This simulation explores the relationship between macroscopic motion, frictional forces, microscopic kinetic energy and temperature:. Fossil Fuels. Nuclear Fuels. Acid Rain. Climate Change. Friction is a force that is around us all the time that opposes relative motion between systems 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.
One of the simpler characteristics of friction is that it is parallel to the contact surface between systems and always in a direction that opposes motion or attempted motion of the systems relative to each other. If two systems 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 objects.
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 1 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. Frictional forces, such as f , always oppose motion or attempted motion between objects in contact. Friction arises in part because of the roughness of the surfaces in contact, as seen in the expanded view. In order for the object to move, it must rise to where the peaks can skip along the bottom surface. Thus a force is required just to set the object in motion. Some of the peaks will be broken off, also requiring a force to maintain motion.
Much of the friction is actually due to attractive forces between molecules making up the two objects, so that even perfectly smooth surfaces are not friction-free. Such adhesive forces also depend on the substances the surfaces are made of, explaining, for example, why rubber-soled shoes slip less than those with leather soles. The magnitude of the frictional force has two forms: one for static situations static friction , the other for when there is motion kinetic friction.
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 , the object will move. As seen in Table 1, the coefficients of kinetic friction are less than their static counterparts. 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.
If the coefficient of static friction is 0. Once there is motion, friction is less and the coefficient of kinetic friction might be 0. 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. The coefficient of the friction depends on the two surfaces that are in contact. 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.
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. In fact, it is the work done by force of friction. This energy converts to heat and warms up brake pads as well as rotors when brakes are applied. Friction does negative work. Example In Example 9 , the work done by force of friction is - ,J.
Use the work formula to calculate the force applied by friction if brakes were used within a distance of 56m. This theorem simply states that "the work done by the net force acting on a mass is equal to the change in the kinetic energy of that mass. The previous two examples combined show an application of this theorem.
In Example 9 , the change in the kinetic energy of the car was calculated. In Example 10 , the work done by the net force was calculated. The change in the K. This shows that when energy is lost in one form, it appears in another form. Both sides are expressed in units of energy, as expected.
Calculate a the net force, and b the engine force if the frictional forces add up to N. Solution: a Using Work-K. Example Starting from rest, a boy pushes a The frictional force between the sled and snow is Calculate a the work done by the boy on the sled, b the work done by the frictional force on the sled, c the work done by the net force on the sled, and d the speed of the sled at the end of the This law states that "Energy is conserved.
It is neither created nor destroyed. It only converts from one form to another. From our point of view, when energy is converted to heat via friction, it is a loss. For example, when we push a heavy crate up a ramp or incline onto a truck, we do some work.
Part of the work stores as P. It is easy to experimentally verify the following energy balance :. From our point of view , the work consumed by friction is a loss. From the Universe point of view, the work consumed by friction returns to the Universe in the form of heat and there is no loss. In the absence of frictional energy losses, the total mechanical energy of a system remains constant. We can always write down an energy balance for a system that goes under a certain process during which its mechanical energy changes from one form to another, even if some energy is lost due to friction.
The following examples apply the law of conservation of energy to a system or an object. Conservation of energy requires total energy to remain constant. Example In the figure shown, neglecting friction, find the speed of the kg car at the bottom of the hill.
Suppose the car is put in neutral and starts from rest from the hill top. Example In the figure shown, if 12 0 ,J of energy is consumed by frictional forces, find the speed of the kg car at the bottom of the hill. A - W fric. Not every term has M ; therefore , M's do not cancel. In this case, part of the available energy at A is be wasted by frictional forces. Example In the figure shown, for the 75 0 kg car in neutral, find a the energy consumed by the 30 0 N frictional force on the car as it coasts down the hill a distance of 40 0 m, and b its speed at the bottom of the hill.
Total Energy at A - W fric. Example An electric motor is capable of delivering 7. Find the power of the motor in watts, kilowatts, and hp. Example Calculate the amount of work or energy that a 4.
Efficiency: When a device receives power from a source, it does not deliver all of it in the intended form and converts a portion of it to other undesired forms. This makes a device to be less than percent efficient. For example, a typical car receives chemical energy in the form of gasoline. The purpose is to convert it to kinetic energy. The efficiency of this conversion is way below percent. A good portion of the original combustion of gasoline in the cylinders coverts to undesirable heat that has to be transferred to the ambient.
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