Transcript Work done
Work Work,W is defined as the product of the applied force,F and displacement of an object in the direction of the applied force,s. w = Fs • S.I unit - Joule Can you define 1 Joule? • 1 Joule is the work done when a force of 1N moves an object through a distance of 1 m in the direction of force. • Sometime the object does not move in the direction of the applied force. F F F Kos s • Since the object is displaced horizontally, the horizontal component of the force is used to calculate the work done by the force. Work done, = ( F cos ) x s Question 1 1. A gardener push a lawn-mover along a horizontal surface with a force of 40 N through a distance 10 m. What is the work done in pushing the lawn mover? Questions • How much work is done if a force of 12N moves an object a distance of 5m? • If you use a 40N force to lift a bag and do 20J of work, how far do you lift it? • A 60 kg of boy pushed a wall with 200N force. How much work has he done? Work is not always done by a applied force! • A student carrying his bag while waiting at the bus stop. Since there is no motion in the direction of the force applied by the student, there will be no displacement in the direction of a force. No work is done by the student on his bag. Work is not always done by a applied • A waiter is carrying a tray of food and walking The applied force act vertically upwards but the displacement of the tray is in a horizontal direction. Since there is no displacement in the direction of the applied force, no work is done on the tray of food. Work is not always done by a applied • A spaceship is moving through space with its engine switched off. There is no work is done if an object moves with uniform velocity without the action of any force. There is no force acting on the spaceship in the forward or backward direction. Therefore, no work is done on the spaceship. Energy transfer when work is done. • • • • Energy – capacity to do work. An object that do work has energy. Energy exist in different forms. Can you name the different forms of energy? Gravitational potential energy Elastic potential energy Kinetic energy Sound energy Forms of energy Electrical energy Light energy Chemical energy Heat energy The work done and the Change in Kinetic Energy • Kinetic energy is the energy of an object due to its motion. Ek = ½ mv2 m = mass of an object v = velocity Work done and Gravitational Potential Energy • Gravitational potential Energy is the energy of an object due to its higher position in the gravitational field. Ep = mgh m = mass g = gravitational field strength h = height Principle of Conservation of energy. • Energy can be changed from one from to another, but it cannot be created or destroyed. • The total energy in the universe is constant. • When the baby falls, the gravitational potential energy changes to kinetic energy. mgh = ½ mv2 Power • When a force is applied on an object and there is motion in the direction of the force, work is said to be done by the force. • Power is defined as the rate at which work is done, or the amount of work done per second. Power = Work done Time taken P =W t • SI unit – Watt (w) Can you define 1 Watt? 1 Watt is a power generated when 1 Joule of work is done in 1 second. Question 1 • In the snatch event of a weightlifting competition, a weightlifter lifts 140 kg from the floor to a height of 1.2 m above the floor in one complete movement in a time of 0.8 s. What is the power generated by the weightlifter during this time? • 2100 W Question 2. • A 50 kg farmer climbed a 8 m tall coconut tree in 5 minute. How much of power has he generated? Work = F x s = 50kg ( 10 ms-1) x 8m = 4000 J Power = Work done Time taken = 4000 J (5 x 60)s = 13.33 Watt Question 2 • The figure shows an electric motor lifting a box of mass 5 kg. The motor takes 4 s to lift the-2box to a height of 0.8 m. What is the power of motor? [Assume g=10 ms ]. • Ans 10W motor load 0.8 m Efficiency Thermal and sound energy lost to the surrounding Chemical energy in petrol Kinetic energy of the car • The engine of a vehicle is unable to change all the chemical energy in the petrol to become the kinetic energy of the vehicle. • Other forms of energy such as thermal energy and sound energy are also obtained from the operation of the engine. Thermal and sound energy lost to the surrounding Chemical energy in petrol • Energy input, Einput= chemical energy in petrol • Unwanted energy = thermal and sound energy • Useful energy, Eoutput = kinetic energy Efficiency = Useful energy output x 100% Energy input Kinetic energy of the car 700 J of thermal and sound energy 1500 J of chemical energy in petrol • Calculate the efficiency of the car = Useful energy output x 100% Energy input = 350 x 100% 1050 = 33.33% 350 J of kinetic energy Question 1 • An electric motor of a crane can lift a 120 kg weight to a height of 4m in 8s. During this time the motor is supplied with 12kJ of electrical energy. Calculate a) the useful power output of the motor b) the efficiency of the motor. • a) 588 W b) 39.2 % The importance of Maximizing the Efficiency of Devices. • Consider two light bulbs: a bulb with a filament and a bulb with a fluorescent tube: Light energy =36 J Light energy =18 J Heat energy =24 J Heat energy =42 J Input energy = 60 J Input energy = 60 J Light energy =36 J Light energy =18 J Heat energy =24 J Heat energy =42 J Input energy = 60 J Input energy = 60 J • The fluorescent bulb converts a higher percentage of the input energy to the useful form of energy. It is said to have a higher efficiency than the filament bulb. When a device is operating at the maximum possible efficiency: • Less input is required to produce the same useful output energy. • The cost of operating the device is reduced • The unwanted output energy is reduced. • The energy resources in this world can be used over a longer period of time. Question • Figure shows an incomplete Sankey diagram for an electric motor that lifts a load. Energy lost to: (i) ___________ Input energy: 16 kJ electrical energy Energy lost as: Useful output energy: 12 kJ of work done to lift a load. (ii) __________ a) Complete figure above b) Calculate the efficiency of the motor. • Explain why the efficiency of devices is always lower than 100% • Give two reason why the efficiency of machines should be maximised.