4.1 Sliding FrictionLaw of sliding frictionWhen two objects in contact with each other have relative sliding orrelative sliding tendency, the force generated between them that hindersrelative sliding or relative sliding tendency is called sliding friction. If thereis only sliding trend but no sliding, the friction force is called static slidingfriction force. If there is relative sliding friction, it is called dynamic slidingfriction.一、Law of static sliding frictionPIP21. When T = O , P and N constitute a7777777Nbalancedforcesystem withoutfrictionThat's F = O, as shown in the figure
Law of sliding friction When two objects in contact with each other have relative sliding or relative sliding tendency, the force generated between them that hinders relative sliding or relative sliding tendency is called sliding friction. If there is only sliding trend but no sliding, the friction force is called static sliding friction force. If there is relative sliding friction, it is called dynamic sliding friction. 一、Law of static sliding friction T P P N 4.1 Sliding Friction 1. When , and constitute a balanced force system without friction. That’s , as shown in the figure. T = 0 P N F = 0
4.1 $Sliding FrictionIP2. Whenthe forceT is small,the objecttends tomovetoTthe rightand is stillin balance,as shownin thefigure.tFZX=O:F=TN3. The object is in critical equilibrium. When the forceTincreases to a certain value, the object is in a criticalP+Tequilibrium state, as shown in the figure. At this time, thevalueofFcan still be determined by the equilibriumequation:FZX=O:F=TNmAt thistime,thefrictionforcereachesthemaximum value,whichis calledthemaximum staticfrictionforce,whichisexpressedby F
P N T F X = 0: F = T 2. When the force is small, the object tends to move to the right and is still in balance, as shown in the figure. T P N T Fm 3. The object is in critical equilibrium. When the force increases to a certain value, the object is in a critical equilibrium state, as shown in the figure. At this time, the value of can still be determined by the equilibrium equation: T F X = 0: F = T At this time, the friction force reaches the maximum value, which is called the maximum static friction force, which is expressed by Fm 4.1 Sliding Friction
4.1Sliding FrictionConclusion: The static sliding friction force changes with the change of thedrivingforce,and its magnitudeis determined by the equilibriumequationandits valueis between zero and the maximum static frictionforce,namelyO≤F≤FmExperiments showthat themaximum staticslidingfrictionforceisproportional to the normalreactionforce between two objects, namely:Fm= fNmThis is thelaw of static slidingfriction.In which f is called staticslidingfrictioncoefficient
Experiments show that the maximum static sliding friction force is proportional to the normal reaction force between two objects, namely: Fm = fN This is the law of static sliding friction. In which is called static sliding friction coefficient. f Conclusion: The static sliding friction force changes with the change of the driving force, and its magnitude is determined by the equilibrium equation, and its value is between zero and the maximum static friction force, namely: 0 F Fm 4.1 Sliding Friction
4.1 Sliding Friction二. Law of dynamic sliding frictionpWhen the force T exceeds Fm , the object-Tbegins to slide, and the friction force is thedynamic sliding friction force, which isFexpressed by F' . As shown in the figure,FNis a fixed value.Experiments show that the friction force of dynamic sliding is directlyproportional to the normal reaction force between two objects, namely:F'=fNThis is the law of dynamic sliding friction. f'is called dynamicslidingfrictioncoefficient
二. Law of dynamic sliding friction When the force exceeds , the object begins to slide, and the friction force is the dynamic sliding friction force, which is expressed by . As shown in the figure, is a fixed value. T Fm F F P N T F Experiments show that the friction force of dynamic sliding is directly proportional to the normal reactionforce between two objects, namely: F = f N This is the law of dynamic sliding friction. is called dynamic sliding friction coefficient. f 4.1 Sliding Friction
4.1Sliding FrictionCriticalstateFStatic stateMotionstateFmaxFdFP0-F=T<FStatic statemMotion state F = F'Critical state - F = F m = f Fn0<F<FThe value of static friction in general:max
Fmax Fd Motion state Static state Critical state F O FP Static state Critical state — F＝F m = f FN Motion state — F＝F’ — F＝T＜F m The value of static friction in general： 0 ≤ F≤Fmax 4.1 Sliding Friction