FMµs NFKµK NApplication of Newton's LawsTYPES OF FORCERev. 0Page 19CP-04Figure 10 Centripetal ForceExperimental evidence shows that the maximum value F of the static-friction force isMproportional to the normal component N of the reaction of the surface, as shown in Equation4-5.(4-5)The term µ is a constant called the coefficient of static friction. Similarly, the magnitude F ofsKthe kinetic-friction force may be expressed in the following form.(4-6)The term µ is a constant called the coefficient of kinetic friction. The coefficients of friction,Kµ and µ , do not depend upon the area of the surfaces in contact. Both coefficients, however,S Kdepend strongly on the nature of the surfaces in contact. Since they also depend upon the exactcondition of the surfaces, their value is seldom known with an accuracy greater than 5 percent.It should be noted that frictional forces are always opposite in direction to the motion (orimpending motion) of the object.CentripetalForceAn object moving at constant speed in a circleis not in equilibrium. Although the magnitudeof the linear velocity is not changing, thedirection of velocity is continually changing.Since a change in direction requiresacceleration, an object moving in a circularpath has a constant acceleration towards thecenter of the circular path.Recalling Newton's second law of motion, F =ma, a force is required to cause acceleration.Therefore, to have constant acceleration towardsthe center of the circular path, there must be a netforce acting towards the center. This force isknown as centripetal force. Without this force, anobject will move in a straight line. Figure 10illustrates the centripetal force.
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