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# Work and Energy : Forms and Changes

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1. Work and Energy: Forms and Changes

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

3. Work • 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

4. Calculating Work • 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 • One newton-meter is equal to one joule so the unit of work is a joule

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

6. When is Work Done? • 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 • Carry books while walking, and your arms are not doing work. Why not?

7. 20 N 1 m 3 m 60 N Examples of Work W = 60 N x 1 m = 60 J (N-m) W = 20 N x 3 m = 60 J (N-m)

8. Power • 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 (J/sec or Watt)= Work (J) Time (sec) • The unit of power is Joules/sec, called a Watt.

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

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

11. 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 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?

12. 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 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! • Eureka! Work

13. 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.

14. Check for Understanding W = F x d but we need to find the gravitational force (weight) of the barbells Fg = m x g Fg = 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

15. History of Work • 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

16. Simple Machines • Ancient people invented simple machines that would help them overcome resistive forces • 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

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

18. Inclined Plane • A flat, slanted surface Eureka! Inclined Plane

19. Lever • Two parts: Fulcrum Bar • Eureka! Levers

20. Wheel and Axle • Two parts: • wheel • bar

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

22. Gears • Wheels with teeth

23. Pulley • Two kinds: Fixed Moveable

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

25. Efficiency • 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

26. Mechanical Advantage • 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 outputforce • Mechanical advantage of a machine is the ratio of the output force to the input force

27. Mechanical Advantage • 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

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

29. Changes Require 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

30. Nature of Energy • 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

31. Nature of Energy • 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

32. Forms of Energy • 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 • Regardless of its form, money is money, and the same goes for energy in that these are only different forms of the same thing

33. States of 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: • Potential • Kinetic

34. Kinetic Energy • Kinetic energy- the energy of motion • 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 Ek = mass x velocity2 2 What has a greater effect on kinetic energy, mass or velocity? Why?

35. Potential Energy • Even motionless objects have energy • 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

36. Elastic Potential Energy • 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 motionbut 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

37. 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

38. 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

39. Gravitational Potential Energy • Depends on mass and height • Ep = m (kg) x g (N/kg) x h (m) 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

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

41. Kinetic Energy Potential Energy

42. Kinetic Energy Potential Energy

43. Check for Understanding • Ek = 1 mv2 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?

44. Check for Understanding • Ek = 1 mv2 2 • What is the kinetic energy of a 100 kg man moving 5 m/s? 1 mv2 =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 mv2 =1 x 0.5kg x (30m/s)2 = 225 J 2 2 • Eureka! Kinetic Energy

45. Check for Understanding • Ep = 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?

46. Check for Understanding • 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/s2 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/s2 x 15m = ~ 75 J • Eureka! Potential Energy

47. 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 • Energy can change from one form to another, but the total amount of energy never changes • The total energy of a system remains constant

48. 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 • 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

49. Mechanical Energy Transformations • 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

50. Mechanical Energy Transformations • 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 intokinetic 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