Once movement is initiated, some abrasion continues to occur, but at a much lower level than during static friction. And the relative velocity between the surfaces provides insufficient time for additional cold welding to occur except in the case of extremely low velocity.
With most of the adhesion and abrasion being overcome to induce movement, the resistance to motion between the surfaces is reduced, and the surfaces are now moving under the influence of kinetic friction, which is much lower than static friction. Friction is an incredibly complex force that manifests differently under various conditions, making it difficult to express in terms of physical laws and mathematic equations. However, there are three assumptions regarding friction that apply in most real-world situations:.
You might notice that assumption 2 friction is independent of area of contact seems to contradict the idea presented earlier that because the points of contact between the asperities are very small, pressure between the surfaces is high and adhesion via cold welding occurs, which increases friction. Using the box example from above: Assume you move the contents of the box to a different box with a larger footprint.
Despite the larger apparent area of contact, the friction force remains the same, as predicted by assumption 2. Note that the discussion above relates to non-lubricated, sliding surfaces. The equation for the force of friction is , where is the coefficient of static friction. The normal force is equal to the mass times acceleration due to gravity, but in the opposite direction negative the force of gravity. Static friction prevents sliding Figure 1.
Cheese resting on an incline because static friction holds it in place. Another example of static friction comes when objects are moving. When you are walking, static friction pushes in the direction of you are trying to move see Figure 2 below. In static friction, the frictional force resists force that is applied to an object, and the object remains at rest until the force of static friction is overcome.
In kinetic friction, the frictional force resists the motion of an object. Different types of motion of the object gives rise to different types of friction. Generally, there are 4 types of friction. They are static friction, sliding friction, rolling friction, and fluid friction.
A sled sliding across snow or ice. Rubbing both hands together to create heat. Skis sliding against snow. A person sliding down a slide is an example of sliding friction. The force resisting the motion of a rolling body on a surface is known as rolling friction or rolling resistance. Rolling of ball or wheel on the ground is an example of Rolling friction.
Pushing a box across the table is an example of Sliding friction. Examples of Sliding Friction A child sliding down through a slide in a park. A coaster sliding against a table. In real world, the phenomenon of friction may be much more complex.
Your reasoning points out that with the velocity very close to zero kinetic friction cannot be much higher than static friction. To be more accurate, if the velocity is infinitely small, the kinetic friction may be higher than static one, but then it may only be infinitely small! An example is air friction, which is zero when a body doesn't move and increases with velocity. In a simple model, for small velocities, air friction is just proportional to velocity.
You start moving an object which increases the static force to prevent any motion. However, you keep applying more force until you reach a maximum value for the static friction. Then, the object begins to move. However, the kinetic friction which is produced when the object moves is less than the applied force. Furthermore, the kinetic friction doesn't depend on the applied force, therefore, you don't need to apply more force to keep moving the object actually, you need less force.
With static friction there is no movement between the surfaces and therefore no work is done, and so the two surfaces remain at the same temperature ambient. However, with dynamic friction there is work done and so one or both surfaces absorb the energy and heat up. With typical friction materials the maximum coefficient of friction is only achieved when the material is heated, and in many not all cases the dynamic coefficient of friction, at elevated temperature and with suitable contact pressure and sliding speed, is in fact HIGHER than the static coefficient of friction with lower ambient temperature and zero sliding speed with the same contact pressure.
Certainly for some types of materials it is demonstrably the case that the static friction coefficient can be lower than the quoted dynamic friction coefficient. I think the answer is yes. Here's one possible explanation for it that's only speculation. Surely that means the coefficient of static friction can be less than that of kinetic friction for a given normal force.
How can that be? I think the theory predicts that for sufficiently low sliding speeds of smooth surfaces, the force of kinetic friction per area varies linearly with sliding speed. In order to prove that the coefficient of static friction can't be less than that of kinetic friction, you have to assume that the force of kinetic friction per area is independent of the sliding speed.
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