How can kinetic friction cause motion

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. The knee joint is formed by the lower leg bone the tibia and the thighbone the femur.

The hip is a ball at the end of the femur and socket part of the pelvis joint. The ends of the bones in the joint are covered by cartilage, which provides a smooth, almost glassy surface. The joints also produce a fluid synovial fluid that reduces friction and wear. A damaged or arthritic joint can be replaced by an artificial joint Figure 2. These replacements can be made of metals stainless steel or titanium or plastic polyethylene , also with very small coefficients of friction.

Figure 2. Artificial knee replacement is a procedure that has been performed for more than 20 years. In this figure, we see the post-op x rays of the right knee joint replacement. Other natural lubricants include saliva produced in our mouths to aid in the swallowing process, and the slippery mucus found between organs in the body, allowing them to move freely past each other during heartbeats, during breathing, and when a person moves.

For example, when ultrasonic imaging is carried out, the gel that couples the transducer to the skin also serves to to lubricate the surface between the transducer and the skin—thereby reducing the coefficient of friction between the two surfaces.

This allows the transducer to mover freely over the skin. A skier with a mass of 62 kg is sliding down a snowy slope. Find the coefficient of kinetic friction for the skier if friction is known to be The magnitude of kinetic friction was given in to be The force acts in the opposite direction to the way an object wants to slide.

The Force of Static Friction keeps a stationary object at rest! Friction occurs essentially in two different modes: sliding and rolling friction. The relationships between the frictional force and the load or weight of the sliding object differ for dry, or unlubricated, surfaces and lubricated surfaces. And the rolling of an object is a type of motion. So, in a way, we can consider rolling friction as a type of kinetic friction. Rolling Friction is extremely smaller than sliding friction. Since the forces that resist rolling are usually very small, the rolling friction turns out to be small.

If the surfaces are not moving relative to each other, we talk of static friction. Question f7a Question Question 0b Question d70af. Question dab6f. See all questions in Frictional Forces.

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