PHY 317K 1st Edition Lecture 5Outline of Last LectureI. Newton’s LawsII. Lecture QuestionsOutline of Current LectureI. FrictionII. Normal ForceIII. TensionCurrent LectureFriction: - Friction acts between surfaces to oppose their relative motion- The strength of friction depends on the normal force acting perpendicular to them- When surfaces aren’t in motion, the force is static frictiono Fs ≤ usN- For surfaces in relative motion, the force is kinetic frictiono Fk = ukN- Friction can slow down an object but not change its direction- For an object to move, the applied force must be greater than the frictional forceNormal Force:- Perpendicular to the surface- Normal force increases with mass- Constant velocity = same normal force- Acceleration changes normal force (example is an elevator going up or down)- Tension decreases normal forceExample: An elevator is not moving and shows 100 kg on a scale. Then the elevator moves down, what happens to the value that you read from the scale? Acceleration is 0.2g.The value will decrease because:Fnet ≠ 0 but Fnet = Fg + Fn (the force of gravity plus the normal force)-0.2 mg = -mg + Fn0.8 mg = FnFn = 80 kg, therefore the normalforce does 80% of the workTension:- The magnitude of tension is the same and independent of direction on a pulley
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