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# Intuition on static and kinetic friction comparisons

Explore the intriguing concept of friction at the atomic level. Understand why static friction can be higher than kinetic friction, and how surface irregularities and temporary chemical bonds play a role. Created by Sal Khan.

## Want to join the conversation?

• In several videos before, you mentioned that a block of ice on ice does not cause a lot of friction. But in the chemistry playlist, water molecules form hydrogen bonds, which take a lot of energy to break. Wouldn't the block of ice slow down even faster?
• So, based on Sal's logic, as the speed of the block got higher and higher, the number of nooks it would get stuck in would approach 0, and the chemical bonds made would get farther between and briefer, to the point where the block passes by the other surfaces' atoms so quickly that there is not enough time for any bonds to be made and held for a significant time. Would that not mean that the force of kinetic friction (1) approaches 0 as the speed approaches infinity, therfore (2) is not constant, and thus (3) should be given for different speeds of an object or in an equation relating the coefficient of friction to the speed of the object?
• Does such a critical point exist where the kinetic friction equals 0 for any materials? If so, then is this the same concept that allows frictionless super-fluids to exist?
• Why do some objects have higher coefficients of friction than others?
• Try touching different surfaces like polished steel, sandpaper, wood etc. You will fell a few of the surfaces are smoother than others. This is because the rougher surfaces have more tiny projection as compared to the smooth surfaces. Therefore, affecting the coeff. of friction.
• Your videos are very interesting but I am unsure on your explanation here.
You state that the theory is it settles and this causes it to take more force to move it. However, settling would be a function of time in one spot (as implied by jumping over ruts comment) so this would seem to imply that the Mk should vary based on how fast an object is moving (less time to settle) which is not part of your equation. Simple experiments also contradict. So, what makes you think of it like this?
Thank
• This is the standard graph of the coefficient of friction vs the force perpendicular to the surfaces that shows up in most elementary physics textbooks.
There aren't just two coefficients of friction, it's a continuum. The two given in tables and problems are just for the two non-transitional states.
• Won't the coefficient of static friction be greater than the coefficient of kinetic friction In all cases, just because additional force is required to override the inertia of rest possessed by the body?
• There is no inertia of rest. An object is only at rest relative to its surroundings when there is no net force, so any unbalanced force will move the object.
(1 vote)
• Is there anything that has 0 friction? If not is it possible? What material do we know of today that has the lowest friction?
• By using materials like lubricants, we can minimise friction to a great extent because they essentially fill up the ruts and grooves on the surface.
As of now, we cannot have zero friction objects under normal conditions on Earth.
Besides, zero friction isn't very ideal for us.
Imagine trying to walk on a surface that has no friction!
(1 vote)
• I have a semi-related question:
Let's say that you have a surface and an object, and you slam down that object with a certain amount of force. Let's also say that theoretically, right before the two surfaces make contact, two electrons from different atoms of different surfaces line up together. So, when the two surfaces "make contact", is it possible for the two electrons to actually touch each other?
• It is possible for electrons to "collide" if they are given enough energy. It is not the case that their opposite charge would have to stop them from colliding, just as magnets can be forced to have their north poles touch one another.

The real problem with thinking about collisions is that electrons aren't really particles. We often think of them as little particles, and sometimes we think of them as waves, depending on what experiment we are trying to do. When you accelerate electrons to speeds that allow them to collide, you can't really describe that collision as if they were two bullets slamming into each other. You have to start to use quantum mechanics to describe what will happen. It gets very weird.
• Would the kinetic friction decrease as the speed of the object increases? The moleculs would have even less time to form bonds and have an easier time bouncing and avoiding those valleys.