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Wea€™ve talked about the ability of friction to add enough heat to an internal combustion engine to fuse the parts of the engine together.
As you might imagine, a lot of research in each of these areas is devoted to investigating the relationship between friction and heat as well as ways to modify the force of friction.
NASCAR TIRES - Race car tires are extremely important in the fast-pace world of professional auto racing.
Friction can be described as the force that opposes the motion between two surfaces because of the mechanical and molecular interaction between the surfaces.
Factors or variables that affect friction were discovered as far back as the 15th a€“ 18th centuries by scientists such as Leonardo Da Vinci and Guillaume Amontons. As you might imagine, there are many types of friction and friction can occur between different phases of matter. However, if you were to look at their surface at the molecular level, you would see that the surfaces are uneven. If we take a look at static friction on the molecular level, you can imagine that the two surfaces in contact with each other may be a€?lockeda€? or a€?adhereda€? to each other because of their asperities or bumps. You can imagine that if the surfaces are locked or adhered to each other, they would be held together with a certain force.
However, if a force is applied to the block that can overcome the force of static friction, then the block will move. The applied force is able to a€?unlocka€? the two surfaces and the asperities begin sliding over one another. Now imagine that you can see what happens at the molecular level as the block is sliding across the table. If the applied force is removed the block will slow and eventually stop because of the force of the asperities snagging and pulling against one another (dynamic friction).
These examples illustrate why friction involves the molecular interaction between two surfaces. Before we discuss friction, leta€™s take a look at each of these areas so that you are familiar with them.
If we take a look at the wheel of a car, we see that the tires make contact with the surface of the road.
If we take a closer look at the wheel of a car, we see that the tire is attached to the wheel. In each of these cases: the piston and cylinder, the tires and the road, the brake rotor and brake pad, friction occurs when the two surfaces rub against each other. In order for a car to move, the static friction between the tires and the road must be OVERCOME.
Once the car is moving, there is another type of friction called rolling friction between the tires and the road. When the car needs to stop, the brakes must function to slow the car enough so that the force of static friction is greater than the force of rolling friction so that the tires will remain stationary on the road. In order to the wheels of a car to slow, the brake pads must come in contact with the brake rotor. In order to answer that question, you need to think back on what you learned in Sector 3.2 about heat, kinetic energy, and temperature. When the kinetic energy of the molecules in a substance changes, the temperature of a substance changes.


If you put this all together, you should see that an event that changes the kinetic energy of molecules in a substance involves heat and a change in the temperature of the substance. In our example of the block and table surface, you would see the box sliding across the table surface. Friction occurs because of the molecular interaction of two surfaces in contact with each other.
You saw examples of this in the 3.3 Journal Club video in the use of oil, different types of piston and cylinder materials, and the force applied to the brake rotors. In addition, some pistons and cylinders are constructed with substances like silicon fused with the metal in order to reduce the friction between the piston and the cylinder. In the wheel hub, brake pads are constructed with materials that will generate enough friction to slow the brake rotor but will not cause an unacceptable rise in the temperature of the rotor.
Youa€™ve watched a video on racing that showed brake rotors glowing from the heat of friction, and tires deformed or melted from the heat of friction. After all, while elimination of friction in an internal combustion engine would be beneficial, its complete elimination on the brake rotors or tires could spell disaster. In class you will work with your group and go over the materials and methods involved in these experiments. In terms of direction, friction acts in the direction that is OPPOSITE to the motion or potential motion of an object. More recent research on the molecular basis of friction came during the 1950a€™s and continues today. One of the easiest ways to understand what is meant by these terms and how these types of friction differ is through an example. Static friction can be described as the friction between two objects that are NOT moving relative to each other. However, you have to keep applying a force to it to keep it moving, otherwise it will slow and stop. Dynamic friction is the friction between two objects that are moving relative to each other. As they move they pull at each other, creating a force that resists the motion of the block along the table. For our discussion today we will focus on three areas: between the piston and cylinder, between the brake rotor and brake pad, and between the tires and the road.
The brake calipers hold the brake pads and can close and open to bring the brake pads in contact with the brake rotor. This friction can stop the motion of the moving object or parts and it can result in the build up of heat. This means that a force must be applied to the piston that OVERCOMES the force of static friction, otherwise the piston will NOT move in the cylinder. When the brake pads come in contact with the rotor, they slide against the rotor causing dynamic friction. It is generated when one surface slides over another or two surfaces slide over each other.
You should remember that heat is defined as the transfer of internal energy because of difference in temperature. At this level, the box has kinetic energy because it is in motion across the surface of the table.


When we do, we see that the surfaces in contact with each other tend to touch only at certain areas or points. You may want to create a list of these terms in your science journal so that you can formulate definitions and application examples as you read. However, in Sector 3.3, we will focus on the concepts of friction that are related to the interaction of two solids as well as the interaction of solids and liquids. However, one of our basic understandings of friction involves asperities or a€?bumpsa€? that make up the surface of materials. As the asperities of the block surface and the table surface move past each other, they exert a force on each other.
This is because it is easiest to understand the relationship between friction, kinetic energy, and heat with examples of dynamic friction. You should also remember that internal energy includes the kinetic and potential energy of a substance. Therefore anything that increases friction would be expected to increase heat and the temperature of the substances. The result is a decrease in the amount of dynamic friction as compared to the friction between the metal of the piston and the metal of the cylinder without the oil. Our exploration of friction will also include some of the events that occur at a molecular level, since it is the change in the kinetic energy of molecules that results in the increase in temperature during friction.
The effect of friction on the motion of an object can be explained by Newtona€™s Three Laws of Motion. Although these molecules are unable to be seen with the human eye, they vibrate and move within each state of matter.
As one surface is rubbed over the other or two surfaces are rubbed together, these bumps interact with one another.
Likewise, anything that decreases friction would be expected to reduce heat and the temperature of the substances. Dynamic friction is the reason that you must continue to apply a force to keep the block moving. As a result, they have kinetic energy and the kinetic energy of these molecules can change. If a force is not continuously applied to the block, the force of dynamic friction will eventually stop the block.
The bumps may push down or compress each other, they may also push the bumps ahead of them into a pile which is then later released so that the pile springs back.
In each of these cases, the compression or piling or rubbing makes the molecules in the substances move faster.
The increase in kinetic energy results in an increase in the temperature of the substances.



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