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PH-211
Andres La Rosa
CHAPTER 6: Friction Force
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View of the contact surface at a
microscopic scale
Surfaces are rough at
microscopic levels
Here we describe an experimental fact:
Stationary surface
Then, it turns out, to keep the mass M moving at constant
velocity, we need to apply a force
Fk somewhat smaller than Fs, max
Interpretation of the experimental fact described above:
Stationary surface
Static
friction
forces
Then, it turns out, to keep the mass M moving at constant
velocity moving at constant, we
need to apply a force Fk
somewhat lower than Fs, max
Example
2N
N'
If m = 1 Kg then,
Exercise
N"
m = 1 Kg
mg
N'
Quantifying the friction force
STATIC CASE
KINETIC CASE
Exercise
μs = 0.15
CH-6, Problem #25
What is the maximum
weight of block-A, for
which the system will
remain stationary?
B
A
No motion along the vertical direction
implies,
Maximum static friction force
No horizontal motion implies
Warm up,
for problems involving inclined planes
1)
2)
Resolving the
force mg into
components
parallel and
perpendicular
to the ramp
3)
4)
5)
Ff
Ff
Ch-6 Problem 79. Block A in Fig. 6-56 has mass mA
block B has mass mB
4.0 kg, and
2.0 kg. The coefficient of kinetic friction
between block B and the horizontal plane is Pk
plane is frictionless and at angle T
0.50. The inclined
30°. The pulley serves only to
change the direction of the cord connecting the blocks. The cord has
negligible mass. Find (a) the tension in the cord and (b) the magnitude of
the acceleration of the blocks.
Problem
#79
General rules for dry sliding friction
1. The friction force is proportional to the normal force at
the surface: f = μs N
f1
f2
N1
N2
This rule is true only if the objects in contact do not
deform appreciable.
2. Usually μs > μk
v v== 00
0.1 < μ <1 for many pair
of materials
v0
However:
μ depends very much on
surface conditions (cleanliness)
v
3. For a given object, the friction force is independent
of the apparent area in contact with the supporting
surface
In both cases the same
force F is needed to drag
the block.
Increasing the load
increases the normal force N, and
increases the actual contact area
fs, max α actual contact area
Actual contact area α
Hence
fs, max α N
More quantitatively,
fs, max = μ N
Key aspects in friction:
- The actual contact area is proportional
to the normal force
- When two objects are forced together,
the high peaks of their surface will crumble,
bringing more and more area into intimate
contact.
N
Metal-metal friction