Examples of liquids that reduce friction. “Friction is useful and harmful

Have you ever wondered why your hands become warm when you rub them together, or why you can create fire by rubbing two pieces of wood together? The answer is friction! When two bodies move relative to each other, a frictional force appears, preventing such movement. Friction can cause energy to be released in the form of heat, warming your hands, starting a fire, and so on. The more friction, the more energy is released, so by increasing the friction between moving parts in mechanical system, you will get a lot of heat!

Steps

Surfaces of rubbing bodies

When two bodies move relative to each other, the following three processes can occur: irregularities on the surface of bodies interfere with the movement of bodies relative to each other; one or both surfaces of bodies may be deformed as a result of such movement; the atoms of each surface can interact with each other. All listed processes participate in the occurrence of friction. Therefore, to increase friction, choose materials with an abrasive surface (for example, sandpaper), with a deformable surface (for example, rubber), or with a surface that has adhesive properties (for example, sticky).

Press the bodies harder against each other to increase friction, since the friction force is proportional to the force acting on the rubbing bodies (force directed perpendicular to the direction of movement of the bodies relative to each other).

If one body is in motion, stop it. So far, we have considered sliding friction that occurs when bodies move relative to each other. Sliding friction is much less than static friction, that is, the force that must be overcome in order to set two contacting bodies in motion. Therefore, it is more difficult to move a heavy object than to control it when it is already moving.

• Do a simple experiment to understand the difference between sliding friction and static friction. Place the chair on a smooth floor (not carpet). Make sure there are no rubber or other pads on the legs of the chair to prevent it from sliding. Push the chair to move it. You will notice that once the chair is in motion, it becomes easier for you to push it because the sliding friction between the chair and the floor is less than the static friction.

1. Get rid of the grease between the two surfaces to increase friction. Lubricants (oils, petroleum jelly, etc.) significantly reduce the friction force between rubbing bodies, because the coefficient of friction between solid bodies is much higher than the coefficient of friction between a solid body and a liquid.

   • Try a simple experiment. Rub your dry hands together and you will notice that their temperature increases (they become warmer). Now wet your hands and rub them again. Now not only is it easier for you to rub your hands together, but they also heat up less (or slower).

2. Get rid of bearings, wheels and other rolling bodies to get rid of rolling friction and get sliding friction, which is much greater than the first (so rolling one body relative to another is easier than pushing/pulling it).

   • For example, imagine that you put bodies of the same mass in a sled and on a wheeled cart. A cart with wheels is much easier to move (rolling friction) than a sled (sliding friction).

3. Increase the viscosity of the fluid to increase frictional force. Friction occurs not only when moving solids, but also in liquids and gases (water and air, respectively). The friction between a liquid and a solid depends on several factors, for example, the viscosity of the liquid - the higher the viscosity of the liquid, the greater the friction force.

   Drag

   1. Increase body surface area. As noted above, when solid bodies move in liquids and gases, a friction force also arises. The force that prevents the movement of bodies in liquids and gases is called drag (sometimes called air resistance or water resistance). Drag is greater with increasing surface area of ​​the body, which is directed perpendicular to the direction of movement of the body through the liquid or gas.

      • For example, take a pellet weighing 1 g and a sheet of paper of the same mass and release them at the same time. The pellet will immediately fall to the floor, and the sheet of paper will slowly fall down. This is where the principle of drag is clearly visible - the surface area of ​​the paper is much larger than that of the pellet, so the air resistance is greater and the paper falls to the floor more slowly.

   2. Use body shape with large coefficient drag. Based on the surface area of ​​the body directed perpendicular to the movement, one can judge the frontal resistance only in general terms. Bodies various shapes interact with liquids and gases in different ways (when bodies move through gas or liquid). For example, round flat plate has more drag than a round spherical plate. The quantity characterizing the drag of bodies of various shapes is called the drag coefficient.

      Use less streamlined bodies. Usually, big bodies cubic shape have high drag. Such bodies have right angles and do not taper towards the end. On the other hand, streamlined bodies have rounded edges and usually taper towards the end.

   3. Use bodies without through holes. Any through hole in the body reduces drag by allowing air or water to flow through the hole (the holes reduce the surface area of ​​the body perpendicular to the movement). The larger the through holes, the less drag. This is why parachutes, which are designed to create a lot of drag (to slow the speed of fall), are made of strong, lightweight silk or nylon rather than gauze.

      • For example, you can increase the speed of a ping pong paddle if you drill a few holes in it (to reduce the surface area of ​​the paddle and therefore reduce drag).

   4. Increase the speed of the body to increase drag (this is true for bodies of any shape and made of any material).

      • The higher the speed of an object, the greater the volume of liquid or gas it must pass through and the greater the drag. Bodies moving at very high speeds experience enormous drag, so they must be streamlined; otherwise the force of resistance will destroy them. For example, consider the Lockheed SR-71, an experimental reconnaissance aircraft built during cold war . This plane could fly with high speed
   • M = 3.2 and, despite its streamlined shape, experienced enormous drag (so great that the metal from which the aircraft fuselage was made expanded when heated by friction).
   • Remember that friction releases a lot of energy in the form of heat. For example, do not touch the car's brake pads directly after braking! Keep in mind that high strength

resistance can lead to the destruction of a body moving in a liquid. For example, if during a boat trip you place a piece of plywood in the water (so that its surface is directed perpendicular to the movement of the boat), then most likely the plywood will break.

• To introduce students to the force of friction, to consolidate their knowledge about forces in nature. Form the concept of “friction” and “friction force”;
• continue the formation of natural scientific ideas;
• continue to develop practical skills in working with equipment;
• promote careful handling of instruments and equipment;
• contribute moral education students through a story about scientists.

Developed skills: work with instruments, observe, compare experimental results, draw conclusions.

Lesson type: combined.

Equipment: dynamometer; wooden blocks; set of loads; sand.

Demos:

• Forces of static and sliding friction.
• Comparison of sliding and rolling friction forces.

During the classes

1. Update background knowledge. Creating situations of success.

1. Frontal survey:

• What is called force?
• What forces have we already studied?
• How to give a complete answer about any force?
• What instrument can you use to measure force?

1. Problem solving. (At the blackboard)

• What force of gravity acts on an apple weighing 120g?
• A spring with a stiffness of 500 N/m was stretched by 2 cm. Under what force was it stretched?

1. Determine what kind of power is spoken about in the text of the book “ Entertaining physics" ME AND. Perelman “We’ve all had to leave the house in icy conditions: how much effort does it take us to keep ourselves from falling, how many funny movements do we have to do in order to stand!”
2. Examples of manifestations of the phenomenon of friction in nature.

1. Explanation of new material.

Presentation . Lesson topic “Friction force” (slide 1)

1. Introduction to Friction (slide 2,3)

Experiment 1. The influence of friction on the movement of bodies. Push the block along the tribometer board. Identify the reason for the rapid stop of the block.

• The force that arises when the surface of one body interacts with the surface of another, when the bodies are stationary or moving relative to each other, is called the friction force. (Video “Power”)
• The friction force is designated by the letter F with the index Ftr

1. A little history (slide 4.5)

The first to study the force of friction was Leonardo da Vinci (1452-1519). This power was later explored by Gilioma Amonton (1663-1705) and Charles Coulomb (1736-1806). Amonton and Coulomb introduced the concept of friction coefficient.

1. Let's take a closer look at the friction force

Exist different kinds dry friction:

Static friction(slide6). The force that holds the cabinet in place is the static friction force. To move the body from the support, you need to apply force. This force balances the friction force. On an inclined support, the friction force holds the body. The static friction force can reach large values. (Video “Friction at rest”)

Task No. 1. Measuring friction force.

Equipment:

Progress:

• Place a wooden block with a 100g load on the tribometer board, attach a dynamometer to the hook of the block, and holding it horizontally, gradually increase the traction force.
• Draw a conclusion.

Conclusion: as long as the traction force is small, the block remains at rest. This means that in addition to the traction force, some other force acts on the block, counteracting this one. This force is called the static friction force.

Sliding friction (slide7). When the body begins to move along the support, a sliding friction force arises, directed in the direction opposite to the movement.

Task No. 2. Measurement of sliding friction force.

Equipment: bars, set of weights, dynamometer, ruler.

Progress:

• Place the block on the table surface. Attach the dynamometer to the block and pull the dynamometer evenly (at the same speed).
• Determine the dynamometer readings. How can you reduce the force of friction? Answer: To reduce friction, liquid lubricant is applied to the smooth surfaces of rubbing bodies.
• Place 1, then 2, and then 3 weights on the block alternately and measure the friction force for each case.
• Record the result.
• Draw a conclusion.

Conclusion: Mutual attractive forces arise between the molecules of contacting bodies, which are the cause of friction. If the bodies are well polished, then the friction force can become very large.

Rolling friction(slide8). Rolling friction is the frictional force that occurs when one body rolls over the surface of another. (video “Rolling friction force”).

In technology, a lubricant is often applied to reduce dry friction forces, or sliding friction is replaced by rolling friction (bearings are used). The rolling friction force is much less than sliding friction.

Task No. 3: The rolling friction force is always less than the sliding friction force.

Equipment: block, dynamometer, roller (instead of a roller you can take a block and wooden pencils), ruler.

Progress:

• Assemble the installation (Fig. 1). (If you don’t have a roller, you can put the block on wooden pencils). Record the friction force values
• Assemble the installation (Fig. 2). Record the friction force values

Fig.1.	Fig.2.

• Compare the values ​​and draw a conclusion.

(Video “Difference between friction forces”)

1. Other friction forces.

When solids move in liquids, a force of viscous friction arises. The amount of viscous friction depends on the shape of the body, the type of liquid and the speed of the body.

1. Features of friction force

• occur when two moving bodies come into contact
• act parallel to the contact surface of the bodies
• directed against body movement

1. Should we get rid of friction? (slide 9,10,11)

Let's imagine our life without friction (conversation with students)

1. Reflective-evaluative stage:

1. Answer the questions:

• Why is any body set in motion in Eventually, stops?
  Answer: A sliding friction force acts on a moving body, which is directed against the movement and reduces the speed of the body.
• Why is it more difficult to move a sled than to carry it?
  Answer: The force of static friction when moving from the place of the sled more power sliding friction.
• Why is the barrel rolled and not carried?
  Answer: B in this case replace the sliding friction force with the rolling friction force, which is significantly less
• How can you reduce friction?
  Answer: Lubrication reduces friction and replaces body sliding with rolling. The rolling friction force is less than the sliding friction force.
• How to increase friction?
  Answer: Make the surface uneven (rough) or increase the pressure.

1. Explain sayings about friction:

• “If you don’t grease it, you won’t go.”
• “It went like clockwork.”
• “What is round rolls easily.”
• “Skis glide with the weather.”
• “Mow, mow, while there is dew, away with the dew - and we’ll go home.”

1. Let's summarize our lesson:

• What phenomenon have we studied?
• What are the causes of friction?
• What does friction depend on?
• What ways are there to reduce and increase friction?
• Does friction depend on the environment in which it occurs?
• What types of friction exist around us?
• Which physical quantities characterize each type of friction?
• What did you like about the lesson? (slide12)
• What was difficult?

1. Homework:

1. §16-17; questions for the paragraph; 10 examples various manifestations friction forces (find from additional literature). Write an essay on the topic: “If there was no friction force.”
2. High level. Challenges for ingenuity:

• There is a stack of books on the table. Which is easier: pulling bottom book, holding the rest, or set the entire stack in motion by pulling the bottom book?
• What is the coefficient of friction of the wheels on the road if the traction force of a car weighing 1 ton is 500 N?

In technology, to reduce the influence of dry friction forces between surfaces, a lubricant is introduced (a viscous liquid that creates thin layer between hard surfaces).

The effect of lubrication is that a layer of viscous liquid is introduced between the rubbing surfaces, which fills all the surface irregularities and, sticking to them, forms two rubbing layers of liquid.

Therefore, instead of friction between two hard surfaces During lubrication, internal friction of the liquid occurs, which is significantly less than the external friction of two solid surfaces. The use of lubricating oils reduces friction by 8-10 times. Typical example The meaning of lubrication represents the running of a speed skater on skates. As a result of the force exerted by the skater on the blade of the skate, the snow melts and water appears under the skate, which freezes again after the skater has run and the pressure has disappeared. However, water is not suitable for lubrication in mechanisms, since due to its low viscosity it would be squeezed out of the gap of irregularities between the rubbing surfaces.

All cars have one common feature: in any of them something is sure to rotate. And everywhere there is an inseparable pair - the axle and its support - the bearing

Since the rolling friction forces are significantly less than the sliding friction forces, in machines and mechanisms, in most cases, sliding bearings are replaced with rolling bearings.

The bearing consists of two rings. One of them - the inner one - is tightly mounted on the axis and rotates with it. The other outer ring is firmly clamped between the base and the bearing cap.

These rings - clips have machined grooves on their surfaces facing each other. Between the clips there are steel balls. When the bearing rotates, the balls roll along the grooves in the cages.

The better the surfaces of the tracks and balls are polished, the less friction. To prevent the balls from running into one heap, they are separated by a separator. Separators are usually made of plastic, steel or bronze.

When rotating, rolling friction appears in such a bearing. Friction losses in a ball bearing are 20-30 times less than in a plain bearing! Rolling bearings are made not only with balls, but also with rollers different shapes. Without rolling bearings modern industry and transport would be impossible.

Currently, a method of reducing friction when moving vehicles is widely used, such as an air cushion.

An air cushion is a layer of compressed air underneath vehicle, which lifts it above the surface of the water or earth. A layer of compressed air is created by fans. The absence of friction on the surface reduces resistance to movement. The ability of such a vessel to move over various obstacles on land or over waves on water depends on the lifting height.

The first idea of ​​such a hovercraft was expressed by K.E. Tsiolkovsky in 1927, in his work “Air Resistance and the Fast Train.” This is a wheelless express that rushes over a concrete road, relying on an air cushion - a layer of compressed air.

Friction plays important role V Everyday life. This force has to be taken into account when designing a wide variety of technical systems, the operating principle of which is based on direct contact of moving parts. Friction is not always a harmful factor, but in most cases, developers try to reduce it in a variety of ways.

Instructions

In the very simple case try to change the degree of roughness of the surfaces of contacting objects. This can be achieved by sanding. Bodies whose interacting surfaces are smooth and glossy will move relative to each other much easier.

If possible, replace one of the contacting surfaces with one that has a lower coefficient of friction. It can be artificial turf - for example, Teflon has one of the lowest coefficients of friction, equal to 0.02. It is easier to change that element of the system that plays the role of a tool.

Use lubricants, introducing them between the rubbing surfaces. This method is used, for example, in skiing, when a special paraffin lubricant corresponding to the temperature of the snow is applied to the working surface of the skis. Lubricants used in other technical systems, can be liquid (oil) or dry (graphite powder).

Consider using “gaseous lubrication.” It's about about the so-called “air cushion”. In this case, the friction force is reduced by creating an air flow between the previously contacting surfaces. The method is used in the design of all-terrain vehicles designed to overcome difficult terrain.

If the system in question uses sliding friction, replace it with rolling friction. Try a simple experiment. Place a regular glass on a flat table surface and try to move it with your hand. Now place the glass on its side and do the same. In the second case, it will be much easier to move the object from its place, since the type of friction has changed.

Use bearings in areas where friction occurs. These elements make it possible to transform the type of movement, thereby significantly reducing friction losses, reducing its force. This method is most widely used in technology.

At first glance, excessive friction is harmful. It reduces the efficiency of mechanisms and wears out parts. But there are cases when the friction force needs to be increased. For example, when wheels roll, it is necessary to improve their grip on the road. See how this can be done.

Instructions

To understand how to increase the friction force, remember what it depends on. Consider the formula: Ftr=mN, where m is the friction coefficient, N is the support reaction force, N. The support reaction force, in turn, depends on the mass: N=G=mg, where G is the body weight, N-m is the mass body, kg - g – acceleration free fall, m/s2.

From the formula we can conclude that the friction force depends on the friction coefficient. The friction coefficient is determined for each pair of interacting materials and depends on the nature of the material and the quality of the surface.

Thus, the first way to increase friction is to change the material of the sliding surface. You've probably noticed that in one shoe it's almost impossible to walk on a wet tiled floor, but in another you don't feel any discomfort. This is because the soles of the shoes are made of various materials. Slippery shoes have a low coefficient of sliding friction between the sole and the wet tile.

The second way is to increase the surface roughness. Example - winter tires for a car have a more prominent tread than summer tires. Due to this, on slippery winter road the car can drive confidently.

The third way is to increase mass. As can be seen from the formula, the friction force directly depends on the mass. This explains why a loaded car in some cases It is easier to get out of the mud than for someone who is light. This rule works when certain quality soil - a heavy machine will sink more into viscous, swampy soil than a light one.

The fourth method is to remove the grease. Imagine a conveyor of a production line consisting of rotating rollers on which a belt is stretched. The conveyor rollers begin to slip along the belt if they are dirty. In this case, the dirt acts as a lubricant. By cleaning the parts of the mechanism, you will increase the friction force and increase the efficiency of the equipment.

The fifth method is polishing. By polishing the surface, you can increase the friction force. This is explained by the fact that when polished surfaces come into contact, intermolecular attractive forces are activated. For example, it is very difficult to move two sheets of glass apart when folded together.

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