Friction Questions: Why does it increase and why does it lag?

[Farzan]

I understand that physics is mostly based on empirical evidence, but sometimes it's interesting to know how some things are explained.

My questions about friction are:

1. Why does friction increase as velocity increases?

2. Why does it take time for friction to become equal to the force?

For example, let's discuss a block that is accelerating. If I start pulling it with 10 N of force, eventually friction will oppose it with 10 N in the opposite direction, but not instantly. Of course, I don't think objects could move if friction was always equal to force so there must be an explanation for why there is a delay. Also, is there an equation to describe the relationship between friction and time?

 

[Doc Al]

For example, let's discuss a block that is accelerating. If I start pulling it with 10 N of force, eventually friction will oppose it with 10 N in the opposite direction, but not instantly.

Can you be more specific? I assume you are talking about air resistance, not the sliding of a block along a surface. Right?

 

[stewartcs]

Give this a read...

http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#ass

It should help explain some of the assumptions on the standard model of friction.

CS

 

[pixel01]

I understand that physics is mostly based on empirical evidence, but sometimes it's interesting to know how some things are explained.

My questions about friction are:

1. Why does friction increase as velocity increases?

2. Why does it take time for friction to become equal to the force?

For example, let's discuss a block that is accelerating. If I start pulling it with 10 N of force, eventually friction will oppose it with 10 N in the opposite direction, but not instantly. Of course, I don't think objects could move if friction was always equal to force so there must be an explanation for why there is a delay. Also, is there an equation to describe the relationship between friction and time?

1) Friction force does not depend on velocity. But in fact, when you increase the velocity to a certain extent, the heat released is increased, the temperature of the surfaces increases then does the friction force.

2) There are two types of friction coefficients: static and dynamic. When you start to pull a mass on a surface, it starts with statics friction force, then dynamic. Transition always takes time.

 

[stewartcs]

Just a quick clarification, Surface Friction does not depend on velocity in the standard model, however, Fluid Friction does depend on velocity.

http://hyperphysics.phy-astr.gsu.edu/hbase/airfri.html#c1

Of course friction is a very complicated phenomenon and thus is difficult to model. Hence, the standard model is used most often especially in physics textbooks.

 

[Shooting Star]

The OP probably wanted to know why an object reaches terminal velocity when it moves through air and such, but why does it take a certain amount of time to reach it.

And for the OP, yes, there is (or rather are) relationship(s) between friction and time, depending upon the circumstances. They are in general not very simple.

 

[pixel01]

The OP probably wanted to know why an object reaches terminal velocity when it moves through air and such, but why does it take a certain amount of time to reach it.

And for the OP, yes, there is (or rather are) relationship(s) between friction and time, depending upon the circumstances. They are in general not very simple.

I see it now.
Then the 1st question can be explained like this: When an object is moving in a media (liquid or air), the drag increases as the velocity increases because at least 2 reasons: 1) the higher the velocity, the more chance of turbulences occur. 2) At higher velocity, the object hits the media molecules harder and more frequently, so it loses its momentum , or velocity more. If it is kept at contant velocity, dragging (lagging) force must be higher.

The 2nd question: Because it takes time for the object to reach the velocity at which the dragging force equals to the force exerts on the object to move (gravity for example)

 

[russ_watters]

Dunno, to me it looked like the OP is talking about a block sliding along a surface with plain ordinary dynamic friction. If that is the case, the premises for both of the questions are wrong.

 

[Farzan]

Alright, thanks for the responses.

So dynamic (sliding) friction stays constant?

 

[pixel01]

Alright, thanks for the responses.

So dynamic (sliding) friction stays constant?

Yes, unless the high velocity increases the temperature, hence changes the surfaces' characteritics.

 

[Shooting Star]

Alright, thanks for the responses.

So dynamic (sliding) friction stays constant?

You really should read again the two links provided above by stewartcs.

 

[TVP45]

Farzan,
Have you by chance been reading advanced textbooks, that is, beyond the General Physics where your post is? Although there is generally a small time dependence of static friction and a small velocity dependence of kinetic friction, both those effects are not seen in most ordinary applications and are completely absent from the standard model of friction. If you are in a General Physics class, you should use the standard model.

 

[rcgldr]

So dynamic (sliding) friction stays constant?

In the idealized "standard model". In real life, it usually decreases as velocity increases, sort of like skis skimming along the surface of water, or "planing", a similar thing happens with dynamic friction if the velocity is high enough (ignoring the drag effect of air here).

In the case of tires, which are flexible, an increase in normal force reduces the coefficient of friction. This is called load sensitivity.

http://en.wikipedia.org/wiki/Tire_load_sensitivity

Then there is some complicated effect with static friction that is sensitive to smaller sizes. Take a look at the second half of video #2 at this web site:

http://www.gyroscopes.org/1974lecture.asp

 

[stewartcs]

In the idealized "standard model". In real life, it usually decreases as velocity increases, sort of like skis skimming along the surface of water, or "planing", a similar thing happens with dynamic friction if the velocity is high enough (ignoring the drag effect of air here).

In the case of tires, which are flexible, an increase in normal force reduces the coefficient of friction. This is called load sensitivity.

http://en.wikipedia.org/wiki/Tire_load_sensitivity

Perhaps they are referring to the http://hyperphysics.phy-astr.gsu.edu/hbase/frict3.html#nor".

Of course rolling friction isn't quite the same as sliding friction though.

CS

 

[rcgldr]

http://en.wikipedia.org/wiki/Tire_load_sensitivity

Perhaps they are referring to the effective coefficient of friciton.

No, just the coefficient of static friction versus normal force. As posted by the wiki article, the maximum horizontal force from friction is equal to the normal force^0.7->0.9 as opposed to the normal force^1.0. Racing cars take advantage of this not quite linear relationship to adjust the oversteer / understeer tendency of a car with anti-roll bar changes. The relatively stiffer end (front or back) of a car ends up with less cornering grip.