Work and Energy: Forms and Changes

What is Work? Remember that a force is a push or a pull. Work requires both force and motion Remember that a force is a push or a pull. Work requires both force and motion – is force applied through a distance –Work is force applied through a distance –If you push against the desk and nothing moves, then you haven't done any work

Work There are two conditions that have to be satisfied for work to be done on an object There are two conditions that have to be satisfied for work to be done on an object –1. The applied force must make the object move –2. The movement must be in the same direction as the applied force Work requires both force AND motion Work requires both force AND motion

Calculating Work The amount of work done depends on the amount of force exerted and the distance over which the force is applied The amount of work done depends on the amount of force exerted and the distance over which the force is applied Work (N-m or Joule) = F (N) x d (m) where d is distance moved in the direction of the force Work (N-m or Joule) = F (N) x d (m) where d is distance moved in the direction of the force One newton-meter is equal to one joule so the unit of work is a joule One newton-meter is equal to one joule so the unit of work is a joule

Weight is a Force! Remember that weight is a force caused by your mass and gravity Remember that weight is a force caused by your mass and gravity –F gravity = mg –To lift something you have to exert a force to overcome this force of gravity, so you are doing work on that object

When is Work Done? Give a book a push and it slides along a table for a distance of 1 m before it stops Give a book a push and it slides along a table for a distance of 1 m before it stops –You did work on the book only while your hand was in contact with it Lift books and your arms apply a force upward to move them, and because the force and distance are in the same direction, your arms have done work on the books Lift books and your arms apply a force upward to move them, and because the force and distance are in the same direction, your arms have done work on the books Carry books while walking, and your arms are not doing work. Why not? Carry books while walking, and your arms are not doing work. Why not?

1 m 60 N W = 60 N x 1 m = 60 J (N-m) W = 20 N x 3 m = 60 J (N-m) 3 m 20 N Examples of Work

Power Running up stairs is harder than walking up stairs and lifting books quickly is harder than slowly Running up stairs is harder than walking up stairs and lifting books quickly is harder than slowly –Why? They both do the same amount of work –Running does the same work more quickly Power is the rate at which work is done and energy is converted Power is the rate at which work is done and energy is converted Power (J/sec or Watt)= Work (J) Time (sec) Power (J/sec or Watt)= Work (J) Time (sec) The unit of power is Joules/sec, called a Watt. The unit of power is Joules/sec, called a Watt.

Check for Understanding What’s work? A scientist delivers a speech to an audience of his peers A scientist delivers a speech to an audience of his peers A body builder lifts 350 pounds above his head A body builder lifts 350 pounds above his head A mother carries her baby from room to room A mother carries her baby from room to room A father pushes a baby in a carriage A father pushes a baby in a carriage A woman carries a 20 kg grocery bag to her car? A woman carries a 20 kg grocery bag to her car?

Check for Understanding What’s work? A scientist delivers a speech to an audience of his peers No A scientist delivers a speech to an audience of his peers No A body builder lifts 350 pounds above his head Yes A body builder lifts 350 pounds above his head Yes A mother carries her baby from room to room No A mother carries her baby from room to room No A father pushes a baby in a carriage Yes A father pushes a baby in a carriage Yes A woman carries a 20 kg grocery bag to her car? No A woman carries a 20 kg grocery bag to her car? No THE FORCE AND THE MOVEMENT MUST THE FORCE AND THE MOVEMENT MUST BE IN THE SAME DIRECTION TO BE WORK! BE IN THE SAME DIRECTION TO BE WORK! Force Distance moved

Check for Understanding How much work does it take to lift a 200 N weight 2 m off the floor? How much work does it take to lift a 200 N weight 2 m off the floor? How much work does it take to hold a 200 N weight 2 m off the floor? How much work does it take to hold a 200 N weight 2 m off the floor? How much work is done if you drop a 2.5 N book 3 meters? What does the work? How much work is done if you drop a 2.5 N book 3 meters? What does the work?

Check for Understanding How much work does it take to lift a 200 N weight 2 m off the floor? 400 J How much work does it take to lift a 200 N weight 2 m off the floor? 400 J How much work does it take to hold a 200 N weight 2 m off the floor? 0 J How much work does it take to hold a 200 N weight 2 m off the floor? 0 J How much work is done if you drop a 2.5 N book 3 meters? 7.5 J What does the work? Gravity! How much work is done if you drop a 2.5 N book 3 meters? 7.5 J What does the work? Gravity! Eureka! Work Eureka! Work Eureka! Work Eureka! Work

Check for Understanding 1. Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately.60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your answers.

W = F x d but we need to find the gravitational force (weight) of the barbells F g = m x g F g = 50kg x 9.8 N/kg = 500 N Both use same force to lift the same barbell Now calculate the work done by each: W = 500N X 6m (total d) Both use same work yet, Ben is the most powerful since he does the same work in less time P = W/d Ben 500J/10sec = 50 watts Bonnie 500J/60sec 8.3 watts Check for Understanding

History of Work Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand Using an animal could help, but what they really needed were some clever ways to either make work easier or faster Using an animal could help, but what they really needed were some clever ways to either make work easier or faster

Simple Machines Ancient people invented simple machines that would help them overcome resistive forces Ancient people invented simple machines that would help them overcome resistive forces A is a machine that does work with only one movement of the machine A simple machine is a machine that does work with only one movement of the machine –Some machines, such as bicycles, increase speed –Some machines, such as an axe, change the direction of force – Some machines, such as a car jack, increase force

Simple Machines Examples of simple machines Examples of simple machines –Inclined Plane –Levers –Wheel and Axle –Wedge and Screw –Gears –Pulley

Inclined Plane A flat, slanted surface A flat, slanted surface Eureka! Inclined Plane Eureka! Inclined Plane

Lever Two parts: Two parts: Fulcrum Fulcrum Bar Bar Eureka! Levers Eureka! Levers Eureka! Levers Eureka! Levers

Wheel and Axle Two parts: Two parts:  wheel  bar

Wedges and Screws Change downward force into sideways force Change downward force into sideways force Eureka! Screw and Wheel Eureka! Screw and Wheel Eureka! Screw and Wheel Eureka! Screw and Wheel

Gears Wheels with teeth Wheels with teeth

Pulley Two kinds: Two kinds:FixedMoveable

Compound Machines A compound machine is one made up of two or more simple machines. A compound machine is one made up of two or more simple machines.

Efficiency - a measure of how much of the work put into a machine is changed into useful output work Efficiency - a measure of how much of the work put into a machine is changed into useful output work –Every machine is less than 100% effective –Not 100% of the work done is useful work, because some gets turned into other forms, like heat –Machines can be made more efficient by reducing friction with a lubricant, such as oil or grease, which is added to surfaces that rub together

Mechanical Advantage Two forces are involved when a machine is used to do work Two forces are involved when a machine is used to do work –One force is applied to the machine and that is the input force –The force applied by the machine is called the output force Mechanical advantage of a machine is the ratio of the output force to the input force Mechanical advantage of a machine is the ratio of the output force to the input force

Mechanical Advantage Window blinds are a machine that changes force Window blinds are a machine that changes force –A downward pull on the cord is changed to an upward force on the blinds –The input and output forces are equal, so the MA is 1 –Eureka! Mechanical Advantage Eureka! Mechanical AdvantageEureka! Mechanical Advantage

Energy Energy is all around you: Energy is all around you: –Light –Heat –Wind You use energy when you: You use energy when you: –hit a softball –lift your book bag –digest food Every change that occurs— large or small—involves energy Every change that occurs— large or small—involves energy

Changes Require Energy When something is able to change its environment or itself, it has energy When something is able to change its environment or itself, it has energy –Anything that causes change must have energy –You use energy to arrange your hair to look the way you want it to –You also use energy when you walk down the halls of your school between classes or eat your lunch

Nature of Energy What is energy that it can be involved in so many different activities? What is energy that it can be involved in so many different activities? –Energy- the ability to do work –If an object or organism does work the object or organism uses energy –Whenever you do work you transfer energy from one thing to another

Nature of Energy Because of the direct connection between energy and work, energy is measured in the same unit as work: joules (J) Because of the direct connection between energy and work, energy is measured in the same unit as work: joules (J) In addition to using energy to do work, objects gain energy because work is being done on them In addition to using energy to do work, objects gain energy because work is being done on them

Forms of Energy The five main forms of energy are: The five main forms of energy are: –Thermal (heat) –Chemical –Electromagnetic –Nuclear –Mechanical If you have $100, you could store it in a variety of forms—cash in your wallet, a bank account, or coins If you have $100, you could store it in a variety of forms—cash in your wallet, a bank account, or coins Regardless of its form, money is money, and the same goes for energy in that these are only different forms of the same thing Regardless of its form, money is money, and the same goes for energy in that these are only different forms of the same thing

States of Energy The most common energy conversion is the conversion between potential and kinetic energy The most common energy conversion is the conversion between potential and kinetic energy All forms of energy can be in either of two states: All forms of energy can be in either of two states: –Potential –Kinetic

Kinetic Energy Kinetic energy- the energy of motion Kinetic energy- the energy of motion Depends on both mass and velocity Depends on both mass and velocity –The faster an object moves, the more kinetic energy it has –The greater the mass of a moving object, the more kinetic energy it has E k = mass x velocity 2 2 What has a greater effect on kinetic energy, mass or velocity? Why?

Potential Energy Even motionless objects have energy Even motionless objects have energy Potential energy- stored energy due to interactions between objects Potential energy- stored energy due to interactions between objects –If the apple stays in the tree, the energy will remain stored –If the apple falls, that stored energy is converted to kinetic energy

Elastic Potential Energy If you stretch a rubber band and let it go, it sails across the room If you stretch a rubber band and let it go, it sails across the room –As it flies through the air, it has kinetic energy due to its motion but where did this kinetic energy come from? – –The stretched rubber band had energy stored as elastic potential energy Elastic potential energy is energy stored by something that can stretch or compress, such as a rubber band or spring Elastic potential energy is energy stored by something that can stretch or compress, such as a rubber band or spring

Chemical Potential Energy Gasoline, food, and other substances have chemical potential energy Gasoline, food, and other substances have chemical potential energy –Energy stored due to chemical bonds is chemical potential energy –Energy is stored due to the bonds that hold the atoms together and is released when the gas is burned

Gravitational Potential Energy Any system that has objects that are attracted to each other through gravity has gravitational potential energy Any system that has objects that are attracted to each other through gravity has gravitational potential energy –Gravitational potential energy (GPE) - energy due to gravitational forces between objects –Water and Earth –Apple and Earth

Gravitational Potential Energy Depends on mass and height Depends on mass and height E p = m (kg) x g (N/kg) x h (m) E p = m (kg) x g (N/kg) x h (m) where g is the force caused by gravity (9.8 N/kg) where g is the force caused by gravity (9.8 N/kg) –If you stand on a 3-meter diving board, you have 3 times the G.P.E, than you had on a 1-meter diving board –A person with 3 times a larger mass has 3 times the potential energy

Gravitational Potential Energy A waterfall, a suspension bridge, and a falling snowflake all have gravitational potential energy A waterfall, a suspension bridge, and a falling snowflake all have gravitational potential energy

Potential Energy Kinetic Energy

Potential Energy Kinetic Energy

Check for Understanding E k = 1 mv 2 2 E k = 1 mv 2 2 –What is the kinetic energy of a 100 kg man moving 5 m/s? –What is the kinetic energy of 0.5 kg ball moving at 30 m/s?

Check for Understanding E k = 1 mv 2 2 E k = 1 mv 2 2 –What is the kinetic energy of a 100 kg man moving 5 m/s? 1 mv 2 = 1 x 100kg x (5m/s) 2 = 1250 J mv 2 = 1 x 100kg x (5m/s) 2 = 1250 J 2 2 –What is the kinetic energy of 0.5 kg ball moving at 30 m/s? 1 mv 2 = 1 x 0.5kg x (30m/s) 2 = 225 J mv 2 = 1 x 0.5kg x (30m/s) 2 = 225 J 2 2 Eureka! Kinetic Energy Eureka! Kinetic Energy Eureka! Kinetic Energy Eureka! Kinetic Energy

Check for Understanding E p = m x g x h E p = m x g x h –A 100 kg boulder is on the edge of the cliff 10 m off the ground. How much energy does it have? –A 0.5 kg ball is thrown 15 m into the air How much potential energy does it have at its highest point?

Check for Understanding E = m x g x h E = m x g x h –A 100 kg boulder is on the edge of the cliff 10 m off the ground. How much energy does it have? 100kg x 9.8 m/s 2 x 10m = ~ 10,000 J –A 0.5 kg ball is thrown 15 m into the air How much potential energy does it have at its highest point? 0.5 kg x 9.8 m/s 2 x 15m = ~ 75 J Eureka! Potential Energy Eureka! Potential Energy Eureka! Potential Energy Eureka! Potential Energy

The Law of Conservation of Energy The Law of Conservation of Energy- energy can be neither created nor destroyed by ordinary means, it can only be converted from one form to another The Law of Conservation of Energy- energy can be neither created nor destroyed by ordinary means, it can only be converted from one form to another Energy can change from one form to another, but the total amount of energy never changes Energy can change from one form to another, but the total amount of energy never changes The total energy of a system remains constant The total energy of a system remains constant

Energy Transformations The law of conservation of energy is a universal principle that describes what happens to energy as it is transferred from one object to another or as it is transformed The law of conservation of energy is a universal principle that describes what happens to energy as it is transferred from one object to another or as it is transformed –You are likely to think of energy as race cars roar past or as your body uses energy from food to help it move, or as the Sun warms your skin on a summer day –These situations involve energy changing from one form to another form

Mechanical Energy Transformations Mechanical energy is the sum of the kinetic energy and potential energy of the objects in a system Mechanical energy is the sum of the kinetic energy and potential energy of the objects in a system –The mechanical energy of a system remains constant or nearly constant –In these cases, energy is only converted between different forms of mechanical energy

Mechanical Energy Transformations An apple-Earth system has gravitational potential energy due to the gravitational force between apple and Earth An apple-Earth system has gravitational potential energy due to the gravitational force between apple and Earth –The instant the apple comes loose from the tree:  It accelerates due to gravity  It loses height so the gravitational potential energy decreases  Its potential energy is transformed into kinetic as the speed of the apple increases  The potential energy that the apple lost is gained back as kinetic energy so the total amount of energy remains the same

Energy Transformation in Projectile Motion Energy transformations also occur during projectile motion when an object moves in a curved path Energy transformations also occur during projectile motion when an object moves in a curved path However, the mechanical energy of the ball-Earth system remains constant as it rises and falls However, the mechanical energy of the ball-Earth system remains constant as it rises and falls

Energy Transformation in a Basketball Ball slows down Ball speeds up Force of gravity

Energy Transformation in a Roller Coaster At the point of maximum potential energy, the car has minimum kinetic energy. Total energy is conserved and constant

Energy Transformations in a Swing When you ride on a swing part of the fun is the feeling of almost falling as you drop from the highest point to the lowest point of the swing’s path When you ride on a swing part of the fun is the feeling of almost falling as you drop from the highest point to the lowest point of the swing’s path –The ride starts with a push that gets you moving, giving you kinetic energy –As the swing rises, you lose speed but gain height –In energy terms, kinetic energy changes to gravitational potential energy

The Effect of Friction You know that if you don’t continue to pump a swing or get a push, your arcs will become lower and you eventually will stop swinging You know that if you don’t continue to pump a swing or get a push, your arcs will become lower and you eventually will stop swinging In other words, the mechanical (kinetic and potential) energy of the swing decreases, as if the energy were being destroyed or lost Is this a violation of the law of conservation of energy? In other words, the mechanical (kinetic and potential) energy of the swing decreases, as if the energy were being destroyed or lost Is this a violation of the law of conservation of energy?

The Effect of Friction NO!!! Energy in the system is conserved NO!!! Energy in the system is conserved –With every movement, the swing’s chains rub on their hooks and air pushes on the rider –Friction and air resistance cause some of the mechanical energy of the swing to change to thermal energy –With every pass of the swing, the temperature of the hooks and the air increases a little, so the mechanical energy of the swing is not destroyed, but transformed into thermal energy, or heat

Conservation of Energy with a Pendulum When is the pendulum moving the fastest? at the lowest point

Conservation of Energy with a Pendulum When does the pendulum have the most kinetic energy? at the lowest point

Conservation of Energy with a Pendulum When does the pendulum have the most gravitational potential energy? at the highest point

E Conservation of Energy with a Pendulum KP

E KP Gravitational potential energy depends on height

EP Kinetic energy depends on speed Conservation of Energy with a Pendulum K

EKP E = P + K

EKP Total Energy Doesn’t Change +=

A Pendulum PE No KE All KE PE No KE

Transforming Electrical Energy Light bulbs transform electrical energy into light so you can see Light bulbs transform electrical energy into light so you can see The warmth you feel around the bulb is evidence that some of that electrical energy is transformed into thermal energy, or heat The warmth you feel around the bulb is evidence that some of that electrical energy is transformed into thermal energy, or heat

Transforming Chemical Energy Fuel stores chemical potential energy Fuel stores chemical potential energy –An engine transforms chemical potential energy of gasoline molecules into the kinetic energy of a moving car or bus –Several energy conversions occur in this process –In a car, a spark plug fires, initiating the conversion of chemical potential energy into thermal energy –As the hot gases expand, thermal energy is converted into kinetic energy

Transforming Chemical Energy Some chemical energy transformations are less obvious because they do not result in visible motion, sound, heat or light Some chemical energy transformations are less obvious because they do not result in visible motion, sound, heat or light –Every green plant you see converts the radiant energy from the Sun into the energy stored in chemical bonds in the plant, like in carbohydrates (sugars) –When we eat we transform the potential energy stored in the carbohydrate bonds and transform it into kinetic energy

Other Energy Conversions In a battery, chemical energy is converted into electromagnetic energy In a battery, chemical energy is converted into electromagnetic energy The mechanical energy of a waterfall is converted to electrical energy in a generator The mechanical energy of a waterfall is converted to electrical energy in a generator Chemical  Thermal  Mechanical

A E D C B Check for Understanding Where is the gravitational potential energy maximum? Where is the kinetic energy maximum? Where is the gravitational potential energy minimum? Where is the kinetic energy minimum?

A E D C B Check for Understanding Where is the gravitational potential energy maximum? A and E Where is the kinetic energy maximum? C Where is the gravitational potential energy minimum? C Where is the kinetic energy minimum? A and E