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2. Resources Chapter Presentation Bellringers Transparencies Standardized Test Prep Visual Concepts Math Skills

3. Work and Energy Chapter 12 Table of Contents Section 1 Work, Power, and Machines Section 2 Simple Machines Section 3 What is Energy? Section 4 Conservation of Energy

4. Section 1 Work, Power, and Machines Chapter 12 Objectives • Definework and power. • Calculatethe work done on an object and the rate at which work is done. • Usethe concept of mechanical advantage to explain how machines make doing work easier. • Calculatethe mechanical advantage of various machines.

5. Section 1 Work, Power, and Machines Chapter 12 Bellringer In your study of motion, you have learned that forces can cause motion. But in some cases, a force that is applied is balanced by another opposite force, and there is no net motion as a result. Look at the following illustrations, and identify the forces and motion in each one. (See illustrations on following slide.) 1. In one drawing, no motion is likely to occur. Which drawing is it? 2. Describe the forces that are acting in this diagram. If the person exerts slightly more force, what happens to the opposite force? Does it increase to match the new force of the person, stay the same, or decrease?

6. Section 1 Work, Power, and Machines Chapter 12 Bellringer, continued

7. Section 1 Work, Power, and Machines Chapter 12 What is Work? • Workis the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force. • Work is done only when a force causes an object to move in the direction of the force. This is different from the everyday meaning of work. • Work Equation work = forcedistance W = Fd

8. Section 1 Work, Power, and Machines Chapter 12 Work

9. Section 1 Work, Power, and Machines Chapter 12 What is Work?, continued • Work is measured in joules. • Because work is calculated as force times distance, it is measured in units of newtons times meters, N•m. • These units are also calledjoules (J). In terms of SI base units, a joule is equivalent to 1 kg•m2/s2. • Definition of joules 1 J = 1 N•m = 1 kg•m2/s2

10. Section 1 Work, Power, and Machines Chapter 12 Math Skills Work Imagine a father playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N? 1. List the given and unknown values. • Given: force, F = 190 N • distance, d = 2.0 m • Unknown: work, W = ? J

11. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for work. • work = force  distance • W = f  d 3. Insert the known values into the equation, and solve. • W = 190 N 2.0 m = 380 N•m • W = 380 J

12. Section 1 Work, Power, and Machines Chapter 12 Power • Poweris a quantity that measures the rate at which work is done or energy is transformed. • Power Equation • Power is measured in watts. • A watt (W) is equal to a joule per second (1 J/s).

13. Section 1 Work, Power, and Machines Chapter 12 Power

14. Section 1 Work, Power, and Machines Chapter 12 Math Skills PowerIt takes 100 kJ of work to lift an elevator 18 m. If this is done in 20 s, what is the average power of the elevator during the process? 1. List the given and unknown values. • Given: work, W = 100 kJ = 1  105 J • time, t = 20 s • The distance of 18 m will not be needed to • calculate power. • Unknown: power, P = ? W

15. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for power. 3. Insert the known values into the equation, and solve.

16. Section 1 Work, Power, and Machines Chapter 12 Machines and Mechanical Advantage • Machines multiply and redirect forces. • Machines help people by redistributing the work put into them. • They can change either the size or the direction of the input force. • Different forces can do the same amount of work. • A machine allows the same amount of work to be done by either decreasing the distance while increasing the force or by decreasing the force while increasing the distance.

17. Section 1 Work, Power, and Machines Chapter 12 Force and Work

18. Section 1 Work, Power, and Machines Chapter 12 Machines and Mechanical Advantage, continued • Mechanical advantagetells how much a machine multiplies force or increases distance. • Mechanical Advantage Equation

19. Section 1 Work, Power, and Machines Chapter 12 Math Skills Mechanical Advantage Calculate the mechanical advantage of a ramp that is 5.0 m long and 1.5 m high. 1. List the given and unknown values. • Given:input distance = 5.0 m • output distance = 1.5 m • Unknown: mechanical advantage = ?

20. Section 1 Work, Power, and Machines Chapter 12 Math Skills, continued 2. Write the equation for mechanical advantage. Because the information we are given involves only distance, we only need part of the full equation: 3. Insert the known values into the equation, and solve.

21. Section 2 Simple Machines Chapter 12 Objectives • Nameand describe the six types of simple machines. • Discussthe mechanical advantage of different types of simple machines. • Recognize simple machines within compound machines.

22. Section 2 Simple Machines Chapter 12 Bellringer You may not think of a door as a simple machine, but it is one. It functions like a lever. Like other levers, when you exert a force on it (an input force), a force is exerted along the entire door (the output force).

23. Section 2 Simple Machines Chapter 12 Bellringer, continued 1. For all levers, one point along the lever stays still while the rest of the lever moves. This point is called the fulcrum. Where is the fulcrum of a door? 2. You can push at any point along the width of a door and it will open. Which position requires the least force: pushing the door near the hinges, in the middle, or near the side farthest from the hinges? (Hint: Which of these feels easiest to do?) 3. If you are trying to prop the door open, but your only doorstop is not very heavy, is it likely to work best near the hinges, in the middle, or near the side farthest from the hinges?

24. Section 2 Simple Machines Chapter 12 The Lever Family • The most basic machines are calledsimple machines. • The six types of simple machines are divided into two families.

25. Section 2 Simple Machines Chapter 12 The Lever Family, continued • Levers have a rigid arm and a fulcrum. • Levers are divided into three classes. • All first-class levers have a fulcrum located between the points of application of the input and output forces. • In a second-class lever, the fulcrum is at one end of the arm and the input force is applied to the other end. • Third-class levers multiply distance rather than force. As a result, they have a mechanical advantage of less than 1.

26. Section 2 Simple Machines Chapter 12 Levers

27. Section 2 Simple Machines Chapter 12 Lever

28. Section 2 Simple Machines Chapter 12 The Lever Family, continued • Pulleys are modified levers. • The point in the middle of a pulley is like the fulcrum of a lever. • A single, fixed pulley has a mechanical advantage of 1. • Multiple pulleys are sometimes put together in a single unit called a block and tackle. • A wheel and axle is a lever or pulley connected to a shaft. • The steering wheel of a car, screwdrivers, and cranks are common wheel-and-axel machines.

29. Section 2 Simple Machines Chapter 12 Pulleys

30. Section 2 Simple Machines Chapter 12 Pulley

31. Section 2 Simple Machines Chapter 12 The Inclined Plane Family • Inclined planes multiply and redirect force. • An inclined plane turns a small input force into a large output force by spreading the work out over a large distance. • A wedge is a modified inclined plane. • A screw is an inclined plane wrapped around a cylinder.

32. Section 2 Simple Machines Chapter 12 Inclined Plane

33. Section 2 Simple Machines Chapter 12 Screws

34. Section 2 Simple Machines Chapter 12 Compound Machines • A machine made of more than one simple machine is called acompound machine. • Examples of compound machines are: • scissors, which use two first class levers joined at a common fulcrum • a car jack, which uses a lever in combination with a large screw

35. Section 3 What is Energy? Chapter 12 Objectives • Explainthe relationship between energy and work. • Definepotential energy and kinetic energy. • Calculatekinetic energy and gravitational potential energy. • Distinguishbetween mechanical and nonmechan-ical energy.

36. Section 3 What is Energy? Chapter 12 Bellringer In the chapter on matter, you learned that energy is conserved. Instead of being created or destroyed, it is just changed from one form to another. The energy of the sunlight that reaches Earth is the ultimate source of most of the energy around us. Look at the illustration below, and identify the types of energy involved.

37. Section 3 What is Energy? Chapter 12 Bellringer, continued 1. How did energy from sunlight provide the energy the girl needed to swing the bat? (Hint: What do you need to have energy?) 2. When the girl hits the ball, she exerts a force on it. Does she do work on the ball in the scientific sense of the term? Explain why. 3. After the girl hits the ball, the ball moves very fast and has energy. When the ball hits the fielder’s glove, it stops moving. Given that energy can never be destroyed but merely changes form, what happened to the energy the ball once had? (Hint: If you are the fielder, what do you hear and feel as you catch the ball?)

38. Section 3 What is Energy? Chapter 12 Energy and Work • Energy is the ability to do work. • When you do work on an object, you transfer energy to that object. • Whenever work is done, energy is transformed or transferred to another system. • Energy is measured in joules. • Because energy is a measure of the ability to do work, energy and work are expressed in the same units.

39. Section 3 What is Energy? Chapter 12 Potential Energy • The energy that an object has because of the position, shape, or condition of the object is calledpotential energy. • Potential energy is stored energy. • Elastic potential energyis the energy stored in any type of stretched or compressed elastic material, such as a spring or a rubber band. • Gravitational potential energyis the energy stored in the gravitational field which exists between any two or more objects.

40. Section 3 What is Energy? Chapter 12 Potential Energy

41. Section 3 What is Energy? Chapter 12 Potential Energy, continued • Gravitational potential energy depends on both mass and height. • Gravitational Potential Energy Equation grav. PE = mass  free-fall acceleration  height PE = mgh • The height can be relative. • The height used in the above equation is usually measured from the ground. • However, it can be a relative height between two points, such as between two branches in a tree.

42. Section 3 What is Energy? Chapter 12 Math Skills Gravitational Potential EnergyA 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? 1. List the given and unknown values. Given: mass, m = 65 kg height, h = 35 m free-fall acceleration, g = 9.8 m/s2 Unknown: gravitational potential energy, PE = ? J

43. Section 3 What is Energy? Chapter 12 Math Skills, continued 2.Write the equation for gravitational potential energy. • PE = mgh • 3. Insert the known values into the equation, and solve. • PE = (65 kg)(9.8 m/s2)(35 m) • PE = 2.2  104 kg•m2/s2 • PE = 2.2  104 J

44. Section 3 What is Energy? Chapter 12 Kinetic Energy • The energy of a moving object due to the object’s motion is called kinetic energy. • Kinetic energy depends on mass and speed. • Kinetic Energy Equation • Kinetic energy depends on speed more than mass.

45. Section 3 What is Energy? Chapter 12 Kinetic Energy Graph

46. Section 3 What is Energy? Chapter 12 Kinetic Energy

47. Section 3 What is Energy? Chapter 12 Math Skills Kinetic EnergyWhat is the kinetic energy of a 44 kg cheetah running at 31 m/s? 1. List the given and unknown values. Given: mass, m = 44 kg speed, v = 31 m/s Unknown: kinetic energy, KE = ? J

48. Section 3 What is Energy? Chapter 12 Math Skills, continued 2.Write the equation for kinetic energy. 3. Insert the known values into the equation, and solve.

49. Section 3 What is Energy? Chapter 12 Other Forms of Energy • The amount of work an object can do because of the object’s kinetic and potential energies is called mechanical energy. • Mechanical energy is the sum of the potential energy and the kinetic energy in a system. • In addition to mechanical energy, most systems containnonmechanical energy. • Nonmechanical energy does not usually affect systems on a large scale.

50. Section 3 What is Energy? Chapter 12 Other Forms of Energy, continued • Atoms and molecules have kinetic energy. • The kinetic energy of particles is related to heat and temperature. • Chemical reactions involve potential energy. • The amount ofchemical energyassociated with a substance depends in part on the relative positions of the atoms it contains. • Living things get energy from the sun. • Plants usephotosynthesisto turn the energy in sunlight into chemical energy.