TAILIEUCHUNG - Mechanical Engineers Handbook Episode 2

Tham khảo tài liệu 'mechanical engineers handbook episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 46 Statics Statics A body has improper supports if it will not remain in equilibrium under the action of the loads exerted on it. The body with improper supports will move when the loads are applied. 5. Dry Friction If a body rests on an inclined plane the friction force exerted on it by the surface prevents it from sliding down the incline. The question is what is the steepest incline on which the body can rest A body is placed on a horizontal surface. The body is pushed with a small horizontal force F. If the force F is suf ciently small the body does not move. Figure shows the free-body diagram of the body where the force W is the weight of the body and N is the normal force exerted by the surface. The force F is the horizontal force and Ff is the friction force exerted by the surface. Friction force arises in part from the interactions of the roughness or asperities of the contacting surfaces. The body is in equilibrium and Ff F. Figure The force F is slowly increased. As long as the body remains in equilibrium the friction force Ff must increase correspondingly since it equals the force F . The body slips on the surface. The friction force after reaching the maximum value cannot maintain the body in equilibrium. The force applied to keep the body moving on the surface is smaller than the force required to cause it to slip. The fact that more force is required to start the body sliding on a surface than to keep it sliding is explained in part by the necessity to break the asperities of the contacting surfaces before sliding can begin. The theory of dry friction or Coloumb friction predicts The maximum friction forces that can be exerted by dry contacting surfaces that are stationary relative to each other The friction forces exerted by the surfaces when they are in relative motion or sliding 5. Dry Friction 47 Static Coefficient of Friction The magnitude of the maximum friction force Ff that can be exerted between two plane dry surfaces in contact .

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