Transcript PartⅣ Friction

Engineering Mechanics
Friction
characteristics [,kærəktə'ristiks]
n. 特性，特征；特色（characteristic的复数）；特质
Friction: Layout





coulomb ['ku:lɒm]
n. 库仑
wedge [wedʒ]
n. 楔子；楔形物；
Types of Friction
Characteristics of Dry Friction
Coulomb’s Friction Model
Friction Angle
Simple Application: Wedge
lubricate ['lubrikeit] adjacent [ə'dʒeisənt] cyclical ['saiklikəl]
vi. 润滑；
adj. 邻近的，毗连的
adj. 周期的，循环的
Types of Friction
Dry Friction
- When the unlubricated surfaces of two solids
are in contact under a condition of sliding or
tendency to slide
• Fluid Friction
- When adjacent layers in a fluid are moving at
different velocities
• Internal Friction
- When solid materials are subjected to cyclical
loading

Characteristics of Dry Friction
Ft: Friction Force
N: Normal Force
critical ['krɪtɪk(ə)l]
adj. 临界的；
impend 英[ɪm'pend]
vi. 即将发生
kinetic [kɪ'netɪk]
adj. [力] 运动的；动力的
coefficient [,kəui'fiʃənt]
n. [数]系数
incline [in'klain] value ['vælju:]
vi. 倾斜；
n. 值；
Friction: Example I

A block of mass m is resting on an
inclined plane. The coefficient of friction
between the block and the plane is µ
=0.25. Determine the maximum value
of the inclining angle θ.
Note: Inclining the plane may
be used to estimate
the coefficient of friction
Friction Force
- Direction
- Magnitude
Coordinate System
- Standard
- Rotated
Friction: Example II

Determine the range of values of force P applied
so that the block of mass m will neither slide
down or move up the inclined plane. Assume that
the coefficient of friction between the block and
the plane is µ =0.25 and the inclining angle of
the plane is θ=45 º.
Friction: Example II
Friction Force: Direction ?
Friction: Example II
square [skwεə] subject ['sʌbdʒikt] tilt [tilt]
adj. 平方的；正方形的； adj. 受制于…的
vi. 倾斜；
Friction: Example III

A square block of mass m resting on the horizontal surface
is subjected to a horizontal force P. The coefficient of
friction between the block and the plane is µ. Prove that
equilibrium is broken by sliding if µ <0.5, and by the block
tilting if µ >0.5.
Friction: Example IV

The three flat blocks are positioned on the 30 º incline and
a force P parallel to the incline is applied to the middle
block. The coefficient of friction for each of the three pairs
of mating surfaces is shown. Determine the maximum
value of P may have before any slipping takes place.
Friction: Example IV
The Angle of Friction
respectively [ri'spektivli]
adv. 分别地；各自地，独自地
Wedge: Example V

The position of the 500kg block is adjusted by the 5 º
wedge under the action of the force P. If the coefficients of
friction for both pairs of wedge surfaces and between the
block and the horizontal plane are 0.3 and 0.6 respectively,
determine the least force P required to move the block.