physics--friction

但我不明if a lubricating oil is added between the block and blench, the block still at rest.
1. the friction acting on the block will not reduced when same force is added to them

You are given that the block is still at rest. As such, the applied force must be balanced by the frictional force at that instant. If not, the block would move and which contradicts what is observed.

2. the limiting friction acting on the block is reduced.

Be aware that after a layer of lubricating oil is added between the block and bench, the block is now in contact with a thin film of oil, not with the bench surface. Since the force required to start the block sliding when it is on a layer of liquid is generally easier (i.e. require less force) because of the fluidity of the liquid, the leads to the large reduction of the maximum (or limiting) frictional force.

Therefore, the reduction in limiting friction is not because of decrease in contanct area, but becuase of change of contacting surface substance (from bench to oil). This is actually the phenomenon of lubrication.

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friction is independent of the apparent
area of contact when the normal reaction is constant.

Consider the base areas of your
shoes are reduced to one-tenth, do you think you can stand still on a slope? (The
normal reaction is the same as you weight the same)

limiting friction is proportional to the normal
force for two contact surfaces.

Yes, shown by experiment



但我想問limiting friction 不是depend on the
normal reaction of the 2 object ma??

既然object都沒變，why limiting friction change?

Limiting friction changes as the contact surface is changed. The block is
not in direct contact with the bench (bench I guess?) any more.

for1, 我想問是不是因為same force added on the block, as
the force is smaller than the limiting friction, friction = force added,
therefore friction unchanged??

Yes. The friction does not exceed limiting friction so you can do it by
thinking about the net force is zero.



我很混亂..到底什麼時候limiting friction change??到底什麼時間friction change??

Limiting friction is the critical friction above which the object will
move. It depends on the contact surfaces and normal reaction.



Friction change (increase) gradually
when the pulling force is increasing (but not above the limiting friction), and
at the same time the normal reaction is the same. In this case the block does
not move.



When the block is sliding with
constant velocity, the friction is constant, known as the kinetic friction.

Kinetic friction is almost proportional
to velocity(Stoke’s law). This also applies to fluid friction(air and water).




2009-12-24 23:11:13 補充：
the NATURE of contact surface is changed
to block-oil-bench
but the AREA of contact is the same

since oil(lubricant) particles can flow quite easily
the minimum frictional force needed to move the block is reduced
hence the limiting friction is reduced

2009-12-24 23:59:09 補充：
No. friction (including static friction, limiting friction and kinetic friction) is proportional to the area of contact

2009-12-24 23:59:13 補充：
recall my first answer
(I rephrase it now)
If you wear a pair of smaller shoes, you probably can't stand on a slope, right?
This is because when the area of contact is reduced, the limiting friction is reduced.
As the limiting friction is reduced, it is easier to slide down the slope.

2009-12-24 23:59:16 補充：
(Or, only when objects are in contact with another material, friction exists.
The more it contacts, the larger the friction)