Work l Work W is defined as the
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Work l. 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 1 N 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 12 N moves an object a distance of 5 m? • If you use a 40 N force to lift a bag and do 20 J of work, how far do you lift it? • A 60 kg of boy pushed a wall with 200 N 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 mv 2 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 mv 2
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 = 50 kg ( 10 ms-1) x 8 m = 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 box to a height of 0. 8 m. What is -2 the power of motor? [Assume g=10 ms ]. • Ans 10 W 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 4 m in 8 s. During this time the motor is supplied with 12 k. J 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
Light energy =36 J Light energy =18 J Heat energy =24 J Heat energy =42 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 k. J electrical energy Energy lost as: Useful output energy: 12 k. J 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.
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- In physics, work is defined as
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