Learning ObjectivesDiscuss the general characteristics of friction.Describe the various species of friction.Calculate the magnitude of static and kinetic friction.
You are watching: In what direction does friction always act
Friction is a force that is around us all the moment that opposes relative motion in between surfaces in contact yet also enables us to relocate (which girlfriend have discovered if girlfriend have ever before tried come walk on ice). If a typical force, the actions of friction is in reality very complicated and is still not completely understood. We need to rely greatly on observations for whatever understandings we deserve to gain. However, we deserve to still resolve its an ext elementary basic characteristics and also understand the circumstances in which that behaves.
One the the simpler characteristics of friction is the it is parallel come the contact surface in between surfaces and always in a direction that opposes movement or attempted movement of the systems family member to every other. If two surfaces are in contact and also moving loved one to one another, climate the friction in between them is dubbed kinetic friction. For example, friction slowly a hockey puck slide on ice. Yet when objects space stationary, revolution friction can act between them; the revolution friction is usually better than the kinetic friction between the surfaces.
If 2 surfaces are in contact and also moving family member to one another, then the friction in between them is called kinetic friction.
Imagine, for example, do the efforts to on slide a hefty crate throughout a concrete floor—you might push harder and also harder ~ above the crate and also not move it in ~ all. This way that the revolution friction responds to what girlfriend do—it increases to be same to and also in the opposite direction of her push. Yet if you finally push hard enough, the crate seems to slip suddenly and starts to move. As soon as in activity it is easier to keep it in motion than it to be to gain it started, indicating the the kinetic friction force is much less than the static friction force. If you include mass come the crate, say by place a crate on optimal of it, you need to push also harder to get it began and likewise to store it moving. Furthermore, if you oiled the concrete girlfriend would uncover it to be simpler to acquire the crate started and keep it going (as you could expect).
(Figure)is a crude photographic representation of just how friction occurs at the interface in between two objects. Close-up investigate of this surfaces reflects them to be rough. So as soon as you press to get an object moving (in this case, a crate), you should raise the object till it deserve to skip along with just the tips of the surface ar hitting, break off the points, or carry out both. A substantial force can be stood up to by friction through no apparent motion. The harder the surfaces are propelled together (such together if one more box is inserted on the crate), the more force is required to move them. Component of the friction is because of adhesive forces between the surface ar molecules the the two objects, which define the dependency of friction top top the nature that the substances. Adhesion varies through substances in contact and is a complex aspect of surface physics. Once things is moving, there space fewer points of contact (fewer molecules adhering), so less pressure is compelled to keep the thing moving. At small but nonzero speeds, friction is almost independent the speed.
Frictional forces, such as
where is the coefficient of revolution friction and also is the size of the normal force (the force perpendicular come the surface).
Magnitude of revolution Friction
Magnitude of static friction is
where is the coefficient of static friction and also is the size of the typical force.
Once an item is moving, the size of kinetic friction
where is the coefficient that kinetic friction. A device in i beg your pardon
Magnitude the Kinetic Friction
The magnitude of kinetic friction is offered by
where is the coefficient of kinetic friction.
As checked out in (Figure), the coefficients that kinetic friction are less than their static counterparts. That values of
|Rubber on dry concrete||1.0||0.7|
|Rubber ~ above wet concrete||0.7||0.5|
|Wood on wood||0.5||0.3|
|Waxed timber on wet snow||0.14||0.1|
|Metal on wood||0.5||0.3|
|Steel on steel (dry)||0.6||0.3|
|Steel on stole (oiled)||0.05||0.03|
|Teflon top top steel||0.04||0.04|
|Bone lubricated by synovial fluid||0.016||0.015|
|Shoes on wood||0.9||0.7|
|Shoes on ice||0.1||0.05|
|Ice on ice||0.1||0.03|
|Steel ~ above ice||0.04||0.02|
The equations given previously include the dependence of friction ~ above materials and the regular force. The direction of friction is constantly opposite the of motion, parallel come the surface between objects, and also perpendicular come the normal force. Because that example, if the crate you shot to press (with a force parallel to the floor) has actually a massive of 100 kg, then the normal pressure would be equal to that is weight,
Find a small plastic thing (such as a food container) and also slide the on a kitchen table by providing it a gentle tap. Currently spray water top top the table, simulating a irradiate shower of rain. What happens now when you offer the object the same-sized tap? Now include a couple of drops that (vegetable or olive) oil top top the surface of the water and give the same tap. What happens now? This latter case is specifically important for vehicle drivers to note, particularly after a light rain shower. Why?
Many human being have proficient the slipperiness of wade on ice. However, numerous parts the the body, particularly the joints, have much smaller coefficients that friction—often 3 or 4 times much less than ice. A share is developed by the ends of two bones, which are linked by special tissues. The knee joint is created by the lower leg bone (the tibia) and also the thighbone (the femur). The hip is a sphere (at the end of the femur) and socket (part of the pelvis) joint. The end of the skeleton in the joint space covered by cartilage, which offers a smooth, nearly glassy surface. The joints likewise produce a fluid (synovial fluid) the reduces friction and also wear. A damaged or arthritic joint have the right to be changed by an artificial joint ((Figure)). These replacements can be make of metals (stainless steel or titanium) or plastic (polyethylene), likewise with very small coefficients that friction.
Artificial knee replacement is a procedure that has been perform for an ext than 20 years. In this figure, we check out the post-op X-rays that the right knee share replacement. (credit: Mike Baird, Flickr)
Other organic lubricants include saliva produced in our mouths to aid in the swallowing process, and the slippery mucus found between organs in the body, permitting them to move openly past every other throughout heartbeats, throughout breathing, and when a human moves. Synthetic lubricants are additionally common in hospitals and doctor’s clinics. Because that example, when ultrasonic imaging is carried out, the gelatin that couples the transducer come the skin likewise serves to to lubricate the surface between the transducer and the skin—thereby reducing the coefficient the friction in between the 2 surfaces. This enables the transducer to move freely over the skin.
A skier through a mass of 62 kg is sliding under a snowy slope. Find the coefficient the kinetic friction because that the skier if friction is recognized to be 45.0 N.
The size of kinetic friction was given in to be 45.0 N. Kinetic friction is pertained to the normal force as ; thus, the coefficient the kinetic friction can be found if we can uncover the normal force of the skier ~ above a slope. The normal force is constantly perpendicular to the surface, and since over there is no activity perpendicular come the surface, the normal force should same the ingredient of the skier’s weight perpendicular come the slope. (See the skier and also free-body diagram in (Figure).)
The motion of the skier and also friction space parallel to the slope and so it is many convenient to job all forces onto a coordinate device where one axis is parallel come the slope and the other is perpendicular (axes presented to left of skier).
This result is a little smaller 보다 the coefficient noted in (Figure) because that waxed lumber on snow, however it is still reasonable due to the fact that values the the coefficients the friction deserve to vary greatly. In instances like this, where an object of fixed
An object will certainly slide down an inclined airplane at a constant velocity if the net force on the thing is zero. We deserve to use this reality to measure up the coefficient of kinetic friction between two objects. As shown in (Figure), the kinetic friction on a slope
Put a coin top top a book and tilt it until the coin slides in ~ a continuous velocity under the book. You can need come tap the book lightly to get the coin to move. Measure up the edge of tilt relative to the horizontal and find . Note that the coin will not begin to slide at all till an angle better than is attained, due to the fact that the coefficient of revolution friction is larger than the coefficient the kinetic friction. Talk about how this may affect the value for and also its uncertainty.
We have debated that when things rests on a horizontal surface, over there is a normal force supporting it equal in magnitude to that is weight. Furthermore, basic friction is constantly proportional to the normal force.
The simpler elements of friction encountered so much are the macroscopic (large-scale) characteristics. Great strides have actually been do in the atomic-scale explanation that friction throughout the previous several decades. Researchers room finding the the atomic nature the friction appears to have several basic characteristics. These features not only define some that the simpler aspects of friction—they likewise hold the potential because that the development of virtually friction-free settings that might save numerous billions the dollars in power which is currently being converted (unnecessarily) to heat.
(Figure) illustrates one macroscopic properties of friction the is defined by microscopic (small-scale) research. Us have noted that friction is proportional to the common force, but not come the area in contact, a somewhat counterintuitive notion. As soon as two turbulent surfaces space in contact, the actual contact area is a tiny portion of the full area since only high spots touch. When a higher normal force is exerted, the actual contact area increases, and also it is uncovered that the friction is proportional come this area.
Two stormy surfaces in call have a lot smaller area that actual contact than their complete area. Once there is a greater normal pressure as a an outcome of a greater applied force, the area the actual call increases together does friction.
But the atomic-scale view assures to explain far more than the much easier features of friction. The device for how warmth is produced is currently being determined. In various other words, why do surfaces get warmer once rubbed? Essentially, atoms are linked with one an additional to kind lattices. Once surfaces rub, the surface atoms adhere and cause atom lattices come vibrate—essentially producing sound waves that penetrate the material. The sound tide diminish with distance and their energy is converted right into heat. Chemistry reactions the are pertained to frictional stay can also occur in between atoms and also molecules ~ above the surfaces. (Figure) shows how the guideline of a probe drawn throughout another material is deformed through atomic-scale friction. The pressure needed to traction the tip can be measured and is discovered to be regarded shear stress, which will be debated later in this chapter. The variation in shear anxiety is impressive (more 보다 a element of
The reminder of a probe is deformed party by frictional pressure as the probe is dragged across a surface. Dimensions of exactly how the pressure varies for various materials room yielding an essential insights right into the atomic nature that friction.
Explore the pressures at work when you try to push a filing cabinet. Create an applied force and see the result friction force and also total pressure acting on the cabinet. Charts present the forces, position, velocity, and acceleration vs. Time. Draw a free-body diagram of every the pressures (including gravitational and also normal forces). Click to open up media in new browser.
Section SummaryFriction is a call force in between systems that opposes the activity or test motion between them. Straightforward friction is proportional come the normal force pushing the solution together. (A normal pressure is always perpendicular to the contact surface between systems.) Friction depends on both of the products involved. The size of revolution friction between systems stationary relative to one one more is given by
wherein is the coefficient of revolution friction, which relies on both that the materials.The kinetic friction force between systems moving relative to one one more is given by
where is the coefficient of kinetic friction, which additionally depends ~ above both materials.
Define normal force. What is its relationship to friction as soon as friction behaves simply?
The glue on a item of tape deserve to exert forces. Deserve to these forces be a type of simple friction? Explain, considering specifically that tape deserve to stick to vertical walls and even come ceilings.
When you discover to drive, you discover that you have to let up slightly on the brake pedal together you pertained to a avoid or the auto will stop with a jerk. Describe this in terms of the relationship between static and also kinetic friction.
When you push a piece of chalk across a chalkboard, it periodically screeches because it swiftly alternates between slipping and sticking to the board. Explain this procedure in more detail, in details explaining just how it is regarded the fact that kinetic friction is less than static friction. (The same slip-grab procedure occurs as soon as tires screech on pavement.)
A physics major is cooking breakfast once he notices the the friction force between his steel spatula and his Teflon frying pan is only 0.200 N. Understanding the coefficient of kinetic friction in between the 2 materials, he quickly calculates the normal force. What is it?
(a) as soon as rebuilding she car’s engine, a physics major must exert 300 N of force to insert a dry steel piston into a steel cylinder. What is the size of the common force between the piston and cylinder? (b) What is the size of the force would she have to exert if the steel components were oiled?
(a) What is the best frictional force in the knee share of a person who supports 66.0 kg of her mass on the knee? (b) during strenuous practice it is possible to exert forces to the joints the are quickly ten times better than the load being supported. What is the maximum pressure of friction under together conditions? The frictional pressures in joints are reasonably small in all circumstances except when the joints deteriorate, such as from injury or arthritis. Enhanced frictional forces can cause further damage and pain.
Suppose you have a 120-kg wooden crate relaxing on a wood floor. (a) What maximum pressure can friend exert horizontally ~ above the crate without relocating it? (b) If you continue to exert this pressure once the crate starts to slip, what will the magnitude of that acceleration then be?
(a) If half of the weight of a little
A team the eight dog pulls a sled through waxed wood runners ~ above wet snow (mush!). The dogs have average masses that 19.0 kg, and also the invited sled v its rider has a massive of 210 kg. (a) calculation the magnitude of the acceleration starting from rest if each dog exerts one average force of 185 N behind on the snow. (b) What is the magnitude of the acceleration once the sled starts to move? (c) for both situations, calculation the magnitude of the pressure in the coupling in between the dogs and also the sled.
Consider the 65.0-kg ice skater being moved by two others displayed in (Figure). (a) uncover the direction and also magnitude of , the complete force exerted on her by the others, offered that the magnitudes
Show the the acceleration of any object down a frictionless incline that provides an edge through the horizontal is
Show the the acceleration of any object under an incline where friction behaves just (that is, whereby ) is
Calculate the deceleration the a snow boarder going increase a
(a) calculate the acceleration that a skier heading under a
If an object is to rest on one incline without slipping, then friction have to equal the component of the load of the thing parallel come the incline. This needs greater and greater friction for steeper slopes. Present that the maximum angle of one incline over the horizontal because that which things will no slide down is
Calculate the maximum deceleration that a auto that is heading down a slope (one that provides an edge of through the horizontal) under the adhering to road conditions. You may assume the the weight of the vehicle is evenly spread on all four tires and that the coefficient of static friction is involved—that is, the tires room not permitted to slip during the deceleration. (Ignore rolling.) calculation for a car: (a) On dried concrete. (b) top top wet concrete. (c) ~ above ice, assuming that
Calculate the maximum acceleration of a auto that is heading up a slope (one that provides an angle of through the horizontal) under the complying with road conditions. Assume that only fifty percent the weight of the automobile is supported by the two drive wheels and that the coefficient of revolution friction is involved—that is, the tires are not enabled to slip throughout the acceleration. (Ignore rolling.) (a) On dried concrete. (b) on wet concrete. (c) top top ice, assuming the
A freight train consists of two
Consider the 52.0-kg mountain climber in (Figure). (a) discover the tension in the rope and the pressure that the hill climber have to exert through her feet top top the vertical rock challenge to continue to be stationary. Assume the the force is exerted parallel to she legs. Also, i think negligible pressure exerted by her arms. (b) What is the minimum coefficient that friction between her shoes and also the cliff?
Part of the climber’s weight is sustained by she rope and component by friction in between her feet and also the rock face.
See more: 100 Oz Is How Many Gallons Converter, 100 Oz To Gallons
A contestant in a winter sporting event pushes a 45.0-kg block that ice across a frozen lake as displayed in (Figure)(a). (a) calculate the minimum force
Repeat (Figure) v the contestant pulling the block that ice through a rope end his shoulder at the exact same angle above the horizontal as shown in (Figure)(b).
Which technique of sliding a block of ice cream requires less force—(a) advertise or (b) pulling in ~ the exact same angle over the horizontal?