Difference between static friction and kinetic friction

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Discussion Overview

The discussion revolves around the differences between static friction and kinetic friction, exploring their definitions, behaviors under various forces, and the conditions under which objects remain at rest or begin to move. Participants examine theoretical concepts and practical examples related to frictional forces on horizontal surfaces.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that static friction equals the applied force when an object remains at rest, while questioning what happens if static friction is greater than the applied force.
  • Others argue that once the applied force exceeds the maximum static friction, the object will begin to move.
  • It is suggested that if the applied force equals the kinetic friction, the object will continue moving at a constant speed, while if the applied force is less than the kinetic friction, the object will slow down.
  • Participants discuss that static friction cannot move an object from rest and always acts to oppose motion.
  • Some clarify that static friction adjusts to match the applied force up to its maximum value, while kinetic friction is constant and does not have a maximum limit.
  • A later reply questions the conditions under which an object remains at rest or moves with constant velocity, particularly in relation to net forces and static friction.
  • There is a discussion about a specific example involving two crates and the forces acting on them, raising questions about the relationship between applied force and static friction.
  • Some participants express uncertainty about whether an object can move when the applied force equals static friction, suggesting it may need to be slightly greater to initiate movement.

Areas of Agreement / Disagreement

Participants generally agree on the definitions and roles of static and kinetic friction, but multiple competing views remain regarding the conditions for movement and the relationship between applied force and frictional forces. The discussion remains unresolved on some specific points, particularly regarding the nuances of when an object transitions from rest to motion.

Contextual Notes

Limitations include assumptions about the conditions of the surfaces involved, the definitions of maximum static friction, and the specifics of applied forces in various scenarios. Some mathematical steps and relationships are not fully resolved.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics, particularly those interested in mechanics, frictional forces, and the behavior of objects under applied forces.

MIA6
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I have some confusions between static friction and kinetic friction:
When the static friction equals to applied force on an object, an object doesn't move and stays at rest I think. How about if the static friction is larger than the applied force? Will the object still stay at the rest, that the object will not be moved to opposite direction of the applied force?
When the applied force is larger than the static friction, then it starts to move?
When the object is moving, it has a kinetic friction. If the kinetic friction is equal to the applied force, will the object come to rest or will move in a uniform motion?
If the kinetic friction is larger than the applied force, will the object move in an opposite direction of the applied force?

Thanks for help.
 
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Static friction indicates the maximum force that can be applied before an object starts to move. If the object is not accelerating, and is on a horizontal plane, then in all cases, friction force is equal in magnitude and opposes the net "horizontal" force on the object, for both static and kinetic forces.
 
MIA6 said:
I have some confusions between static friction and kinetic friction:
When the static friction equals to applied force on an object, an object doesn't move and stays at rest I think. How about if the static friction is larger than the applied force? Will the object still stay at the rest, that the object will not be moved to opposite direction of the applied force?
When the applied force is larger than the static friction, then it starts to move?
Let's keep it simple. Imagine an object resting on a horizontal surface. Apply a horizontal force. As long as the force is less that the maximum value for static friction (which is [itex]\mu N[/itex]), then the applied force will equal the friction and the object will remain at rest. When the force is greater than the maximum value for static friction, then the object starts moving.

When the object is moving, it has a kinetic friction. If the kinetic friction is equal to the applied force, will the object come to rest or will move in a uniform motion?
If the kinetic friction is larger than the applied force, will the object move in an opposite direction of the applied force?
Once the object is moving, kinetic friction appears. If the applied force equals the kinetic friction, then there's no net force and thus no acceleration--it keeps moving at whatever speed it has. Realize that kinetic friction always opposes the slipping between the surfaces, so it always acts opposite to the way the object is moving. If the applied force is less than the kinetic friction, then the object will slow down.
 
Static friction cannot move the object from the rest. Friction makes objects slower or come to the rest or keep their rest position. Friction is against the motion. That's what I can add Doc Al's great answer.
 
Doc Al said:
Let's keep it simple. Imagine an object resting on a horizontal surface. Apply a horizontal force. As long as the force is less that the maximum value for static friction (which is [itex]\mu N[/itex]), then the applied force will equal the friction and the object will remain at rest. When the force is greater than the maximum value for static friction, then the object starts moving.


Once the object is moving, kinetic friction appears. If the applied force equals the kinetic friction, then there's no net force and thus no acceleration--it keeps moving at whatever speed it has. Realize that kinetic friction always opposes the slipping between the surfaces, so it always acts opposite to the way the object is moving. If the applied force is less than the kinetic friction, then the object will slow down.

So if the horizontal force=static friction, the object still doesn't move, right? As long as the static friction is not greater than horizontal force, then the object will remain at rest.
 
MIA6 said:
So if the horizontal force=static friction, the object still doesn't move, right?
Right, since the net force would be zero it would remain at rest.

As long as the static friction is not greater than horizontal force, then the object will remain at rest.
The point is that the static friction will never be greater than the applied force.

Let's use an example to hammer this home. Assume that the maximum value of static friction is 100 N (thus [itex]\mu N[/itex] = 100 N). If you push with a force of 10 N, what will be the static friction? Answer: 10 N. Push with a force of 75 N. What's the friction? 75 N.

Static friction will always be whatever it needs to be to prevent slipping, up to its maximum value.
 
Doc Al said:
Right, since the net force would be zero it would remain at rest.


The point is that the static friction will never be greater than the applied force.

Let's use an example to hammer this home. Assume that the maximum value of static friction is 100 N (thus [itex]\mu N[/itex] = 100 N). If you push with a force of 10 N, what will be the static friction? Answer: 10 N. Push with a force of 75 N. What's the friction? 75 N.

Static friction will always be whatever it needs to be to prevent slipping, up to its maximum value.

OHh, sorry, I was meant to say that as long as the horizontal force is not greater than the maximum value of static friction, not the static friction greater ... okay. I got it. But kinetic friction doesn't have maximum value, right? I think static friction has because when it gets over to the maximum, it will become kinetic friction, and the object starts to move.
 
Right. Static friction is always [itex]\leq \mu_s N[/itex], while kinetic friction always [itex]= \mu_k N[/itex].
 
Well, I have one more problem here about static friction that I just discovered today.
I just posted a new topic. https://www.physicsforums.com/showthread.php?p=1467721#post1467721
Or I just put here. Much easier to see.
I know that static friction happens only when the object doesn't move. And if the maximum static friction=applied horizontal force, the object still remains at rest. I have already posted a topic about friction yesterday. But there is a problem here:
Two crates are stacked on top of each other on a horizontal floor. The coefficient of static friction between Box A and Box B is 0.6. The coefficient of static friction between Box A and the floor is 0.2. A force of 100 N is applied parallel to the floor on Box B. The boxes move together along the floor. What's the ratio of the mass of Box A to Box B.
My teacher said if there is net force 0 acted on an object, the object is either at rest or moves with constant velocity. So she said the force 100 = the static friction for Box B. I have two questions here: If an object is at rest, then zero net force will still make it rest, right? But is there a possibility that it will move with constant velocity according to what my teacher said? Second, if the applied force = static friction, then the object is not supposed to move? Only if the force is larger than the static friction, then it will move?
 
Last edited:
  • #10
I presume the teacher meant that the force of 100N was just enough to overcome static friction between block and floor. Once overcome, static friction is replaced by the lower kinetic friction.
 
  • #11
Doc Al said:
I presume the teacher meant that the force of 100N was just enough to overcome static friction between block and floor. Once overcome, static friction is replaced by the lower kinetic friction.

So when the applied force=static friction, the object may move? If 100 N was just enough to overcome the static friction, it can be equal to the static friction? I mean at least it should be slightly bigger than static friction, maybe as my friend said, we assumed it to be just equal to static friction?
 
  • #12
Yes, the applied force needs to be slightly greater than the maximum static friction, at least until it starts to move. But if the maximum static friction is 100 N, how much applied force do you need to overcome it? 100.1 N? 100.01 N? Just call it 100 N! (But I think you have the right idea.)
 
  • #13
Wish teachers knew that often they word things differently than they really mean to
 

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