Chapter 5

1. Chapter 5 Work, Machines and Energy

2. VOcAbuLary Compound machine – Joule – Machine – Mechanical advantage – Mechanical efficiency Power – Simple machine – Watt - Work –

3. Section 5.1 Work and Power

4. 5.1 Work and Power • Objectives: • Describe the conditions which must be met to do work. • Distinguish between work and power. • Calculate work and power. • Interpret data from a sample electric bill.

5. 5.1 Work and Power • MAKE A LIST OF FIVE ACTIVITIES THAT YOU CONSIDER WORK: • 1. • 2. • 3. • 4. • 5.

6. Work • In science the word ‘work’ relates to forces, motion and energy. • More specifically work is a force acting on an object in the direction in which it moves. When you rake leaves, you do work.

7. Work • There are two conditionsthat must be met in order for work to be done: • The object must move. • A force must act on the object in the direction the object moves. When you rake leaves, you do work.

8. Energy and Work • Energy is needed to rake leaves or to do any kind of work. • In fact, energy is defined as the ability to do work. • For work to be done, a force must move an object through a distance. When you run laps around a track you do work. Your legs apply a force against the track to move your body forward.

9. Measuring Work • To determine the amount of work done, you use a mathematical formula. • The formula relates work to force and distance. • Two measurements are needed to use this formula. • The amount of force exerted in the direction of motion. • The distance moved. Write the equation

10. Unit of Measurements • The amount of force exerted on an object is measured in • The distance the object moves is measured in meters. • As a result, the unit for work is sometimes called the newton meter (Nm) or joule. Newtons. The downward force does not do work. Only the force applied in the same direction the lawn mower moves does work.

11. Unit of Measurement - Joules • One joule (J) equals the work done by a force of 1 N that moves an object a distance of 1 m. • The joule is a unit that is also used to measure energy. • As you can see, energy & work are related. Lifting a glass of water from a counter top to your mouth, for example, would require you to use about 1 J of energy and to perform about 1 J of work.

12. SAMPLE PROBLEMS • How much work is done if I apply a force of 5N to move an object 2 meters across a desk? • W=F*d • W=(5)(2) • W= 10J • How much energy was used to move it? • 10J of energy

13. SAMPLE PROBLEMS • I mowed my lawn this weekend and I used 15 N to mow my lawn a distance of 10 m – how much work did I do? • W=(15)(10) • W= 150J • How much work did I do if I used a force of 30 N to pull a table a distance of 3 m? • W=(30)(3) • W= 90J • My two kids worked together to move a heavy ladder to the garage a distance of 2 m. They used a combined force of 25 N to complete the task. How much work did they do together? • W=(25)(2) • W=50J • DO Practice Problems 1-5 pg. 110

14. Power • Power is the rate at which work is done. • To calculate power, you divide the amount of work done by the time it took to do the work. • Write the equation: • When the work is in joules and the time in seconds, the power is in joules per second (J/s). • Another name for a joule per second is the watt (W).

15. Watt (W). • One watt is equal to 1 J/s. • The SI unit of power was named for James Watt, a Scottish engineer. • Watt coined the term "horsepower“ (hp), which is the amount of work a horse could do in one second. • One horsepower is equal to 745.56 W

16. WATT One watt is about the amount of power it takes for you to lift one glass of water one meter in one second.

17. SAMPLE PROBLEMS • If I run up several flights of stairs in 1.5 min., and the work I do is equal to 9000J, how much power did I use. • P= work (J)/time (s) • p=(9000)/(1.5)(60) • p=(9000)/(90) • p=100J/s or 100 W (watts)

18. SAMPLE PROBLEMS • I decided to go rowing this weekend. I rowed the boat for 14 min. And did a 168,000 J of work, how much power did I exert? • P=168000/(14)(60) • P=168000/(840) • P=200 W • DO Practice Problems 7-9 pg. 110

19. CHECK & EXPLAIN pg.111 • 1. No; even though the push required effort, by definition work is done only if an object moves. • 2. Both are measurements that relate force and distance. Power is the rate at which work is done so it includes the measurement of time. • 3. Work=500N X 1.5 m = 750 J Power= 750J/2s=375 W • The charge for the electricity usage is $63.43.

21. Section 5.2 Work and Machines

22. 5.2 Work and Machines • Objectives • Explain how machines make work easier. • Calculate mechanical advantage. • Infer the relationship between energy use and mechanical efficiency.

23. Machines • A machine is a device that makes work easier. • Machines make work easier by changing the direction or the size of the force needed to do work.

24. Machines and Forces Two forces are involved when you use a machine. • The force applied to the machine is called the effort force . • The force opposing the effort force is called the resistant force. The lid on the paint can is the resistance, or opposing, force. Often resistance is the weight of the object. When you push down on a screwdriver to remove the lid, you apply effort force to the screwdriver.

25. Mechanical Advantage • Most machines multiply the force of your efforts. • The number of times a machine multiplies an effort force is its mechanical advantage (M.A.). • A machine with an M.A. of 2 doubles your effort force. • As a result, you only have to use half of the effort force needed to do the same amount of work without a machine.

26. Mechanical Advantage • To find out a machine's mechanical advantage, divide the resistance force by the effort force. * For example, if you can lift a 300 N object by applying only 20 N of force to a lever, the M.A. of the lever is 15.

27. SAMPLE PROBLEMS • John likes to work on old automobiles. He has constructed a pulley system to help her lift the engine out of the car. She can lift a 600 N engine by applying only 150 N of force. What is the M.A. of her pulley system? • M.A. =resistance force/effort force • M.A. = 600 N/150 N • M.A. = 4 • Do problem 1-3 0n pg. 113

28. Mechanical Efficiency • The amount of work put into a machine, or work input, isalways greater than the amount of work done by the machine, or work output because some of the work put into the machine is used to overcome friction. • The mechanical efficiency of a machine compares its work output with the work input. In general: Because the work output is always less than the work input, the mechanical efficiency of a machine is always less than 100%.

30. Section 5.3 Simple and Compound Machines

31. 5.3 Simple and Compound Machines • Objectives • Explain how six simple machines make work easier. • Classify the simple machines in a compound machine you are familiar with. • Predict the mechanical advantages of simple machines.

32. SIMPLE MACHINES • Machines are almost everywhere you look. • Below are two groups of devices. • Group 1: Ramp, bottle opener, pulley, wheelbarrow • Group 2: Car, escalator, lawn mower, hair dryer • Which group of items contains machines? • All devices in both groups make work easier. • Each changes the size or direction of a force applied to it. • The devices in Group 1 are examples of simple machines. • Simple machines do work with one movement

33. SIMPLE MACHINES • There are six types of simple machines: the inclined plane, wedge, screw, lever, wheel and axle and pulley.

34. Inclined Plane • A simple machine that has a sloping surface is an inclined plane. A ramp makes moving a heavy crate easier. A wedge is an inclined plane that can move. A screw is also an inclined plane. Many car jacks are screws. An ax is a wedge.

35. Levers • A balance, a wheelbarrow, and a shovel are all machines that move when a force is applied. • They all have a straight point that moves when a force is applied & one point that does not move, called a fulcrum. • Machines that do work by moving around a fixed point are called levers. Shovel Wheelbarrow Balance

36. TYPES OF LEVERS - First Class Lever First class levers multiply the effort force and also change its direction. • The fulcrum is always between the effort force and the resistance force.

37. TYPES OF LEVERS - Second Class Lever A second class lever is defined as having the resistance force between the fulcrum and the effort force. • Second class levers multiply the effort force without changing its direction. • A wheelbarrow and nut cracker are examples of a second class lever.

38. TYPES OF LEVERS -Third Class Lever A third class lever is defined as having the effort force is between the fulcrum and the resistance force. • When you use a third class lever, the effort force is greater than the resistance force. • The reason for using a third class lever is to increase the distance moved, not to reduce the force. • Your forearm is an example of a third class lever.

39. M.A. of Levers The mechanical advantage of a lever is calculated by dividing the effort distance by the resistance distance. • The M.A. of first and second class levers is usually greater than 1. • The M.A. of third class levers is less than 1. • Third class levers do not multiply force.

40. Wheel and Axle • This type of simple machine consists of two circular objects called a wheel and an axle. • The wheel has a larger radius than the axle does. • The radius is the distance from the center of the wheel to the edge. An effort force applied at the wheel is multiplied at the axle to overcome a resistance force. The effort force applied to the wheel moves over a greater distance than the resistance force does.

41. M.A. of Wheel and Axle The mechanical advantage of a wheel and axle is equal to the radius of the wheel divided by the radius of the axle. • The M.A. of a wheel and axle is always greater than 1.

42. A fixed pulley is attached to a stationary structure. • It can make lifting an object easier by changing the direction of the effort force. • The M.A. of a fixed pulley is one because it does not multiply the effort force. Pulleys • A pulley is a rope wrapped around a grooved wheel. • The two main types of pulleys are called fixed pulleys & movable pulleys.

43. Pulleys A movable pulley is hung on a rope and hooked to a resistance. • Movable pulleys can multiply your effort force. • They have a M.A. of 2 • Notice that this pulley system also changes the direction of force. When two or more pulleys are used together, a pulley system is formed.

44. Pulleys This pulley system below has a M.A. of 3. The mechanical advantage of a pulley system is equal to the number of rope segments pulling up on the resistance force. http://www.historyforkids.org/scienceforkids/physics/machines/pulley.htm

45. Compound Machines • A compound machine is a systemof two or more simple machines. • The mechanical advantage of a compound machine is much greater than that of a simple machine. • The combination of simple machines in the compound machine multiplies the total mechanical advantage. How many simple machines are there in this pencil sharpener?

46. CHECK & EXPLAIN pg.121Answer 1 & 4 • 1. Inclined plane - You use less force to pull an object up an inclined plane, but go a greater distance. It reduces the effort force. wedge – an inclined plane that can move. screw – an inclined plane that produces a far greater force than the force needed to turn the screw. lever – a machines that does work by moving around a fixed point. wheel & axle – a machine consisting of a wheel and axle. The effort force applied at the wheel moves over a greater distance than the resistance force does. pulley - a machine that makes lifting an object easier by changing the direction of the effort force . • The M.A. is 4. Even though you cut down on the effort needed to lift something., you now have to increase the distance you have to pull the rope. In other words, if you use four pulleys, it takes 1/4 the effort to lift something, but the distance you have to pull the rope is four times as far.

48. Section 5.4 Energy and Its Forms

49. 5.4 Energy and Its Forms • Objectives • Name and describe five forms of energy. • Give examples of energy conversions. • Describe the Law of Conservation of Mass and Energy. • Infer the role of energy conversions in an everyday situation.

50. Machines, Work & Energy • Machines can make your work easier, but machines can't save work or energy. The amount of work you get out of a machine can never be greater than the amount of work or energy put into it. • For example, the work a lawn mower engine does is less than the energy in the gasoline that is burned. Most of the energy is converted to heat, due to friction between moving parts in the motor. • Remember, work and energy are related. Energy is the ability to do work.