B Why does friction decrease with repeated force application?

[rudransh verma]

Suppose you have a heavy body placed on a surface. We apply a horizontal force such that it is on the verge of moving (maximum static friction). We measure this force and note it down. Now we cut the body from the top and remove a thin layer and again do the same thing. We apply a force and measure it. We repeat this process a couple of times. Each time there will be lesser force. So what is going on?

I think in the starting the weight of the whole body is applying a pressure on the molecules that are at the base. They are weakly bonded with the surface and when we try to move the body by applying a force on the top half of the body the bonding between the surface atoms and the base of the body opposes it.
Now as we decrease the weight of the body the actual contact area decrease and so the number of bonding decrease. Now if we try to move the upper half of the body which is attached to the base molecules we can move it with lesser force. This is how friction works.

Any thought on this would be appreciated.

 

[Vanadium 50]

Given that your premise isn't true, I see little need for explanation.

 

[rudransh verma]

Given that your premise isn't true, I see little need for explanation.

Share your thoughts.

 

[phinds]

Share your thoughts.

No, you THINK about what you said in the first paragraph. follow it through step by step and maybe you'll figure out why what you said is a false premise. Asking us to do your thinking for you isn't going to help you.

 

[Arjan82]

mymy guys, a little more helpful would not hurt anyone... If you don't have the knowledge you can think as hard as you like, but it won't help...

The premise isn't true? So you don't measure a lesser force with decreasing vertical force? There is a common model in mechanics which says the maximum static friction force is equal to the normal force times some constant. So this premise is not out of this world crazy. See, I also don't understand what you mean here. Aren't we suppose to help each other on this forum?

But the underlying question is easy to reformulate: is dry friction mostly dependent on molecular bonding forces or not? If you have two very flat and well polished peaces of metal, then you indeed get some bonding if you put them together, depending on the finish and cleanliness of the surfaces (cold welding, it's a problem in space even).

But I think in general it is not the most dominant force. Not all materials bond that easily with each other, certainly not if they are not of the same kind. I think friction is mostly due to the roughness (micro scale possibly) of the surface. Where troughs and crests of the surface just bump into each other. A higher weight will bring a larger part of the surface in close proximity with each other as you always get some micro-scale deformation. That's how I understood things when studying mechanical engineering anyway.

 

[jack action]

Yes, you are partly right.

First, you have to understand that friction can be a very complicated process and can go past the bonding at the molecular level. There could also be mechanical obstructions between peaks and valleys or a partial vacuum created between the two surfaces. Think also about products like scotch tape: When does a force stop being considered friction and begin to be a sticky glue?

But this area idea you are exploring has its merits. With little normal force applied, the two surfaces will tend to rest peaks-on-peaks, thus a smaller contact area compared to the geometric total area of the contact patch. By adding more force, the materials will compress and squish, thus increasing the contact area (more molecule bonding), but also forcing more peaks to go inside valleys and creating more mechanical restrictions.

Figure [5.1.4] illustrates one macroscopic characteristic of friction that is explained by microscopic (small-scale) research. We have noted that friction is proportional to the normal force, but not to the area in contact, a somewhat counterintuitive notion. When two rough surfaces are in contact, the actual contact area is a tiny fraction of the total area since only high spots touch. When a greater normal force is exerted, the actual contact area increases, and it is found that the friction is proportional to this area.

5.1.4: Two rough surfaces in contact have a much smaller area of actual contact than their total area. When there is a greater normal force as a result of a greater applied force, the area of actual contact increases as does friction.​

 

[hutchphd]

the actual contact area decrease and so the number of bonding decrease.

With due respect to all, this is an almost meaningless phrase. What is "actual contact"??
There are many types of "friction" and each is more or less useful as a concept. There is little to be gained by formulating and debating general models. For a specific system a model may be of utility.
Friction is sometimes roughly proportional to the normal force., and if the surfaces move relatively energy often escapes (to auxiliary degrees of freedom). Done.

 

[Vanadium 50]

Share your thoughts.

• I think the quesion as posed is unanswerable, as the premise is not generally true.
• I think you have a history of asking us to do your thinking for you. It's disrespectful.
• I think you learned nothing from your two temporary bans, which is one more than most people get. I for one would not shed any tears if your next ban is permanent. And that's really your loss, isn't it.
• I think your habit of immediately responding to people's messages without thinking about what they say is rude, arrogant and obnoxious. It's also in effective,
• I think you are your own biggest obstacle to learning physics.

That's what I think. And remember, you did ask.

 

[phinds]

• I think the question as posed is unanswerable, as the premise is not generally true.
• I think you have a history of asking us to do your thinking for you. It's disrespectful.
• I think you learned nothing from your two temporary bans, which is one more than most people get. I for one would not shed any tears if your next ban is permanent. And that's really your loss, isn't it.
• I think your habit of immediately responding to people's messages without thinking about what they say is rude, arrogant and obnoxious. It's also in effective,
• I think you are your own biggest obstacle to learning physics.

That's what I think. And remember, you did ask.

 

[PeterO]

Suppose you have a heavy body placed on a surface. We apply a horizontal force such that it is on the verge of moving (maximum static friction). We measure this force and note it down. Now we cut the body from the top and remove a thin layer and again do the same thing. We apply a force and measure it. We repeat this process a couple of times. Each time there will be lesser force. So what is going on?

I think in the starting the weight of the whole body is applying a pressure on the molecules that are at the base. They are weakly bonded with the surface and when we try to move the body by applying a force on the top half of the body the bonding between the surface atoms and the base of the body opposes it.
Now as we decrease the weight of the body the actual contact area decrease and so the number of bonding decrease. Now if we try to move the upper half of the body which is attached to the base molecules we can move it with lesser force. This is how friction works.

Any thought on this would be appreciated.

Perhaps you could consider the following reverse experiment.
Place an empty beaker on a block of wood, placed on a surface, and establish the static friction.
Now progressively add some water to the beaker, and note the increased friction that results. Would your thoughts on what was happening as you pared down the original body be any different to they way you considered the increased friction was occurring?

 

[rudransh verma]

But I think in general it is not the most dominant force. Not all materials bond that easily with each other, certainly not if they are not of the same kind. I think friction is mostly due to the roughness (micro scale possibly) of the surface. Where troughs and crests of the surface just bump into each other. A higher weight will bring a larger part of the surface in close proximity with each other as you always get some micro-scale deformation.

Ok that seems an important point. So instead of bonding generally it’s the amount of actual area that is in contact(the peaks and valleys). I have read the actual contact area is different from the apparent area by a factor of $10^4$.
As we cut down the layers weight decrease and the actual contact decrease decreasing the friction. When applied force on top half part of the body the Interlocking resists the movement of the upper portion.
Materials like sand paper, doormat, tractor tyres on soil.

 

[rudransh verma]

First, you have to understand that friction can be a very complicated process and can go past the bonding at the molecular level. There could also be mechanical obstructions between peaks and valleys or a partial vacuum created between the two surfaces.

I have come to realize things like road and tiled floors and glass seems so smooth yet they are not. They are rough and produce friction. Newton’s first law applies all the time.