Engineering Mechanics

Friction

Friction: Layout

Types of Friction Characteristics of Dry Friction Coulomb’s Friction Model Friction Angle Simple Application: Wedge

characteristics [,kær kt 'ristiks] ə ən. 特性，特征；特色（ characteristic 的复数）；特质

coulomb ['ku:l m] ɒn. 库仑

wedge [wed ] ʒn. 楔子；楔形物；

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

lubricate ['lubrikeit] vi. 润滑；

adjacent [ 'd eis nt] ə ʒ əadj. 邻近的，毗连的

cyclical ['saiklik l] əadj. 周期的，循环的

Characteristics of Dry Friction

Ft: Friction Force N: Normal Force

critical ['kr t k( )l]ɪ ɪ əadj. 临界的；

impend 英 [ m'pend] ɪvi. 即将发生

kinetic [k 'net k]ɪ ɪadj. [ 力 ] 运动的；动力的

coefficient [,k ui'fi nt] ə ʃən. [ 数 ] 系数

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

incline [in'klain] vi. 倾斜；

value ['vælju:] n. 值；

Friction Force- Direction

- MagnitudeCoordinate 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

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.

square [skwε ] əadj. 平方的；正方形的；

subject ['s bd ikt] ʌ ʒadj. 受制于…的

tilt [tilt] vi. 倾斜；

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

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.

respectively [ri'spektivli] adv. 分别地；各自地，独自地