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
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.
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 is
where is the coefficient of static friction and is the magnitude of the normal force (the force perpendicular to the surface).
Magnitude of Static Friction
Magnitude of static friction is
where is the coefficient of static friction and 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. 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, the object will move. Thus
Once an object is moving, the magnitude of kinetic friction is given by
where is the coefficient of kinetic friction. A system in which is described as a system in which friction behaves simply.
Magnitude of Kinetic Friction
The magnitude of kinetic friction is given by
where 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 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.
|System||Static friction||Kinetic friction|
|Rubber on dry concrete||1.0||0.7|
|Rubber on wet concrete||0.7||0.5|
|Wood on wood||0.5||0.3|
|Waxed wood on wet snow||0.14||0.1|
|Metal on wood||0.5||0.3|
|Steel on steel (dry)||0.6||0.3|
|Steel on steel (oiled)||0.05||0.03|
|Teflon on 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 on ice||0.04||0.02|
The equations given earlier include the dependence of friction on materials and the normal force. The direction of fr