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1 7.1 Work and Energy 7.1 Work and energy 2 1 Work a Meaning of work When a force moves something, work is done. In everyday language, work may mean anything that people do. 7.1 Work and energy a meaning of work What is work then? In physics, work is done whenever A transfers energy to B by forcing B to have a displacement. 7.1 Work and energy 3 a meaning of work Work is calculated by: force in the Work = direction of displacement displacement W = F//s F s 7.1 Work and energy 4 a meaning of work 3 things happen when work is done: 1 Application of force 2 Displacement of object 3 Energy transfer 7.1 Work and energy 5 a meaning of work Work Scalar (has magnitude only, no direction) Unit: N m or J 1 J of work is done when a force of 1 N moves a distance of 1 m in the direction of the force. 7.1 Work and energy 6 a 7 meaning of work If there is no motion along the force direction, no work is done on the object even though a force is exerted on it. F E.g.: Carrying a load is tiring, but no work is done on it. Why? There is no displacement (s = 0) along the direction of force. 7.1 Work and energy standing still E b work (non-parallel cases) When force makes with displacement, only component of the force in the direction of displacement counts F s W = Fs cos F sin F F cos this component // forward motion W = F//s = (F cos )s = F (s cos ) = Fs// 7.1 Work and energy 8 W = Fs cos When = 0o, W = Fs When = 90o, W = 0 When = 180o, W = -Fs 7.1 Work and energy 9 3 Energy transferred and work 10 Sign of work: the sign of work does not indicate direction. +ve work: in energy of the object. ve work: in energy of the object. Work done by resistive force e.g. friction Energy is dissipated in the form of heat and sound. 7.1 Work and energy 3 Energy transferred and work Sign of work: e.g. when braking a car, the braking force does ve work on the car. F F KE of car 7.1 Work and energy S 11 2 Mechanical energy 12 Energy exists in many different forms. But we shall only study the different forms of mechanical energy. kinetic energy Gravitational potential energy 7.1 Work and energy Elastic potential energy a Kinetic energy (KE) A moving object has kinetic energy. The faster it moves, the more KE it possesses. kinetic energy chemical energy 7.1 Work and energy 13 b Gravitational potential energy (PE) 14 When you lift a box up, it gains gravitational potential energy (重力勢能). put the box higher 7.1 Work and energy box possesses more PE c Elastic potential energy (彈力勢能) Another form of potential energy When an elastic object is stretched, compressed or bent object gains elastic potential energy 7.1 Work and energy 15 3 Energy transferred and 16 work Work is done whenever energy is transferred from one form to another. energy in one form work done 7.1 Work and energy energy in another form 3 Energy transferred and work energy in one form work done energy in another form potential energy 10 J work done 10 J kinetic energy 10 J Work done = energy transferred 7.1 Work and energy 17 18 Example 1 Work done in lifting a suitcase A man lifts a 10-kg suitcase on his shoulder through a vertical distance of 1.5 m. (a) How much work does he do? What is the energy transfer in this process? (b) Does he do work when he transports the suitcase by carrying it on his shoulder? 7.1 Work and energy 1.5 m 10 kg 19 Example 1 Work done in lifting a suitcase (a) Force = weight of suitcase = 10 10 = 100 N 100 N Work = 100 × 1.5 = 150 J Chemical energy of the man is converted to the potential energy of the suitcase. 7.1 Work and energy 100 N 1.5 m 10 kg 20 Example 1 Work done in lifting a suitcase (b) Force on suitcase is at right angle to the motion. Displacement in the direction of force is zero. work done is zero. 5m 7.1 Work and energy E 21 Example 2 Work done in pulling a suitcase How much work does he do? What is the energy transfer in this process? 20 N 30 10 kg Man's chemical energy suitcase's KE 5m 20 N 30 10 kg Component of force in the direction of motion = (F cos ) s = 20 cos 30 Work = 20 cos 30 5 = 86.6 J 7.1 Work and energy 22 Work done by tension of the string = 0 7.1 Work and energy Q1 Which of the following... 23 Which of the following statements about work is INCORRECT? A Work = applied force in the direction of displacement displacement B Since displacement and force are vectors, work done is also a vector. C Work is done when a force moves something. D Our muscles do work when we walk. 7.1 Work and energy E 24 Q2 A block of weight W... A block of weight W slides down an inclined plane over a distance d. Find work done by gravity on the block. A Work = W d sinθ B Work = W d cosθ C Work = W d d θ 7.1 Work and energy Q3 The following screw... 25 The following screw jack is used to raise heavy objects by turning the handle. 30 cm load 50 kg 7.1 Work and energy handle 26 Q3 The following screw... What is the work done in one complete revolution if the turning force is 20 N? The work done of the turning force = force × displacement = _______ 20 2 ________ 0.3 37.7 = ________________ J 7.1 Work and energy 30 cm 27 The End 7.1 Work and energy 28 Return 7.1 Work and energy