A Review of Friction
Defining Friction:
Friction is a force that opposes motion in a particular direction. Friction occurs when the surfaces of 2 materials come into contact with each other. Since no surface is completely smooth, there will always be a frictional force that opposes motion. The magnitude of this force is dependent upon the material of the 2 surfaces and the normal force. (Physics Encyclopedia, "Friction", 1998).
Defining Friction:
Friction is a force that opposes motion in a particular direction. Friction occurs when the surfaces of 2 materials come into contact with each other. Since no surface is completely smooth, there will always be a frictional force that opposes motion. The magnitude of this force is dependent upon the material of the 2 surfaces and the normal force. (Physics Encyclopedia, "Friction", 1998).
Normal Force
The normal force is always perpendicular to the surface of motion. On a flat surface, the normal force (N) is equal to the weight, mg, of the object in motion. However, on an inclined plane, the normal force is equal to mgcos? In the physics of hockey, the surface of the ice pad is parallel to a level ground. (Physics Encyclopedia, "Friction", 1998)
The Two Types of Friction
There are two types of friction that can occur when an object moves along a surface: Static and Kinetic. Static friction prevents motion from occurring altogether and occurs when objects are stationary. Kinetic friction, on the other hand, impedes motion already in progress.
The normal force is always perpendicular to the surface of motion. On a flat surface, the normal force (N) is equal to the weight, mg, of the object in motion. However, on an inclined plane, the normal force is equal to mgcos? In the physics of hockey, the surface of the ice pad is parallel to a level ground. (Physics Encyclopedia, "Friction", 1998)
The Two Types of Friction
There are two types of friction that can occur when an object moves along a surface: Static and Kinetic. Static friction prevents motion from occurring altogether and occurs when objects are stationary. Kinetic friction, on the other hand, impedes motion already in progress.
Calculations for Static and Kinetic Friction:
Ffr = µN
This is the coefficient of friction, and it changes depending on the materials and the surfaces in contact. Static friction (µs) has a different coefficient then kinetic friction (µk) The amount of force needed to overcome an object's inertia is greater then the amount of force needed to keep it moving. This is due to the fact that,
in general,
µs >µk.
Ffr = µN
This is the coefficient of friction, and it changes depending on the materials and the surfaces in contact. Static friction (µs) has a different coefficient then kinetic friction (µk) The amount of force needed to overcome an object's inertia is greater then the amount of force needed to keep it moving. This is due to the fact that,
in general,
µs >µk.
THE PHYSICS OF HOCKEY!
Sliding Friction and Momentum on Ice
Coefficient of Friction
Misconceptions
Many texbooks make an assumption that ice is a frictionless surface: this is entirely untrue. People mistake ice for being frictionless because the ice seems to be a transparent, flat and smooth (it's slippery when you run your hand across the ice).
"At a macroscopic level, smooth surfaces exert frictional forces at least as large as do rough ones." (Sir Robert Robinson, 1965, p. 110)
Many texbooks make an assumption that ice is a frictionless surface: this is entirely untrue. People mistake ice for being frictionless because the ice seems to be a transparent, flat and smooth (it's slippery when you run your hand across the ice).
"At a macroscopic level, smooth surfaces exert frictional forces at least as large as do rough ones." (Sir Robert Robinson, 1965, p. 110)
Varying Coefficients
The coefficient of friction on ice is not constant; it is just too complicating to find the exact value. The reason for this is that the friction depends on many factors such as asperities, the mass of load, temperature, sliding speed, normal reaction, and the type of material.
Major Factors Affecting the Coefficient of Friction
1)Sliding Speed
(D.C.B Evans, "The Kinetic Friction of Ice." 493, 1976)
Click to view graph A
The graph above shows the relationship between the coefficient of friction of various materials according to different velocities. The low values of sliding friction arise when the speed exceeds 0.1 m/s. The other factors are kept constant.
For all three materials, they share a linear relationship on Graph A: it shows that the coefficient of friction of a slider on ice decreases in an straight line (inclined) as the speed increases. However you can see that the horizontal grid from for speed
We can conclude from this graph that the coefficient is extremely great when the sliding speed is too small; further, the friction drops as the speed increases.
The second graph below illustrates what happens when the sliding speed is less than 1m/s.
Click to view graph B
(Perssonn, 1998, p. 496)
This graph conveys the extreme high coefficient of friction when the slider is moving very slow.
For instance, at speeds around 10, exponent -7, m/s glass and granite are 0.3 and 0.9 respectively. Perssonn believes that adhesion occurs since there is no melting of ice at low speeds. Also, the data suggests the shearing of ice due to the slide which breaks open the adhesive contact.
2)Friction Depends on Asperities
Asperities, which are microscopic projections from the "average surface," play a major role in determining the coefficient of friction between materials.
The friction depends on the asperities of the surfaces in contact. The pressure on an asperity is greater than the normal force, that it may deform the contact area "plastically" (asperities can weld together). Therefore, frictional resistance arises from sliding objects breaking and creating bonds created by asperities.
In addition, you may notice that after a long hockey game, the surface of the pond or artificial pad you skated on is no longer smooth and shiny. You may see various sizes of scratches on the ice --your skates will have a difficult time, sliding on the roughness. It is time to call it "quits", or you simply have to ask the Zamboni man to clean the ice surface.
3)Temperature
(D.C.B Evans, p. 512, 1976)
Click to view graph C
This third graph is the relationship for the coefficient of friction of various materials at different temperatutes with a fixed velocity ( 3.14m/s on the graph ).
The temperature proportionally affects the amount of frictional force. From -1 to -25*C , the coefficient of static fricition for all three solids increases; thus there needs to be more energy exerted to slide an object at these temperatures.
Once again, mu, the coefficient of friction, rises with the falling temperature at a given speed; likewise, mu decreases as the temperature approaches the melting point of ice.
4)Load and Area of Real Contact
(N.J.Perssonn, 1998, p.444)
click to view graph D
This fourth graph indicates the relationship of a load on ice at two different temperatures. Since there was a 1kg load on the tip of a diamond, the area of real contact depends on the time the load is sitting. The two arrows on the graph indicates the approximation of the areas in contact if pressure-melting was the case.
The contact area increases linearly with the time of loading; similarly, the slower the object moves, the longer the time of loading, which leads to a more suface contact.
The amount of surface area which is in contact with the ice directly affects the coefficient of friction; a greater surface area will increase the amount of contact area between the load and the ice, therefore, allowing the area of friction increase.
Amonton's law states that the coefficient of friction is independent of the normal force.
However, Bowden (1953) proved that Amonton's law strictly applies when there is a lower load. The reason for this independence at lower loads is that there is less pressure for the asperities in contact to deform. Perssonn concludes from his extensive research that the amount of pressure influences asperities to deform "instantaneously."
The coefficient of friction on ice is not constant; it is just too complicating to find the exact value. The reason for this is that the friction depends on many factors such as asperities, the mass of load, temperature, sliding speed, normal reaction, and the type of material.
Major Factors Affecting the Coefficient of Friction
1)Sliding Speed
(D.C.B Evans, "The Kinetic Friction of Ice." 493, 1976)
Click to view graph A
The graph above shows the relationship between the coefficient of friction of various materials according to different velocities. The low values of sliding friction arise when the speed exceeds 0.1 m/s. The other factors are kept constant.
For all three materials, they share a linear relationship on Graph A: it shows that the coefficient of friction of a slider on ice decreases in an straight line (inclined) as the speed increases. However you can see that the horizontal grid from for speed
We can conclude from this graph that the coefficient is extremely great when the sliding speed is too small; further, the friction drops as the speed increases.
The second graph below illustrates what happens when the sliding speed is less than 1m/s.
Click to view graph B
(Perssonn, 1998, p. 496)
This graph conveys the extreme high coefficient of friction when the slider is moving very slow.
For instance, at speeds around 10, exponent -7, m/s glass and granite are 0.3 and 0.9 respectively. Perssonn believes that adhesion occurs since there is no melting of ice at low speeds. Also, the data suggests the shearing of ice due to the slide which breaks open the adhesive contact.
2)Friction Depends on Asperities
Asperities, which are microscopic projections from the "average surface," play a major role in determining the coefficient of friction between materials.
The friction depends on the asperities of the surfaces in contact. The pressure on an asperity is greater than the normal force, that it may deform the contact area "plastically" (asperities can weld together). Therefore, frictional resistance arises from sliding objects breaking and creating bonds created by asperities.
In addition, you may notice that after a long hockey game, the surface of the pond or artificial pad you skated on is no longer smooth and shiny. You may see various sizes of scratches on the ice --your skates will have a difficult time, sliding on the roughness. It is time to call it "quits", or you simply have to ask the Zamboni man to clean the ice surface.
3)Temperature
(D.C.B Evans, p. 512, 1976)
Click to view graph C
This third graph is the relationship for the coefficient of friction of various materials at different temperatutes with a fixed velocity ( 3.14m/s on the graph ).
The temperature proportionally affects the amount of frictional force. From -1 to -25*C , the coefficient of static fricition for all three solids increases; thus there needs to be more energy exerted to slide an object at these temperatures.
Once again, mu, the coefficient of friction, rises with the falling temperature at a given speed; likewise, mu decreases as the temperature approaches the melting point of ice.
4)Load and Area of Real Contact
(N.J.Perssonn, 1998, p.444)
click to view graph D
This fourth graph indicates the relationship of a load on ice at two different temperatures. Since there was a 1kg load on the tip of a diamond, the area of real contact depends on the time the load is sitting. The two arrows on the graph indicates the approximation of the areas in contact if pressure-melting was the case.
The contact area increases linearly with the time of loading; similarly, the slower the object moves, the longer the time of loading, which leads to a more suface contact.
The amount of surface area which is in contact with the ice directly affects the coefficient of friction; a greater surface area will increase the amount of contact area between the load and the ice, therefore, allowing the area of friction increase.
Amonton's law states that the coefficient of friction is independent of the normal force.
However, Bowden (1953) proved that Amonton's law strictly applies when there is a lower load. The reason for this independence at lower loads is that there is less pressure for the asperities in contact to deform. Perssonn concludes from his extensive research that the amount of pressure influences asperities to deform "instantaneously."
Summary:
From these graphs, we can see that many factors play a role in determining the coefficient of friction on ice.
In particular, these graphs were created by the results of the experiments which were conducted by Evans and Perssonn. Their data have a great degree of accuracy and precision due to the careful procedure and to the utilization of advanced tools and apparatus. These graphs demonstrate the relationship of many conditions combined to showcase a better understanding of sliding friction on ice.
It is imperative to note that the science community can learn a great deal from the collected data of these scientist; further, these experimental results direct the nature of the theory. Certainly, with this data we can choose which theory or theories are pinpointing the true nature of sliding friction on ice.
That is why I conducted my own experiment, using the methods and tools that are accessible, to find similarities or irregularities with the results.
From these graphs, we can see that many factors play a role in determining the coefficient of friction on ice.
In particular, these graphs were created by the results of the experiments which were conducted by Evans and Perssonn. Their data have a great degree of accuracy and precision due to the careful procedure and to the utilization of advanced tools and apparatus. These graphs demonstrate the relationship of many conditions combined to showcase a better understanding of sliding friction on ice.
It is imperative to note that the science community can learn a great deal from the collected data of these scientist; further, these experimental results direct the nature of the theory. Certainly, with this data we can choose which theory or theories are pinpointing the true nature of sliding friction on ice.
That is why I conducted my own experiment, using the methods and tools that are accessible, to find similarities or irregularities with the results.
More Examples:
Values for Other Objects source: University Textbook
Values for Other Objects source: University Textbook
Coefficient Table
Notice on the table how static friction is greater than the kinetic friction. When an object moves there is kinetic friction, and this value is ower than the static friction in which an object is not moving.
For "teflon on teflon", the values for static and kinetic friction are both 0.04. My guess is that static friction friction is greater than the kinetic friction at a thousandth decimal place of a mu.
We can always assume that static friction is greater than the actual kinetic friction.
The value of synovial joints in humans is about 0.003 mu.
This is no surprise if you think carefully: it should have the lowest value possible so that the friction doesn't deteriorate the cartilage (promoting arthritis at a young age).
Teflon on teflon (coating on skis) is low as well. Also, ice on ice is low too.
Note that steel on steel is 0.57, whereas it is 0.06 when lubricated.
Further, another reason for the synovial joints to be smoothly sliding is that globules of synovial fluid are readily available to lubricate the joints.
f you have a look at the first graph, prior to this page, you can see the values of coefficient range from 0.01 to 0.06 (depending on the speed) for copper, mild steel and persplex on ice. Since ice (one material of the two in contact) itself has a low value, while the other materials have a greater value, we can hypothesize that a lubricant is indeed one of the factors. As discussed earlier with the properties of ice, developments, theories, and more, a lubricant does exist because of the heat created by the friction (and therefore melting the ice).
For "teflon on teflon", the values for static and kinetic friction are both 0.04. My guess is that static friction friction is greater than the kinetic friction at a thousandth decimal place of a mu.
We can always assume that static friction is greater than the actual kinetic friction.
The value of synovial joints in humans is about 0.003 mu.
This is no surprise if you think carefully: it should have the lowest value possible so that the friction doesn't deteriorate the cartilage (promoting arthritis at a young age).
Teflon on teflon (coating on skis) is low as well. Also, ice on ice is low too.
Note that steel on steel is 0.57, whereas it is 0.06 when lubricated.
Further, another reason for the synovial joints to be smoothly sliding is that globules of synovial fluid are readily available to lubricate the joints.
f you have a look at the first graph, prior to this page, you can see the values of coefficient range from 0.01 to 0.06 (depending on the speed) for copper, mild steel and persplex on ice. Since ice (one material of the two in contact) itself has a low value, while the other materials have a greater value, we can hypothesize that a lubricant is indeed one of the factors. As discussed earlier with the properties of ice, developments, theories, and more, a lubricant does exist because of the heat created by the friction (and therefore melting the ice).
Screenshot of Original
