient μ It has nothing to do with the apparent contact area and sliding speed of two objects. The experimental results show that the friction coefficient of the machined surface is higher than that of the machined surface μ It has nothing to do with the surface roughness of the object; For very rough surfaces, because the convex and concave parts of the contact surface mesh alternately, the friction coefficient will decrease μ enlarge; For very smooth surfaces, especially for particularly clean surfaces, the friction coefficient decreases e to the increase of the real contact area and the bond strength at the contact points μ It will also increase. The smoother the surface, the higher the friction coefficient μ The higher the value is, which is different from common sense.
3. The friction coefficient of the material the surface finish of the object the pressure on the friction surface
4. A large number of experiments show that the sliding friction is only related to the pressure and the roughness of the contact surface; The coarser the contact surface is, the greater the sliding friction is.
(3) sliding friction is the force that hinders the relative movement of objects in contact, not necessarily the force that hinders the movement of objects, It should be clear that "relative motion" is hindered by objects in contact with each other as reference objects. "Object
Motion" may be based on other objects as reference objects. For example, in the experiment, when a weight is placed on the wood block and the spring scale is used to pull the wood block for uniform linear motion
, The specific situation is: when the wooden block is pulled forward by the pulling force
from static, the weight will slide backward relative to the wooden block, and the wooden block will give the weight a friction to prevent it from sliding backward, The direction of the friction is forward, so the weight does not slide relative to the wood block, and the friction is static friction.
(4) the sliding friction has nothing to do with the speed of the movement of the object, and has nothing to do with the size of the contact area between the objects.
(5) in order to simplify the practical problems, the "idealized" method is often used, If an object is placed on the smooth surface of another object, this means that there is no friction between two objects when they move relative to each other.
3. Methods to increase beneficial friction
(1) increase the pressure on the contact surface, Bicycles are easy to stop.
(2) rough the contact surface. For example, the uneven tread patterns on the tires of automobiles, tractors and bicycles are designed to increase the roughness of the contact surface, so as to increase the friction between the tire and the ground.
(3) in addition to the above two methods to increase the beneficial friction, The method of changing rolling to sliding can also be used to increase the beneficial friction. For example, when the train or car is in emergency braking, the wheel only slides but does not roll, and the car stops quickly.
4. The method of recing harmful friction.
(1) recing the pressure on the contact surface.
(2) recing the roughness of the contact surface.
(3) using rolling instead of sliding, The rolling friction is much smaller than sliding friction. For example, there are many smooth steel balls or steel columns between the inner and outer rings of the bearing. When the wheel rotates, it drives the outer ring to rotate, and the steel balls or steel columns roll between the inner and outer rings, so the friction is greatly reced, The moving parts only slide on the oil film, which greatly reces the friction.
(5) using compressed gas to form a layer of air cushion between the friction surfaces, which makes the friction surface out of contact, can make the friction smaller, such as hovercraft
(3) sliding friction is the force that hinders the relative movement of objects in contact, not necessarily the force that hinders the movement of objects, It should be clear that "relative motion" is hindered by objects in contact with each other as reference objects. "Object
Motion" may be based on other objects as reference objects. For example, in the experiment, when a weight is placed on the wood block and the spring scale is used to pull the wood block for uniform linear motion
, The specific situation is: when the wooden block is pulled forward by the pulling force
from static, the weight will slide backward relative to the wooden block, and the wooden block will give the weight a friction to prevent it from sliding backward, The direction of the friction is forward, so the weight does not slide relative to the wood block, and the friction is static friction.
(4) the sliding friction has nothing to do with the speed of the movement of the object, and has nothing to do with the size of the contact area between the objects.
(5) in order to simplify the practical problems, the "idealized" method is often used, If an object is placed on the smooth surface of another object, this means that there is no friction between two objects when they move relative to each other.
3. Methods to increase beneficial friction
(1) increase the pressure on the contact surface, Bicycles are easy to stop.
(2) rough the contact surface. For example, the uneven tread patterns on the tires of automobiles, tractors and bicycles are designed to increase the roughness of the contact surface, so as to increase the friction between the tire and the ground.
(3) in addition to the above two methods to increase the beneficial friction, The method of changing rolling to sliding can also be used to increase the beneficial friction. For example, when the train or car is in emergency braking, the wheel only slides but does not roll, and the car stops quickly.
4. The method of recing harmful friction.
(1) recing the pressure on the contact surface.
(2) recing the roughness of the contact surface.
(3) using rolling instead of sliding, The rolling friction is much smaller than sliding friction. For example, there are many smooth steel balls or steel columns between the inner and outer rings of the bearing. When the wheel rotates, it drives the outer ring to rotate, and the steel balls or steel columns roll between the inner and outer rings, so the friction is greatly reced, The moving parts only slide on the oil film, which greatly reces the friction.
(5) using compressed gas to form a layer of air cushion between the friction surfaces, which makes the friction surface out of contact, can make the friction smaller, such as hovercraft
5. The faster the relative velocity of the object and the air, the greater the friction
the higher the density of the air, that is, the closer it is to the bottom of the atmosphere, the greater the friction.
the higher the density of the air, that is, the closer it is to the bottom of the atmosphere, the greater the friction.
6.
Friction is affected by factors such as friction coefficient and surface pressure. The greater the vertical force, the greater the friction. Similarly, the greater the coefficient of friction, the greater the friction, the greater the force required. The specific performance is as follows:
1. When other conditions remain unchanged, the size of friction is related to the pressure acting on the surface of the object. The greater the pressure on the surface, the greater the friction
7.
bus line: Metro Line 1 → No.80, the whole journey is about 11.4km
1. Walk about 260m from Hangzhou bus passenger transport center station to the passenger transport center station
2. Take Metro Line 1, pass 6 stations, and reach zhalangkou station
3. Walk about 190m to jishencun station
4. Take No.80, pass 2 stations, and reach Huajiachi Campus Station of Zhejiang University
5 Walk about 440 meters to Huajiachi cadre training center of Zhejiang University
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