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Chapter 5 Work, Energy and Power

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Section 5.1 Work Work - The quantity of force times distance, as long as the force is parallel to the direction of motion Example: If you push a box with a force of 1 Newton for a distance of 1 meter, you did exactly 1 Joule of Work.

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Units of work Work is measured in Joules (J) Work = Force X Distance W=F x d Units for Force are Newtons (N) Units for distance are Meters (m) Units for Work are Joules (J) Newtons times Meters equals Joules or N times m = J

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Input and Output Work As an example, consider a block and tackle machine to lift a load of 10 Newtons and suppose you lift the load half of meter. Because W = F x d

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W= 10N x.5m = 5J Work equals 5 Joules

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What about input work? You pulled a string with a force of 5 Newtons because the machine gave you advantage of 2, However, you had to pull the string twice as far as you lifted the object. The work input is the force you apply times the distance you pulled the string. Because W= F x d

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W= 5N x 1m Work equals 5 Joules

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THE WORK OUTPUT OF A SIMPLE MACHINE CAN NEVER EXCEED THE WORK INPUT You can never get more out of a machine than what you put into it.

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Efficiency Efficient machine- all or most of the work input becomes work output. The efficiency of a machine is the ratio of work output to work input. Ex. Block and tackle all 5 joules of input were transformed into 5 joules of output

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Efficiency = Work Output Work Input X100 = % Efficiency A Bicycle is one of the most efficient machines ever made 95% efficient!!!!!

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Friction –the force that opposes motion –In real machines, work output is always less than the work input because of friction.

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Power Power - the rate at which work is done is called POWER Power = work/time Units for power are watts –It makes a difference how fast you do your work! –The faster you work the greater the power

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Units for Power Power is measured in watts Power is work divided by time P= W/t Units for work are Joules Units for time MUST be in seconds

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Ex. 2 people drag an object with a force of 100 Newtons for a distance of 10 meters. 1 person took 10 seconds and the 2nd person took 60 seconds Calculate the Power for each person Work = force x distance Power = work/time

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W= F x d W = 100N x 10m = 1000 joules –Both people did the same amount of work Power = Work/time P = W/t Person # 1 P = 1000j / 10 sec Power equals 100 watts Person #2 P = 1000j / 60 sec Power equals 16.7 watts

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Named after James Watt ( ) Inventor of the steam engine One watt is equal to one joule of work done in one second. Another unit for power is the Horsepower One horsepower is equal to 746 watts

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Section 5. 2 Energy Conservation Energy - is the ability to work. –Any object that has energy has the ability to create force –Ex. Gasoline has energy because it can be burned in an engine to make force toe move a car.

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Units of energy Energy is measured in joules, the same units as work

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Potential Energy - Energy that comes from the position of an object relative to the Earth. Ex. Consider a marble lifted off a table, since earths gravity pulls on the marble, we must apply a force to lift it the marble up. Applying a force over a distance requires work, which gets stored as the potential energy of the marble.

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Pe= mXgXh Pe is the gravitational potential energy (J) m is the mass of the object and is in Kg g is the acceleration due to gravity 9.8 m/s/s h is the height of the object and is in meters

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Sample Problem: You put a 1-kilogram mass that is on the floor, up on a shelf that is 3 meters high. How much Potential Energy (Pe) does this store? –Step #1 You know the mass and height –Step #2 The equation for Pe = mgh –Step #3 Get equation in right format –Step #4 Plug in numbers Pe = (1Kg) x (9.8m/sec/sec) x (3meters) = 29.4J

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Kinetic Energy Kinetic energy is energy in motion Kinetic energy increases with speed Kinetic energy increases with mass Kinetic energy has units of Joules

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Ke = ½ mv 2 Ke is kinetic energy and is measured in J m is the mass of the object in Kg v is the velocity of the object in m/s

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Sample Problem: A 50Kg boy runs at a rate of 5 m/s. How much Kinetic energy does he have? –Step #1 You know the mass and velocity –Step #2 The equation for Ke = ½ mv 2 –Step #4 Plug in the numbers Ke = ½ (50Kg)(5m/sec) 2 = 625 Joules

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Conservation of Energy The law of conservation of energy - Energy can never be created or destroyed, just transformed from one form into another form. When you throw a ball up in the air, its energy transformed from kinetic to potential and back to kinetic energy.

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Section 5.3 Energy Transformations The Conservation of Energy: This is a theory that states that energy is never created nor destroyed, it just transforms from one type of energy to another form. Anything you do involves the transformation of energy For Example: When you exercise you transform chemical energy from your food to Kinetic and Potential Energy

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Energy has the ability to flow between various forms of energy such as from Potential Energy to Kinetic Energy. There are 6 different forms of energy

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Chemical to Potential Energy The energy you use when you exercise comes from food. The chemical potential energy stored in the food that you ate is converted into simple sugars. These sugars are burned as your muscles do work. If you were climbing a hill the chemical potential energy is converted into potential energy at the top of the hill.

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Types of Energy Mechanical Energy - This is energy that is possessed by an object due to its motion or stored up potential energy. Mechanical Energy involves moving parts For Example: A simple machines has mechanical energy. Mechanical energy is the ability to do work with a machine

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Radiant Energy - Also known as Electromagnetic Radiation. Light is a Radiant Energy, as well as Radio waves, microwaves, infrared light, visible light, UV light, X-Rays, Gamma Rays Its the Radiant Energy from the sun that helps keep us warm!

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Electrical Energy - Electricity that we use in our everyday lives. –In a natural gas power plant, energy starts off as chemical energy in which gas is burned to produce heat energy, the heat energy in turn produces high temperature steam, which turns a turbine, which transforms the heat energy into mechanical energy. Finally the turbine turns and generates electricity. Chemical to Heat to Mechanical to Electrical.

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Chemical Energy - Chemical reactions either release or absorb energy. Chemical energy can be stored in batteries and then converted into electrical energy.

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Nuclear Energy - Energy that comes from Fusion and Fission. Nuclear Reactors produce Electricity form Fission Reactions –Reactors use Fission to produce heat and turn turbines, which in return generate electricity

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Thermal Energy - Heat is a form of energy. –Heating contractor measure the amount of heat in BTUs or British Thermal Units. One BTU is the same amount of energy as 1055 Joules

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