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Academic Cheating. My Slide Shows. First Q Review. My Forms. Home Work. My Blog. My Puzzles. They are static friction, sliding friction, rolling friction, and fluid friction. The next sections will explore these forces and when they are applied. Static friction exists between a stationary object and the surface on which it is resting.
It prevents an object from moving against the surface. Example: Static friction prevents an object like a book from falling of the desk, even if the desk is slightly tilted. It helps us pick up an object without it slipping through our fingers. When we want to move an object first we must overcome the static friction acting between the object and the surface on which the object is resting. Example: When a man is pushing an object on a rough surface the force acting is called "sliding friction".
Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. It is also called rolling resistance. When a force or torque is applied to a stationary wheel, there is a small static rolling friction force holding back the rolling motion.
However, resistance from static sliding friction is what really causes the wheel to start rolling. 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.
For every general rule about friction, there are just as many exceptions. For instance, while two rough surfaces such as sandpaper rubbing against each other sometimes have more friction, very smoothly polished materials such as plates of glass that have been carefully cleaned of all surface particles may actually stick to each other very strongly.
There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren't moving relative to each other, while kinetic friction acts between objects in motion. In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water. The atoms inside a solid material can experience friction as well.
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