presentation on topic friction

5.1 Friction

Learning objectives.

By the end of this section, you will be able to:

• Discuss the general characteristics of friction.
• Describe the various types of friction.
• Calculate the magnitude of static and kinetic friction.

Friction is a force that is around us all the time that opposes relative motion between surfaces in contact but also allows us to move (which you have discovered if you have ever tried to walk on ice). While a common force, the behavior of friction is actually very complicated and is still not completely understood. We have to rely heavily on observations for whatever understandings we can gain. However, we can still deal with its more elementary general characteristics and understand the circumstances in which it behaves.

Friction is a force that opposes relative motion between surfaces in contact.

One of the simpler characteristics of friction is that it is parallel to the contact surface between surfaces and always in a direction that opposes motion or attempted motion of the systems relative to each other. If two surfaces are in contact and moving relative to one another, then the friction between them is called kinetic friction . For example, friction slows a hockey puck sliding on ice. But when objects are stationary, static friction can act between them; the static friction is usually greater than the kinetic friction between the surfaces.

Kinetic Friction

If two surfaces are in contact and moving relative to one another, then the friction between them is called kinetic friction.

Imagine, for example, trying to slide a heavy crate across a concrete floor—you may push harder and harder on the crate and not move it at all. This means that the static friction responds to what you do—it increases to be equal to and in the opposite direction of your push. But if you finally push hard enough, the crate seems to slip suddenly and starts to move. Once in motion it is easier to keep it in motion than it was to get it started, indicating that the kinetic friction force is less than the static friction force. If you add mass to the crate, say by placing a box on top of it, you need to push even harder to get it started and also to keep it moving. Furthermore, if you oiled the concrete you would find it to be easier to get the crate started and keep it going (as you might expect).

Figure 5.2 is a crude pictorial representation of how friction occurs at the interface between two objects. Close-up inspection of these surfaces shows them to be rough. So when you push to get an object moving (in this case, a crate), you must raise the object until it can skip along with just the tips of the surface hitting, break off the points, or do both. A considerable force can be resisted by friction with no apparent motion. The harder the surfaces are pushed together (such as if another box is placed on the crate), the more force is needed to move them. Part of the friction is due to adhesive forces between the surface molecules of the two objects, which explain the dependence of friction on the nature of the substances. Adhesion varies with substances in contact and is a complicated aspect of surface physics. Once an object is moving, there are fewer points of contact (fewer molecules adhering), so less force is required to keep the object moving. At small but nonzero speeds, friction is nearly independent of speed.

The magnitude of the frictional force has two forms: one for static situations (static friction), the other for when there is motion (kinetic friction).

When there is no motion between the objects, the magnitude of static friction f s f s is

where μ s μ s is the coefficient of static friction and N N is the magnitude of the normal force (the force perpendicular to the surface).

Magnitude of Static Friction

Magnitude of static friction f s f s is

where μ s μ s is the coefficient of static friction and N N is the magnitude of the normal force.

The symbol ≤ ≤ means less than or equal to , implying that static friction can have a minimum and a maximum value of μ s N μ s N . Static friction is a responsive force that increases to be equal and opposite to whatever force is exerted, up to its maximum limit. Once the applied force exceeds f s ( max ) f s ( max ) , the object will move. Thus

Once an object is moving, the magnitude of kinetic friction f k f k is given by

where μ k μ k is the coefficient of kinetic friction. A system in which f k = μ k N f k = μ k N is described as a system in which friction behaves simply .

Magnitude of Kinetic Friction

The magnitude of kinetic friction f k f k is given by

where μ k μ k is the coefficient of kinetic friction.

As seen in Table 5.1 , the coefficients of kinetic friction are less than their static counterparts. That values of μ μ in Table 5.1 are stated to only one or, at most, two digits is an indication of the approximate description of friction given by the above two equations.

The equations given earlier include the dependence of friction on materials and the normal force. The direction of friction is always opposite that of motion, parallel to the surface between objects, and perpendicular to the normal force. For example, if the crate you try to push (with a force parallel to the floor) has a mass of 100 kg, then the normal force would be equal to its weight, W = mg = ( 100 kg ) ( 9 . 80 m/s 2 ) = 980 N W = mg = ( 100 kg ) ( 9 . 80 m/s 2 ) = 980 N , perpendicular to the floor. If the coefficient of static friction is 0.45, you would have to exert a force parallel to the floor greater than f s ( max ) = μ s N = 0.45 ( 980 N ) = 440 N f s ( max ) = μ s N = 0.45 ( 980 N ) = 440 N to move the crate. Once there is motion, friction is less and the coefficient of kinetic friction might be 0.30, so that a force of only 290 N ( f k = μ k N = 0 . 30 980 N = 290 N f k = μ k N = 0 . 30 980 N = 290 N ) would keep it moving at a constant speed. If the floor is lubricated, both coefficients are considerably less than they would be without lubrication. Coefficient of friction is a unit less quantity with a magnitude usually between 0 and 1.0. The coefficient of the friction depends on the two surfaces that are in contact.

Take-Home Experiment

Find a small plastic object (such as a food container) and slide it on a kitchen table by giving it a gentle tap. Now spray water on the table, simulating a light shower of rain. What happens now when you give the object the same-sized tap? Now add a few drops of (vegetable or olive) oil on the surface of the water and give the same tap. What happens now? This latter situation is particularly important for drivers to note, especially after a light rain shower. Why?

Many people have experienced the slipperiness of walking on ice. However, many parts of the body, especially the joints, have much smaller coefficients of friction—often three or four times less than ice. A joint is formed by the ends of two bones, which are connected by thick tissues. The knee joint is formed by the lower leg bone (the tibia) and the thighbone (the femur). The hip is a ball (at the end of the femur) and socket (part of the pelvis) joint. The ends of the bones in the joint are covered by cartilage, which provides a smooth, almost glassy surface. The joints also produce a fluid (synovial fluid) that reduces friction and wear. A damaged or arthritic joint can be replaced by an artificial joint ( Figure 5.3 ). These replacements can be made of metals (stainless steel or titanium) or plastic (polyethylene), also with very small coefficients of friction.

Other natural lubricants include saliva produced in our mouths to aid in the swallowing process, and the slippery mucus found between organs in the body, allowing them to move freely past each other during heartbeats, during breathing, and when a person moves. Artificial lubricants are also common in hospitals and doctor’s clinics. For example, when ultrasonic imaging is carried out, the gel that couples the transducer to the skin also serves to lubricate the surface between the transducer and the skin—thereby reducing the coefficient of friction between the two surfaces. This allows the transducer to move freely over the skin.

Example 5.1

Skiing exercise.

A skier with a mass of 62 kg is sliding down a snowy slope. Find the coefficient of kinetic friction for the skier if friction is known to be 45.0 N.

The magnitude of kinetic friction was given in to be 45.0 N. Kinetic friction is related to the normal force N N as f k = μ k N f k = μ k N ; thus, the coefficient of kinetic friction can be found if we can find the normal force of the skier on a slope. The normal force is always perpendicular to the surface, and since there is no motion perpendicular to the surface, the normal force should equal the component of the skier’s weight perpendicular to the slope. (See the skier and free-body diagram in Figure 5.4 .)

Substituting this into our expression for kinetic friction, we get

which can now be solved for the coefficient of kinetic friction μ k μ k .

Solving for μ k μ k gives

Substituting known values on the right-hand side of the equation,

This result is a little smaller than the coefficient listed in Table 5.1 for waxed wood on snow, but it is still reasonable since values of the coefficients of friction can vary greatly. In situations like this, where an object of mass m m slides down a slope that makes an angle θ θ with the horizontal, friction is given by f k = μ k mg cos θ f k = μ k mg cos θ . All objects will slide down a slope with constant acceleration under these circumstances. Proof of this is left for this chapter’s Problems and Exercises.

An object will slide down an inclined plane at a constant velocity if the net force on the object is zero. We can use this fact to measure the coefficient of kinetic friction between two objects. As shown in Example 5.1 , the kinetic friction on a slope f k = μ k mg cos θ f k = μ k mg cos θ . The component of the weight down the slope is equal to mg sin θ mg sin θ (see the free-body diagram in Figure 5.4 ). These forces act in opposite directions, so when they have equal magnitude, the acceleration is zero. Writing these out:

Solving for μ k μ k , we find that

Put a coin on a book and tilt it until the coin slides at a constant velocity down the book. You might need to tap the book lightly to get the coin to move. Measure the angle of tilt relative to the horizontal and find μ k μ k . Note that the coin will not start to slide at all until an angle greater than θ θ is attained, since the coefficient of static friction is larger than the coefficient of kinetic friction. Discuss how this may affect the value for μ k μ k and its uncertainty.

We have discussed that when an object rests on a horizontal surface, there is a normal force supporting it equal in magnitude to its weight. Furthermore, simple friction is always proportional to the normal force.

Making Connections: Submicroscopic Explanations of Friction

The simpler aspects of friction dealt with so far are its macroscopic (large-scale) characteristics. Great strides have been made in the atomic-scale explanation of friction during the past several decades. Researchers are finding that the atomic nature of friction seems to have several fundamental characteristics. These characteristics not only explain some of the simpler aspects of friction—they also hold the potential for the development of nearly friction-free environments that could save hundreds of billions of dollars in energy which is currently being converted (unnecessarily) to heat.

Figure 5.5 illustrates one macroscopic characteristic of friction that is explained by microscopic (small-scale) research. We have noted that friction is proportional to the normal force, but not to the area in contact, a somewhat counterintuitive notion. When two rough surfaces are in contact, the actual contact area is a tiny fraction of the total area since only high spots touch. When a greater normal force is exerted, the actual contact area increases, and it is found that the friction is proportional to this area.

But the atomic-scale view promises to explain far more than the simpler features of friction. The mechanism for how heat is generated is now being determined. In other words, why do surfaces get warmer when rubbed? Essentially, atoms are linked with one another to form lattices. When surfaces rub, the surface atoms adhere and cause atomic lattices to vibrate—essentially creating sound waves that penetrate the material. The sound waves diminish with distance and their energy is converted into heat. Chemical reactions that are related to frictional wear can also occur between atoms and molecules on the surfaces. Figure 5.6 shows how the tip of a probe drawn across another material is deformed by atomic-scale friction. The force needed to drag the tip can be measured and is found to be related to shear stress, which will be discussed later in this chapter. The variation in shear stress is remarkable (more than a factor of 10 12 10 12 ) and difficult to predict theoretically, but shear stress is yielding a fundamental understanding of a large-scale phenomenon known since ancient times—friction.

PhET Explorations

Forces and motion.

Explore the forces at work when you try to push a filing cabinet. Create an applied force and see the resulting friction force and total force acting on the cabinet. Charts show the forces, position, velocity, and acceleration vs. time. Draw a free-body diagram of all the forces (including gravitational and normal forces).

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PHYSICS FRICTION.

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Slides from a lecture presentation covering the following topics: Friction, friction coefficient, ? varies as a function of the sliding distance, scale issues in tribology, friction measurement lab, microscale friction as a function of coating, relative friction forces in MEMS, attractive forces in MEMS devices, friction at macroscale sliding contacts, coefficient of friction versus sliding distance, effect of removing wear particles, friction at dry sliding interface, experimental setup, Specimens, wear track, particle (SiC) insertion, six stages in the frictional force versus distance slid relationship, two interacting surface asperities, geometrically compatible slip-line field, effect of boundary lubrication, friction in geometrically confined space, friction at polymeric interfaces, structure of some thermoplastics, friction coefficient of low density polyethylene, frictional behavior of composites, effect of coatings on friction, and conclusions.

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The topic of friction can be a little dry. Consider lubricating students' interest with these two examples

Friction around Pole

What it shows :  Many people have probably observed that, by wrapping a rope around a post, a person can hold in check a much larger force than would ordinarily be possible. In this experiment a flexible thick rope is wound around a horizontal pipe. Due to the interaction of the frictional forces and tension, there can be a considerable difference in tension between the two ends of the rope. In the demonstration, one end of the rope supports a (heavy) load and the other end is held by (small) "holding" force. The demonstration models the principle by which a capstan works.

How it works :  The Euler-Eytelwein equation (1)  relates the tension of the two ends of the rope:  T 2  = T 1  e μθ , where T 2  is the tension in the rope due to the load it's supporting, T 1  is the tension necessary to hold the load without slipping, μ is the coefficient of friction between the rope and the pipe, and θ is the total angle (measured in radians) made by all the windings of the rope (one full winding is 2∏ radians). The tension force increases exponentially with the coefficient of friction and the number of turns around the pipe. Note that the diameter of the pipe does not come into play. However, the pipe diameter cannot be too small because a significant amount of force would be lost in bending the rope around the pipe, especially if the rope is a little stiff.

Setting it up :  Secure a 2"-diameter steel pipe to the edge of the lecture bench with two C-clamps, as shown in the photographs. Use the 3/8"-diameter nylon rope. Our 65-lb wrecking ball makes an impressive heavy load. Tie one end of the rope to the ball and wrap several turns of the rope around the pipe. Upon slowly unwrapping the rope, you'll discover that 4½ turns is enough to hold the ball with only the weight of the loose rope supplying the "holding" force. That length of loose rope weighs about 0.11 lb. With θ=9∏, this suggests that μ≈0.23 (not an unreasonable number). With 2½ turns you can easily hold the ball with just your fingers. The 3rd photo shows a 100-lb dumbbell held with just 4½ turns of rope and the fingers supplying the tension. Using μ≈0.23, we estimate a 0.15 lb finger holding force.

Comments :  Exponential relationships are always astounding and this is no exception. Many devices based on belt or wrap friction are used in rappelling, rock climbing (so-called top-roping in which one can hold (belay) a heavy person to prevent a fall), sailing, and rigging of equipment. The ubiquitus V-belt and pulley is another example, cleverly enhanced by the V shape. The relation between tightside and slackside tension for a V-belt is similar: 

T 2  = T 1  e μθ/sin(β/2) , where β is the angle of the V in the pulley (note that for β=180 degrees, we are back to the flat belt relationship).

footnotes : (1) The equation is derived in most mechanical engineering textbooks.  For example: R. Becker,  Introduction to Theoretical Mechanics , (McGraw-Hill, NY, 1954) p 45-46.  L. Goodman and W. Warner,  Statics and Dynamics , (Wadsworth, Belmont CA, 1964) pp 308 - 314.

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Lesson Discovering Friction

Grade Level: 7 (6-8)

(one class period before the associated activity and one class period after)

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Whether engineers are trying to design a better set of automobile brakes or a more efficient wind turbine, a thorough understanding of friction is a vital prerequisite. Engineers must understand how friction affects all sorts of everyday situations, from the bottom of skis in which friction is a disadvantage to hiking boots where friction provides important traction.

After this lesson, students should be able to:

• Describe friction as a force that impedes motion and generates heat.
• Distinguish between static friction and kinetic friction.
• Explain why friction occurs.
• Describe common occurrences of friction, including those in which friction can be used to advantage in everyday life.
• Describe ways in which friction can be reduced.

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A basic understanding of the concept of force; an awarenes of the existence of the gravitational force.

For better or for worse, friction is an inescapable force we encounter every moment of our lives. We depend on friction in order to walk, we take advantage of friction in order to light a match, we try to reduce friction in our car engines and door hinges, and friction is generated as the muscle fibers of our hearts contract and relax with each heart beat. most often, physicists and engineers invest a lot of time and energy into trying to reduce or eliminate friction within the moving parts of machinery, but sometimes they look for ways to increase friction. Whether an engineer is trying to design a better set of automobile brakes or a more efficient wind turbine, a thorough understanding of friction is a vital prerequisite. You can easily master the fundamentals of friction, and even conduct simple experiments in order to discover most of the fundamental ideas for yourselves.

Demonstration: Start by making an inclined plane at a shallow angle using a flat piece of plywood, a kitchen cutting board or even a large book. Place two coffee cups on the board -- one cup on its rough, unglazed bottom; the other on its smooth, glazed side (see Figure 1). Ask the students to predict what will happen when you slowly raise the plane to a steeper angle. You can then perform the experiment.

Ask students if they can explain what makes the cup on its side slide down while the cup on its bottom does not (until you increase the plane to a steeper angle). The point is to give students only a basic definition of friction and then let them see what they can find out about it for themselves. Depending on their prior knowledge, they may answer that the force of gravity makes the smooth cup slide, but that gravity isn't strong enough to make the rough cup slide until you make the angle steeper. You can point out that the force of gravity must be great enough to overcome another force, the force of friction, in order for the cups to move. Friction is a force that occurs between two surfaces, and it acts to impede motion.

If the students suggest inertia as the reason why the cups do not slide, you might introduce Newton's first law of motion: An object at rest will stay at rest unless it is acted on by an outside force. Then you can point out that friction can be what keeps an object at rest until another, stronger force, gravity, causes it to begin moving. Conversely, once an object is in motion, it will stay in motion until a force acts to stop it. Friction is one such force. For example, the friction a cup encounters as it slides across a flat table will eventually stop the cup; the cup will not keep sliding forever. Please note, however, that if students do not bring up the subject of inertia, it is not necessary to discuss it at this point. In fact, if your students are relatively naïve, they may be confused by trying to understand inertia at this time. (That discussion can wait until after the students have had time to explore friction through experimenting with different surfaces as presented in the associated activity, Sliding and Stuttering ).

Then ask the students what they think would happen if you changed the bottom of the cup by gluing sandpaper to it. Would the cup slide more or less freely down the inclined plane. In other words, would there be more or less friction between the cup and surface of the plane? They will probably predict that the cup would slide less freely. Instead of asking for their reasoning, however, tell them that if they want to compare the amounts of friction involved between the two surfaces, there is a way to measure it directly. They can use this method to experiment with different surfaces and see for themselves what they can discover about friction in the associated activity, Sliding and Stuttering.

Lesson Background and Concepts for Teachers

Friction occurs whenever two surfaces are in contact with each other, and in general, it is the roughness of the surfaces that determines the amount of friction that results. Even surfaces that look and feel smooth may contain thousands of irregular bumps, pits, ridges and valleys, although a microscope may be required to see them. When two such surfaces slide past one another, the tiny bumps and ridges on one surface can get hung up briefly in the pits and ridges on the other surface. It is the brief locking together of the surface irregularities that creates friction and impedes their motion.

Static friction is the force that must be overcome in order to set a body in motion. Kinetic friction is the force that must be overcome in order to keep a body in motion. Kinetic friction is usually less than static friction, but both types occur mainly because of the surface macro- and microscopic imperfections.

When an object such as a coffee cup is at rest on a table top, some of its surface imperfections are pressed up against the similar imperfections of the table, with the tiny peaks of one surface nestled into the tiny valleys of the other. To set the cup in motion and make it slide across the table, enough force must be applied to get the peaks and valleys on the upper surface up and out of the valleys and peaks on the stationary surface below. Static friction is the force that must be overcome to disengage these peaks and valleys in order for the cup to begin sliding across the table.

While friction is primarily caused by surface roughness, many modern synthetic materials have exceptionally smooth surfaces. For these materials, the friction that results from surface roughness can be very, very small. However, another source of friction can become important in these materials. Although the mechanisms are not yet well understood, molecular attraction between two very smooth surfaces can create a surprising amount of friction. These commonly occur between some types of plastics, and can also occur with some glass surfaces.

Also, soft materials can deform and thereby produce increased resistance to motion. Sliding a coffee cup across a carpet is one example of deformation friction. In this case, the surface roughness contributes relatively little to the frictional force observed; instead, the weight of the cup bends the carpet fibers down, making the cup sink into the carpet's pile. In order to move the cup across the carpet, the unbent fibers adjacent to it must be pushed aside and/or compressed by the cup, and this "plowing through" may require considerable force.

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After students have completed the associated activity, Sliding and Stuttering, ask each group for its answer to the first question on the student data sheet. Expect students to be able to make some general observations, such as, "There is usually less friction when two smooth surfaces slide past each other than when one (or both) of the surfaces is rough." Once this has been established, ask them why they think the rougher surfaces create more friction than the smoother ones. Expect them to answer to the effect that rough surfaces are bumpy and the bumps on the two surfaces hit each other and make it harder for one surface to slide past the other. This is exactly right, but you can point out that it happens on a microscopic scale, too (see the Lesson Background & Concepts for Teachers section). If students found that some of the smooth-feeling surfaces generated a lot of friction also, point out that molecular attraction and deformation are also responsible for friction.

Ask students for examples of situations in which people try to take advantage of either reduced or increased amounts of friction between two surfaces. Good examples include oiling a squeaky door hinge, going down a water slide, using a bath mat in the tub or shower, opening a jar with the help of a rubber gripper, and taping the end of a baseball bat.

Also, ask students to note what happens when they vigorously rub their palms together, or rub their palms against their thighs—this works especially well for those wearing jeans. Expect them to notice that heat is generated. This is always true of friction: friction generates heat. The reason is that some of the kinetic energy of the moving object is reduced by the force of friction. Since energy cannot be lost from a system, that kinetic energy is converted to heat energy.

We take advantage of the heat generated by friction every time we light a match. And although we keep our mammalian bodies warm by the metabolic, fuel-burning activity of our millions of cells, we feel particularly hot during strenuous activity. This is not only due to the fact that we are burning fuel faster when we exercise, but it is also partly due to the friction created by large blocks of muscles moving back and forth next to each other. When we run, for example, the muscles in the fronts of our thighs, known collectively as the quadriceps, or "quads", rub back and forth against those in the backs of our thighs, known as the "hamstrings." Heat is also generated by the movement of hundreds of thousands of muscle cells and their protein components as they slide past one another when we alternately contract and relax our muscles. These sources of friction build up heat and cause us to sweat and fan ourselves in an effort to cool off.

On the other hand, if we are too cold, we shiver. Shivering is a special type of involuntary, cyclical pattern of muscle contraction and relaxation. It is a physiological adaptation that causes us to burn fuel and produce heat whether we want to or not, but it also lets friction help us maintain our body temperature when our clothing and shelters are not sufficient. Regardless of the type of situation-—physiological or mechanical—the amount of heat produced is proportional to the amount of friction generated.

friction: A resistance to motion that occurs when two surfaces are in contact with each other.

kinetic friction: The resistance to motion that occurs once one surface is in motion, sliding against another surface.

static friction: The resistance to motion that must be overcome in order to allow one surface to begin sliding against another surface.

Questions : In the form of a writing assignment or quiz, ask students the following questions to assess their comprehension of the lesson subject matter.

• Define friction.

Lesson Extension Activities

• Rolling friction is the type of resistive force that can slow the motion of a wheel, tire or ball bearing. Rolling friction is generally much less than kinetic friction, which is why wheels and ball bearings are such remarkable inventions. In rolling friction, both the deformation and the molecular attractions of the materials involved are important determinants of the amount of rolling resistance produced. Students can learn more about rolling friction through simple experiments, library and/or Internet research, and taking apart ball bearings to see how they work.
• Take a field trip to an ice skating rink to let students experience movement in a low-friction environment, or take a trip to an indoor rock climbing facility for a challenging way to experience the interactions of gravity and friction.

Based on what students have already learned about friction, they formulate hypotheses concerning the effects of weight and contact area on the amount of friction between two surfaces.

High school students learn how engineers mathematically design roller coaster paths using the approach that a curved path can be approximated by a sequence of many short inclines. They apply basic calculus and the work-energy theorem for non-conservative forces to quantify the friction along a curve...

Contributors

Supporting program, acknowledgements.

This content was developed by the MUSIC (Math Understanding through Science Integrated with Curriculum) Program in the Pratt School of Engineering at Duke University under National Science Foundation GK-12 grant no. DGE 0338262. However, these contents do not necessarily represent the policies of the NSF, and you should not assume endorsement by the federal government.

This lesson and its associated activity were originally published, in slightly modified form, by Duke University's Center for Inquiry Based Learning (CIBL). Please visit the http://ciblearning.org/ website for information about CIBL and other resources for K-12 science and math teachers.

Last modified: May 10, 2021

Friction, Gravity, and Elastic Forces PowerPoint Presentation

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When we throw a ball on the floor, it starts moving with some velocity. But ideally, no force should be acting in the direction of motion, and according to Newton’s first law, the ball should keep rolling, but this does not happen. Instead, the ball stops after moving a certain distance, so a force must be acting on it. We call this force “friction.”

What Is Friction?

Friction is defined as the resistance offered by the surfaces that are in contact when they move past each other.

Friction provides traction that is needed to walk without slipping. Friction is helpful in most cases. However, they also offer a great measure of opposition to the motion. In addition, about 20 per cent of the engine power of automobiles is consumed in overcoming frictional forces in the moving parts. In the next section, let us go through some of these factors.

Factors Affecting Friction

Friction is a force that is dependent on external factors. Following are the two factors on which friction depends:

What Causes Friction?

Friction is caused due to the irregularities on the two surfaces in contact. So, when one object moves over the other, these irregularities on the surface get entangled, giving rise to friction. The more the roughness, the more irregularities and more significant will be the friction.

Types of Friction

There are four types of friction and they are classified as follows:

• Static friction
• Sliding friction
• Rolling friction
• Fluid friction

All four types of frictions are different from each other. For example, static friction is the friction that acts between the surfaces when they are at rest with respect to each other. In contrast, sliding friction is the resistance created between any two objects when sliding against each other. Similarly, understand the nature of different types of friction from the article below.

Read more: Types of Friction

Watch the Video and Learn about Friction, Coefficient and Properties of Friction

Applications of Friction

• Friction finds application when matchsticks are ignited.
• Motion of pistons in a cylinder is an application of friction.
• It is possible to write on books and board as there is friction between pen and the board.

Friction Problem with Solution

Block A is kept on top of block B; the coefficient of static friction between A and B is 0.6 and between B and ground is 0.5. Also, the coefficient of kinetic friction between A and B is 0.4 and between B and ground is 0.3. If a force of 60 N is applied on block B, Find the acceleration of both the blocks. Mass of block A is 5 kg, and Block B is 10 Kg.

The first step should always be to draw the F.B.D of the given setup:

Let’s consider that both blocks are moving with the same acceleration, then A + B can be considered as a system, so the force equation in the horizontal direction will be,

60–f1 = 15a —— (1)

Since block B is moving over the surface, hence it is sliding friction.

Therefore, in this case we will use coefficient of friction as 0.3, f1 = 0.3 × 15 × 10 N f1 = 45 N —— (2)

From (1) and (2)

we have, a = 1m/s 2 From F.B.D of block A, f = 5.a

Putting the value of a, we get, f = 5 N

Maximum value of static friction between A and B = 0.6 × 5 × 10 = 30 N

As f lies within the range, the two blocks can move together with an acceleration of 1 m/s 2 . We can also be asked to find the maximum force applied to block B so that both blocks move together. In that case, since both blocks will move together, there will be static friction between the two, and hence the maximum value that can act is 30 N.

Therefore, acceleration of B, a = 305 m/s 2 = 6 m/s 2

For Block A,F – 45 – 30 = 10 × 6 F = 135 N

So if a force of more than 135 N is applied, both blocks will have different accelerations.

Understand the Common Misconception about Friction by Watching the Video

Videos Related to Friction

Demonstration of friction.

Self-Adjusting Nature of Friction

Role of Friction in Walking

Is Friction Impulsive?

Frequently Asked Questions – FAQs

How does friction produce heat, how is friction useful, why is friction a non-conservative force, will friction increase as the speed increases, can friction be zero.

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CHAPTER - 12 FRICTION

Aug 14, 2012

16.96k likes | 50.72k Views

CHAPTER - 12 FRICTION. 1) Force of friction :-. Force of friction is the force which opposes the motion of an object over a surface. The force of friction acts between the object and the surface. Eg :- A ball rolling on ground gradually slows down and comes to

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• fluid friction
• better grip
• daily activities
• ball bearings
• sometimes undesirable

Presentation Transcript

CHAPTER - 12FRICTION

1) Force of friction :- Force of friction is the force which opposes the motion of an object over a surface. The force of friction acts between the object and the surface. Eg :- A ball rolling on ground gradually slows down and comes to rest due to force of friction between the ball and the ground. If we stop pedalling a bicycle, it gradually slows down and comes to a stop due to force of friction between the wheel and the road.

2) Factors affecting friction :- Friction depends upon two factors. They are :- i) Nature of the surfaces in contact. ( The smoothness of the surfaces). ii) How hard the surfaces press together. Friction is less on a smooth surface. Friction is more on a rough surface.

Friction is more if the surfaces are pressed harder. Friction is less if the surfaces are not pressed harder.

3) Cause of friction :- Friction is caused due to the interlocking of irregularities between the two surfaces in contact. Smooth surfaces have minute irregularities between the two surfaces. Rough surfaces have larger irregularities between the two surfaces. So force of friction is more if the surfaces are rough.

4) Types of friction :- There are three main types of friction. They are static friction, sliding friction and rolling friction. a) Static friction is the friction exerted on a object at rest. . b) Sliding friction is the friction exerted when an object slides over a surface. c) Rolling friction is the friction exerted when an object rolls over a surface. Sliding friction is slightly less than static friction. Rolling friction is less than sliding friction and static friction.

5) Friction is sometimes useful :- Friction is useful for many of our daily activities. It is possible to hold a tumbler due to friction between the hand and the tumbler. Friction between the feet and ground helps us to walk on the ground. It is possible to write with a pen or pencil on a paper due to friction between the pen or pencil and the paper. It is possible to write on a blackboard due to the friction between the chalk and black board. Friction between the tyres and the road helps automobiles to move on roads. Friction between the bricks helps in the construction of buildings.

6) Friction is sometimes undesirable :- Soles of shoes wear out due to friction. Tyres of bicycles and automobiles wear out due to friction. Steps of staircases and foot over bridges in railway stations wear out due to friction. Knives and razors lose their sharp edges due to friction.

7) Increasing friction :- Friction can be increased by increasing the roughness of the surfaces in contact. Eg :- The soles of shoes are grooved to have a better grip on the floor. The tyres of vehicles are treaded to increase the grip on the road. The break pads of vehicles are rough to stop moving vehicles when the breaks are applied. Gymnasts apply some coarse substance on their hands for a better grip.

8) Reducing friction :- Friction can be reduced by :- I) Using lubricants like powders or oils and grease. ii) Using rollers or wheels. iii) Using ball bearings. We sprinkle powder on a carrom board to reduce friction. Oil or grease is applied between moving parts of machines to reduce friction. Rollers are used in luggage bags to reduce friction. Wheels are used in vehicles to reduce friction. Ball bearings are used in ceiling fans, bicycles and vehicles to reduce friction.

9) Fluid friction :- Fluid friction is the force of friction exerted by liquids and gases on objects moving through them. Fluid friction depends upon :- i) The speed of the object. ii) Shape of the object. iii) The nature of the fluid. Birds flying in air have streamlined body to reduce fluid friction. Fishes living in water have streamlined body to reduce fluid friction. Aeroplanes and spacecrafts have streamlined body to reduce fluid friction.

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How to Present to an Audience That Knows More Than You

• Deborah Grayson Riegel

Lean into being a facilitator — not an expert.

What happens when you have to give a presentation to an audience that might have some professionals who have more expertise on the topic than you do? While it can be intimidating, it can also be an opportunity to leverage their deep and diverse expertise in service of the group’s learning. And it’s an opportunity to exercise some intellectual humility, which includes having respect for other viewpoints, not being intellectually overconfident, separating your ego from your intellect, and being willing to revise your own viewpoint — especially in the face of new information. This article offers several tips for how you might approach a roomful of experts, including how to invite them into the discussion without allowing them to completely take over, as well as how to pivot on the proposed topic when necessary.

I was five years into my executive coaching practice when I was invited to lead a workshop on “Coaching Skills for Human Resource Leaders” at a global conference. As the room filled up with participants, I identified a few colleagues who had already been coaching professionally for more than a decade. I felt self-doubt start to kick in: Why were they even here? What did they come to learn? Why do they want to hear from me?

• Deborah Grayson Riegel is a professional speaker and facilitator, as well as a communication and presentation skills coach. She teaches leadership communication at Duke University’s Fuqua School of Business and has taught for Wharton Business School, Columbia Business School’s Women in Leadership Program, and Peking University’s International MBA Program. She is the author of Overcoming Overthinking: 36 Ways to Tame Anxiety for Work, School, and Life and the best-selling Go To Help: 31 Strategies to Offer, Ask for, and Accept Help .

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