Difference between Static and Kinetic coefficients of friction?

In summary, static coefficient of friction refers to the friction between two stationary objects, while kinetic coefficient of friction refers to the friction between two objects in motion. In most cases, only one coefficient is used in a calculation, based on the situation described in the question. However, in certain mechanical engineering problems, both coefficients may be used. It's important to note that an object may be in motion with respect to the ground, but not in motion with respect to the surface it is in contact with, in which case the static coefficient of friction would still be used.
  • #1
Air
203
0

Homework Statement


What is the difference between Static and Kinetic coefficients of friction?

2. The attempt at a solution
I think Static coefficient of friction is one which refers to friction which is considered when the object is stationary and Kinetic coefficient of friction is one which refers to the friction which is considered when the object is in motion.

3. The assistance that I require
Is this correct, is my wording correct too, how can my wording be improved?
 
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  • #2


Pretty much sounds good to me. I guess the only clarification you could give is static is when an objective is stationary relative to the other contact object and then kinetic is when the objective is rubbing/moving relative to the other contact object.
 
  • #3


Basically yes, when two bodies in contact are not moving relative to each other, we're talking about the static coefficient (which, multiplied by the normal reaction to the surface, produces the tangential frictional force which prevents motion), and when there exists motion, you're talking about the kinetic coefficient (which is usually less than the static coefficient). It's pretty much a fundamental concept, and can be found everywhere on the net. Try wiki, for example.
 
  • #4
Hi Air! :smile:
Air said:
I think Static coefficient of friction is one which refers to friction which is considered when the object is stationary and Kinetic coefficient of friction is one which refers to the friction which is considered when the object is in motion.

You could also say which coefficient is greater, and whether the friction force is equal to the normal force times the coefficient. :wink:
 
  • #5


You can never have both situations in one calculations, right? If given a question and we are given both values, when the object is stationary we use static coefficient and when in motion we use kinetic coefficient. In a calculation it's either one or the other? It's based on the question and we make judgements?
 
  • #6


Air said:
You can never have both situations in one calculations, right? If given a question and we are given both values, when the object is stationary we use static coefficient and when in motion we use kinetic coefficient. In a calculation it's either one or the other? It's based on the question and we make judgements?

In introductory physics, that is correct; you'll have one or the other at each interface. You may have both in a problem (professors are fond of thinking up problems where one block is able to slide while another does not but just look at each individual interface).

In mechanical engineering, there is a situation where both are present, but that is a unique problem and, to the best of my knowledge, never presented in physics.
 
  • #7


Air said:
You can never have both situations in one calculations, right? If given a question and we are given both values, when the object is stationary we use static coefficient and when in motion we use kinetic coefficient. In a calculation it's either one or the other? It's based on the question and we make judgements?
I just want to emphasize what has already been pointed out: The static coefficient is used when the object is stationary with respect to the surface it is in contact with . An object might be in motion with respect to the ground, but not in motion with respect to the surface it is in contact with.
 
  • #8


Theoretically, you could have an incline with a growing angle A (imagine a box on the back of an unloading kipper truck). Since the angle is growing, the component of the weight of the box which opposes the force of static friction, G*sin(A), is growing too, and can overcome the force of friction. Once the box starts sliding down, the coefficient of kinetic friction is "activated".
 

What is the difference between static and kinetic coefficients of friction?

The static coefficient of friction is the maximum amount of force that must be applied to an object to keep it from moving when it is resting on a surface. Kinetic coefficient of friction, on the other hand, is the amount of force needed to keep an object moving at a constant speed on a surface. Essentially, static friction deals with objects at rest, while kinetic friction deals with objects in motion.

How are static and kinetic coefficients of friction measured?

Static and kinetic coefficients of friction are measured by conducting experiments in which the force needed to move an object on a surface is measured. The ratio of this force to the weight of the object gives the coefficient of friction.

Why are static and kinetic coefficients of friction important?

Understanding the difference between static and kinetic coefficients of friction is important in many areas of science and engineering. It can help engineers design structures that can withstand forces, and it is also important in fields such as transportation, where reducing friction can improve efficiency.

Can the static and kinetic coefficients of friction ever be equal?

Yes, there are certain situations where the static and kinetic coefficients of friction can be equal. This occurs when an object is just on the verge of moving, or when it is moving at a constant speed with no acceleration. In these cases, the force needed to keep the object from moving or to keep it moving at a constant speed is equal to the weight of the object.

How do the static and kinetic coefficients of friction vary for different surfaces?

The values of both static and kinetic coefficients of friction can vary for different surfaces. For example, a rough surface will have a higher coefficient of friction compared to a smooth surface. Additionally, the coefficient of friction can also depend on the materials of the two surfaces in contact.

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