PHY 101 1nd Edition Lecture 12 Outline of Last Lecture I Newton s Third Law II Application of Newton s Laws III Objects in Equilibrium Outline of Current Lecture VI 4 6 Forces of Friction a What causes friction VII Static Friction VIII Kinetic Friction Current Lecture 4 6 Forces of Friction Friction the resistance an object experiences when moving on a surface or through a viscous medium Friction is not necessarily a bad thing o It is important because without friction we would not be able to walk drive slow down a car etc What causes friction o Roughness of contact surface o Less friction on a smooth contact surface o Normal force easier to push forward lighter objects Ex Bicycle brakes vs car brakes Ex Pulling a crate o Before pulling a crate no friction force o Start pulling with small force crate may not move These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute o Magnitude of friction force tension opposite direction o Pull harder the friction force on the crate grows until it reaches the max o Then it will start moving Static Friction The max friction force fs max sFn o s coefficient of static friction o Fn normal force Therefore before an object moves o F Fs o Fs Fs max Fs and Fs max are always opposite in direction Kinetic Friction Once an object starts moving friction force is constant called kinetic friction force o Fk kFn o k coefficient of kinetic friction General k s need greater force to push an object to start moving than to keep it moving Example a block of 2 5kg is at rest on an incline with angle 30 o a what is the magnitude of the friction force Fs acting on the block o Analysis object at rest equilibrium net force zero o Draw coordinate system label forces indicate important angles o How many forces acting on the block Free body diagram o Forces all along the x direction 1st component of gravitational force Fgx mgsin30 mgcos60 Equilibrium condition Fgx Fs 0 Mgsin30 Fs 0 Fs mgsin30 2 5 9 8 0 5 12 3N NOTE Fgx is not a separate force it is the x component of gravitational force o B what is the magnitude of the normal force Fn Use equilibrium condition in the y direction Fn Fgy 0 Fgy mgcos30 mgsin60 Fn mgcos30 2 5 9 8 0 866 21 2N
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