PHY 101 1nd Edition Lecture 13 Outline of Last Lecture VI. 4.6 Forces of Frictiona. What causes friction?VII. Static FrictionVIII. Kinetic FrictionOutline of Current Lecture I. EnergyII. 5.1 WorkIII. Work Done by Force of Kinetic FrictionIV. 5.2 Kinetic Energy, Work – Energy TheoremCurrent LectureChapter 5: Energy, Work, Power- Energyo One of the most important concepts in science- Various formso Kinetico Potentialo Chemicalo Thermalo Electromagnetico Nuclear- Energy can be transformed from one kind to anothero Light bulb: electrical to light/heato Electric motor: electrical to mechanicalo Nuclear power: nuclear to electricalo Solar cell: light to electrical- Conservation of energy: total amount of energy in the universe is a constantThese 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.5.1 Work- Work: the amount of energy transferred by a force acting through a displacemento If force and displacement are along the same directiono W = FΔx SI Unit: Joule (J) 1J = 1 Newton · metero “work” in physics is very different from its everyday meaning **if the block does NOT move, the force does NO work on the block- What happens if the force is not in the same direction with the displacement?o W = FΔx = FcosΘ(Δx)- Can be positive, negative or zero- You carry a bucket of water, walking with a constant velocity horizontallyo Fx = FcosΘ = 0o W = 0 (No work is done)- Work done by a force on an object can be NEGATIVEo W = FΔx = FcosΘ(Δx)o cosΘ < 0 if 90° < Θ < 270°o ** Θ is the angle between the force and the displacement- If there are multiple forces acting on an object, the total work done is the algebraic sum of the amount of work done by each forceo W = F1cosΘ1(Δx) + W = F2cosΘ2(Δx) +…Example- A force of 100N at 30° is applied to pull a block. The block moves along the x-direction through the displacement of 5 m. what is the work done by this force?o Fx = FcosΘ = (100)(0.866) = 86.6No W = Fx(Δx) = (86.6)(5) = 433J- What is the work done by the force if it is along the x-direction?o W = FΔx = (100)(5) = 500JWork Done by Force of Kinetic Friction- Suppose the coefficient of kinetic friction μk is 0.2- The object moves 5m along the x-direction- Mass m = 5kg- F = 100N- What is the work done by the force of kinetic friction on the block?- Force analysis: magnitude of the friction forceo Fk = μkFn = μkmg = (0.2)(5)(9.8) = 9.8No Wk = FkcosΘΔx = (9.8)(-1.0)(5) = -49J- Δx = 5m- Mass = 5kg- F = 100N- What is the net force work done by all forces?o Wf = FΔx = (100)(5) = 500Jo Wk = -49Jo Wnet = Wf + Wk = 500 – 49 = 451J- From the previous example, we learned that the work done by force of kinetic friction could be negative- It tends to slow down the objects motion (i.e dissipates energy)- Force of kinetic energy is a kind of dissipative force- Where does the energy go?o Transformed into heat, sound, etc.o Ex. Sanding a surface can be very noisy and the surface can get really hoto Ex. Rubbing your hands together in the cold5.2 Kinetic Energy, Work – Energy Theorem- A moving object has a certain amount of “energy” – kinetic energy- The kinetic energy of mass m moving with a speed v is KE = (1/2)mv2- SI Unit: J (same as unit of work)- 1J = 1N·m = 1kg·m2/s2- What is the relation between work and energy?- Work – energy theorem: the net work done on an object is equal to the change in the object’s kinetic energyo Wnet = KEf – Kei - Positive net work means that the object’s final kinetic energy Kef is greater than it’s initial kinetic energy
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