3.1: Work and Power Work

• Work is done when a force moves an object over a distance. • Example, you do work when you lift a textbook. • If an object does not move, no work is done. • The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement. Calculating Work

• You can calculate work by multiplying the force exerted on the object times the distance the object moves: • Work = Force X Distance • The joule (J) is the SI unit of work. One joule is the amount of work done when a force of 1 newton moves an object a distance of 1 meter in the direction of the force. •Work = Force x Distance

•The force must be in the direction of the motion, or no work is done. The Statue of Liberty

• The statue of liberty has been holding up her torch for an awfully long time. How much work has she done? • Answer: 0! Although it takes a force to hold the torch against the force of gravity, there is no motion so no work is done. James Prescott Joule

• The unit of work is the Joule • 1 Newton x 1 meter = 1 Newton meter • 1 Newton meter = 1 Joule • 1 Newton=kg*m/s2 • 1 Joule = kg*m2/s2 Written Response #1:

• A high jumper weighs 700 newtons. What work does the jumper perform in jumping over a bar 2.0 meters high? • W = F x d • W = 700 N x 2.0 m = 1400 nm = 1400 Joules Written Response #2

• A force of 200N is required to push a lawn mower. If 4000 J of work is performed on the lawnmower, how far does it move? • W = F x d • d = W / F • D = 4,000 Nm / 200 N = 20 m Power

• Power is the rate (speed) of doing work. • To do work faster, you must use more power. • To increase power, you can do more work in the same time or you can do the same work in less time. Calculating Power

• You can calculate power by dividing the amount of work done by the time needed to do the work: • Power = Work/Time • Power = (Force X Distance) / Time

• The watt (W) is the SI unit of power. • One watt equals one joule per second (1 W = 1 J/s) Written Response #3

• A crane lifts a car into a junk pile in 10 seconds. What is the crane’s power if 120,000 J of work are performed? • Power = Work / Time • Power = 120,000 J / 10 sec = 12,000 J/s = 12,000 Watts Written Response #4

• A 750 N diver does a somersault off a 10m platform. It takes her 1.5 seconds to hit the water. What is her power? • Power = Work / Time • Power = (Force X Distance) / Time • Power = (750 N X 10 m) / (1.5 sec) = (7,500 N m) / (1.5 sec) = 5,000 Watts James Watt and Horsepower

• Another unit of power is the horsepower (hp). • One horsepower equals about 746 watts. • The horsepower was invented by Scottish engine builder James Watt around 200 years ago. • Watt defined one horsepower as the power output of a very strong horse. 3.1 - Work and Power Practicing Work and Power - Handout 3.2: Types of Energy Energy and Work

• Work = a force moves an object through a distance. • Energy: the ability to do work • Energy is transferred by a force moving an object through a distance. • When work is done on an object, energy is transferred to that object. • Measured in joules (J) • Energy has different forms and comes from different sources: sunlight, food, etc. • Many forms of energy can be classified into two general types: kinetic and potential energy. Kinetic Energy

• Kinetic energy: the energy of motion • The kinetic energy of any moving object depends upon its mass and speed. • Kinetic energy (KE) = 1/2mv2 • m = mass (kg) • v = velocity / speed (m/s) • How will changing the mass of an object affect its kinetic energy? How will changing the speed of an object changes its kinetic energy? • Doubling the mass will double the kinetic energy • Doubling the speed will quadruple the kinetic energy Written Response #5

• Scenario: a 0.10-kilogram bird is flying at a constant speed of 8.0 m/s. What is the bird’s kinetic energy? • Given: m = 0.1 kg v = 8.0 m/s • KE = ½mv2 • KE = ½(0.1 kg)(8.0 m/s)2 • KE = 3.2 kg * m2/s2 = 3.2 J Potential Energy

• Potential energy: energy that is stored as a result of position or shape and is converted to kinetic energy • Gravitational potential energy: potential energy that depends upon an object’s height • Elastic potential energy: potential energy of an object that is stretched or compressed • “elastic”: something springs back to its original shape after being stretched or compressed. • Examples: rubber bands, springs Gravitational Potential Energy

• An object’s gravitational potential energy depends on its mass, its height, and the acceleration due to gravity. • Potential energy (PE) = mgh m = mass h = height g = acceleration due to gravity (9.8 m/s2) Kinetic and Potential Energy - Handout Energy and Its Forms

• The major forms of energy are: • Mechanical energy • Thermal energy • Chemical energy • Electrical energy • Electromagnetic energy • Nuclear energy. • Each form can be converted into other forms of energy. Potential and Kinetic Energy

1. What is the potential energy of a rock that weighs 100 Newtons that is sitting on top of a hill 300 meters high? 2. What is the kinetic energy of a bicycle with a mass of 14 kg traveling at a velocity of 3 m/s? 3. A flower pot weighing 3 Newtons is sitting on a windowsill 30 meters from the ground. Is the energy of the flower pot kinetic or potential? How many joules is this? 4. When the flower pot in problem 3 is only 10 meters from the ground, what is its potential energy? 5. A 1,200 kg automobile is traveling at a velocity of 100 m/s. Is it energy kinetic or potential? How much energy does it possess? Types of Energy Poster

• Forms of Energy: • For each form of • Mechanical energy energy: • Thermal energy • Label the form of • Chemical energy energy • Electrical energy • Describe each form of • Electromagnetic energy energy • Nuclear energy • Two examples of this form of energy • Draw a picture showing this form of energy Mechanical Energy

• Mechanical energy: the energy associated with the motion and position of everyday objects • Mechanical energy does not include thermal energy, chemical energy, or other forms of energy Thermal Energy

• Thermal energy: the total potential and kinetic energy of all the microscopic particles in an object. • As an object’s atoms move faster, its thermal energy increases and the object becomes warmer. • Objects that are hot enough can emit light. Written Response #6

• Based on the diagrams below, which beaker of water has more thermal energy? • B - same temperature, more mass

80ºC 80ºC B A 400 mL

200 mL Chemical Energy

• Chemical energy: the energy stored in chemical bonds. • When bonds are broken, the released energy can do work. • All chemical compounds store energy (including fuels). Electrical and Electromagnetic Energy • Electrical energy: the energy associated with electrical charges • They can exert forces that do work • Electromagnetic energy: a form of energy that travels through space in the form of waves (visible light, x-rays) • Electromagnetic waves are often used for communication b/c they travel long distances. Thermal: internal motion of particles

Chemical: Nuclear: bonding of changes in atoms Energy: the the nucleus ability to cause change (measured in joules)

Electrical: Mechanical: motion of motion of electric objects charges 3.3: Energy Conversion Energy Conversion

• Energy can be converted from one form to another. • Energy conversion: the process of changing energy from one form to another Conservation of Energy

• When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur. • The law of conservation of energy states that energy cannot be created or destroyed. Energy Conversions

• One of the most common energy conversions is between potential energy and kinetic energy. • The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. • Conversions between kinetic and potential energy can happen in both directions. Energy Conversion in Pendulums

• A weight swinging back and forth from a rope or string • Example: clocks • Kinetic energy and potential energy undergo constant conversion as a pendulum swings. • At highest point, pendulum is briefly motionless while it changes direction. • The weight of the pendulum at that point has zero kinetic energy and maximum potential energy. Energy Conversion in Pendulums

• Pendulum swings down: potential energy is converted to kinetic energy • Pendulum has maximum kinetic energy and zero potential energy as it reaches the bottom of the swing. (process repeats) Energy Conversion Calculations

• When friction is small enough to be ignored, and no mechanical energy is added to a system, the system’s mechanical energy does not change. • ME = KE + PE • Conservation of Mechanical Energy • (KE+ PE) beginning = (KE + PE) end Section 15.2 Energy and Mass

• Einstein: developed “Theory of Relativity” in 1905 • Einstein’s equation, E = mc2, says that energy and mass are equivalent and can be converted into each other. • E=energy; m=mass; c=speed of light • Energy is released as matter is destroyed, and matter can be created from energy. • However, if was found that mass and energy together are always conserved (law of conservation of energy) Conservation of Energy - Handout

• ME = KE + PE • KE = ½ * m * v2 • m = mass • v = velocity • PE = m * g * h • m = mass • g = gravity (9.8 m/s2) • h = height 3.4: Thermal Energy Work and Heat

• Heat: the transfer of thermal energy from one object to another because of a temperature difference • Heat flows spontaneously from hot objects to cold objects. Temperature

• Measured by a thermometer. • A measure of how hot or cold an object is compared to a reference point • Celsius scale: freezing and boiling points of water • Kelvin scale: absolute zero (temperature of 0 kelvins) • Measured by a thermometer Temperature

• Temperature is related to the average kinetic energy of the particles in an object due to their random motions through space. • Particles move faster as heat increases (average kinetic energy of particles increase; temperature also increases) Temperature

• Question: why does heat flow from high to low temperature? • Typically, high-energy particles lose energy; low-energy particles gain energy in collisions. • Collisions transfer thermal energy from hot to cold objects. Thermal Energy

• Total potential and kinetic energy of all the particles in an object • Thermal energy depends on the mass, temperature, and phase (solid, liquid, or gas) of an object. Thermal Energy

• Thermal energy depends on mass. • More particles = more thermal energy • Higher kinetic energy = more thermal energy (cup of hot tea vs. cup of cold tea) • Lower kinetic energy = more thermal energy (cup of hot tea vs. pitcher of cold lemonade) *lemonade has more particles so it has more thermal energy Thermal Expansion

• The increase in volume of a material due to a temperature increase • Occurs because particles of matter tend to move farther apart as temperature increases. Thermal Expansion

• Gases expand more than liquids • Gas particles have a weaker attraction for each other so they expand more easily. • Liquids usually expand more than solids. Measuring Heat Changes

• Calorimeter: an instrument used to measure changes in thermal energy • Uses the idea that heat flows from a hotter object to a colder object until both reach the same temperature. A Calorimeter Specific Heat

• Some materials absorb heat more readily than others. • Specific heat: the amount of heat needed to raise the temperature of one gram of a material by one degree Celsius Specific Heat Specific Heat

• The lower a material’s specific heat, the more its temperature increases when heat is absorbed. • Heat is measured in joules (J) or calories (cal). • Calorie: the energy needed to raise the temperature of 1g of water by one degree Celsius Specific Heat

• One calorie = 4.184 joules • Calorie for food labels = kilocalorie. • Units for specific heat: J/g * °C • Q = m * c * ∆T Q (heat absorbed by a material) m (mass) c (specific heat) ∆T (change in temperature) Specific Heat - Handout Lab: Thermal Energy Beakers 3.5: Heat and Thermodynamics Heat and Thermodynamics - Conduction • Conduction: the transfer of thermal energy with no overall transfer of matter • Occurs within a material or between materials that are touching. • In conduction, collisions between particles transfer thermal energy, without any overall transfer of matter. • Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often. Heat and Thermodynamics – Thermal Conductors • Thermal conductors: a material that conducts thermal energy well • Do not have to be hot. • Can be cold and transfer thermal energy from warm/hot objects. Heat and Thermodynamics – Thermal Insulators • Thermal Insulators: a material that conducts thermal energy poorly • Example: air (good) Heat and Thermodynamics - Convection • Convection: the transfer of thermal energy when particles of a fluid (air or liquid) move from one place to another. • Convection current: occurs when a fluid circulates in a loop as it alternately heats up and cools down • Examples: how temperature is kept uniform throughout a room that is heated, oven, ocean temperature Heat and Thermodynamics – Convection • Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior. Heat and Thermodynamics - Radiation • Radiation: the transfer of energy by waves moving through space • All objects radiate energy. • As an object’s temperature increases, the rate at which it radiates energy increases. Thermodynamics

• Thermodynamics: the study of conversions between thermal energy and other forms of energy • James Prescott Joule (1818-1889): given credit for discovering the first law of thermodynamics First Law of Thermodynamics

• The first law of thermodynamics states that energy is conserved. • All the energy in a system can be accounted for (regardless of an addition of energy to the system). Second Law of Thermodynamics

• The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. • Example: refrigerator • Heat engine: any device that converts heat into work. • A heat engine converts most of the input energy into useful work. • Waste heat: thermal energy that is not converted into work. • Waste heat is lost to the surrounding environment Third Law of Thermodynamics

• The third law of thermodynamics states that absolute zero cannot be reached. • Example: the efficiency of a heat engine is always less than 100% (due to the cold outside environment not being at absolute zero). Lab: Heat and Thermodynamics 3.6: Using Heat Using Heat

• Heat engine: any device that converts heat into work • The two main types of heat engines are the external combustion engine and the internal combustion engine. External Combustion Engine

• External Combustion Engine: an engine that burns fuel outside the engine • Example: steam engine Internal Combustion Engine

• Internal Combustion Engine: a heat engine in which the fuel burns inside the engine • Example: most cars use internal combustion engines that burn gasoline. • Most use pistons that move up and down inside cylinders. (stroke) • Waste energy is released to the environment. Internal Combustion Engine Heating Systems

• Wood burning fireplaces were the main method used to heat buildings. • Central heating system: a heating system that heats many rooms from one central location • They differ in how they transfer thermal energy to the rest of the building. Heating Systems

• Most heating systems use convection to distribute thermal energy. • Energy sources: electrical energy, natural gas, oil, and coal Heating Systems: Hot-Water Heating • Water is heated by a boiler → carried to radiators in each room → thermal energy transferred by hot water to the radiator by conduction → as pipes heat up, room air is heated by conduction and radiation • Hot air rises (sets up a convection current); cooled water returns to the boiler and the cycle repeats • Temperature is controlled by a thermostat. Heating Systems: Steam Heating

• Very similar to hot-water heating except steam is used instead of hot water. • Transfer of heat still occurs by conduction and radiation. • Often used in older buildings or when many buildings are heated from one central location. Heating Systems: Electric Baseboard Heating • Uses electrical energy to heat a room. • Electrical energy is converted to thermal energy. • Hot coil heats the air near it by conduction and radiation—then convection circulates warm air to heat the room. • Example: space heaters Heating Systems: Forced-Air Heating • Use fans to circulate warm air through ducts to the rooms of a building. • Convection circulates air in each room. • Advantage: cleans air as it passes through filters Cooling Systems

• Most are heat pumps • Refrigerators and air conditioners • Heat pump: a device that reverses the normal flow of thermal energy • Example: circulate a refrigerant through tubing • Refrigerant: a fluid that vaporizes and condenses inside the tubing of a heat pump. • Refrigerant absorbs heat, vaporizes, and turns into a gas • When giving off heat, condenses or turns back into a liquid • Heat pumps must do work on a refrigerant in order to reverse the normal flow of thermal energy. Cooling Systems: Refrigerators

• A heat pump that transfers thermal energy from the cold food compartment to the warm room. • Refrigerator’s motor has to do work to move refrigerant through tubing inside the refrigerator walls Cooling Systems: Air Conditioners

• Actually heats the outdoor air • Hot air comes from inside the house • Compressor raises the temp. and pressure of the refrigerant (hot, high- pressure gas); compressor coil temp. is higher than outside air temp. so heat flows from coil to the outside. Cooling Systems: Air Conditioners

• Refrigerant cools as thermal energy is released and condenses into a liquid → flows through expansion valve and decreases in temperature → while flowing through evaporator coil it absorbs thermal energy from the warm room air → a fan sends cold air back into room 3.7: Machines Mechanical Advantage

• The number of times that the machine increases an input force. • Example: you place a nut in a nutcracker near the hinge. The nutcracker exerts a 7X force greater than what you exert on the nutcracker. • Actual Mechanical Advantage (AMA): the actual forces acting on a machine • Ideal Mechanical Advantage (IMA): the mechanical advantage in the absence of friction • Because friction is always present, the AMA is always less than the IMA Mechanical Advantage - Handout Efficiency

• Efficiency: the percentage of the work input that becomes work output • Efficiency = (Work Output / Work Input) x 100 Written Response #7 - Efficiency

1. A man expends 100 J of work to move a box up an inclined plane. The amount of work produced is 80 J. Calculate efficiency (in percent) 2. A pulley system operates with 40% efficiency. If the work put in is 200 J, how much useful work is produced? 3. A boy pushes a lever down 2 meters with a force of 75 N. The box at the other end with a weight of 50 N moves up 2.5 meters. Calculate efficiency 4. Using a lever, a person applies 60 N of force and moves the lever 1 meter. This moves a 200 N rock at the other end 0.2 meters. Calculate efficiency. Machines

• Types of Simple Machines: • Lever • Wheel and Axle • Inclined Plane • Wedge • Screw • Pulley Levers

• Lever: a rigid bar that is free to move around a fixed point (fulcrum) • Input arm: distance between the input force and the fulcrum • Output arm: distance between the output force and the fulcrum • Ideal Mechanical Advantage (IMA): input arm / output arm Types of Levers

• First-Class: the fulcrum is always located between the input force and the output force • Examples: seesaw, scissors, tongs • IMA can be greater than one, equal to one, or less than one Types of Levers

• Second-Class: the output force is always located between the input force and the fulcrum • Example: wheelbarrow • IMA is always greater than one Types of Levers

• Third-Class: the input force is located between the fulcrum and the output force • Examples: baseball bats, golf clubs, hockey sticks • IMA is always less than one Written Response #8 – Types of Levers • Classify the type of lever being shown in each picture. Wheel and Axle

• Simple machine that consists of two disks or cylinders, each with a different radius • Example: a steering wheel consists of a large wheel attached to a narrow axle • To calculate IMA, divide the radius (or diameter) where the input force is exerted by the radius (or diameter) where the output force is exerted • Can be less than one or greater than one Inclined Planes

• A slanted surface along which a forces moves an object to a different elevation • Example: using a ramp to load a four-wheeler into the bed of a truck, wheelchair ramps in front of buildings • IMA: distance along the inclined plane divided by the change in height Wedges

• Wedge: a v-shaped object whose sides are two inclined planes sloped towards each other • Examples: knife blade, zipper • A thin wedge of a given length has a greater IMA than a thick wedge of the same length Screws

• Screw: inclined plane wrapped around a cylinder • Examples: nuts, bolts • Screws with threads that are closer together have a greater IMA Pulleys • Simple machine that consists of a rope that fits into a groove on a wheel • IMA of a pulley or pulley system is equal to the number of rope sections supporting the load being lifted • Fixed pulleys: a wheel attached in a fixed location. They only rotate in one place. IMA is always one • Example: pulley at the top of a flag pole Pulleys • Moveable pulleys: attached to an object being moved rather than to a fixed location. Moveable pulleys are used to reduce the input force needed to lift a heavy object • Example: sailors use moveable pulleys to pull in sails Pulleys

• Pulley system: combination of fixed and moveable pulleys to achieve a large mechanical advantage Written Response #9 • What types of simple machines are shown in the pictures to the right? Written Response #10 - Mechanical Advantage Mechanical Advantage of Simple Machines - Handout Compound Machines

• Combination of two or more simple machines that operate together • Examples: car, washing machine, clock Lab: Machine Stations