PHY 101 1nd Edition Lecture 13 Outline of Last Lecture VI 4 6 Forces of Friction a What causes friction VII Static Friction VIII Kinetic Friction Outline of Current Lecture I Energy II 5 1 Work III Work Done by Force of Kinetic Friction IV 5 2 Kinetic Energy Work Energy Theorem Current Lecture Chapter 5 Energy Work Power Energy o One of the most important concepts in science Various forms o Kinetic o Potential o Chemical o Thermal o Electromagnetic o Nuclear Energy can be transformed from one kind to another o Light bulb electrical to light heat o Electric motor electrical to mechanical o Nuclear power nuclear to electrical o Solar cell light to electrical Conservation of energy total amount of energy in the universe is a constant 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 5 1 Work Work the amount of energy transferred by a force acting through a displacement o If force and displacement are along the same direction o W F x SI Unit Joule J 1J 1 Newton meter o 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 F x Fcos x Can be positive negative or zero You carry a bucket of water walking with a constant velocity horizontally o Fx Fcos 0 o W 0 No work is done Work done by a force on an object can be NEGATIVE o W F x Fcos x o cos 0 if 90 270 o 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 force o W F1cos 1 x W F2cos 2 x 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 Fcos 100 0 866 86 6N o W Fx x 86 6 5 433J What is the work done by the force if it is along the x direction o W F x 100 5 500J Work Done by Force of Kinetic Friction Suppose the coefficient of kinetic friction k 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 force o Fk kFn kmg 0 2 5 9 8 9 8N o Wk Fkcos x 9 8 1 0 5 49J x 5m Mass 5kg F 100N What is the net force work done by all forces o Wf F x 100 5 500J o Wk 49J o 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 hot o Ex Rubbing your hands together in the cold 5 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 energy o Wnet KEf Kei Positive net work means that the object s final kinetic energy Kef is greater than it s initial kinetic energy KEi
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