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Objectives: Describe the conditions which must be met to do work. Distinguish between work and power. Calculate work and power. Interpret data from.

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Presentation on theme: "Objectives: Describe the conditions which must be met to do work. Distinguish between work and power. Calculate work and power. Interpret data from."— Presentation transcript:

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4 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 MAKE A LIST OF FIVE ACTIVITIES THAT YOU CONSIDER WORK:

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 conditions that 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 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 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. 1.The amount of force exerted in the direction of motion. 2.The distance moved. Write the equation

10 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. The downward force does not do work. Only the force applied in the same direction the lawn mower moves does work. Newtons.

11 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 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 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 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 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 W

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

17 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 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 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 4.The charge for the electricity usage is $63.43.

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

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

25 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 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 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 The amount of work put into a machine, or work input, is always 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%.

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

34 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. An ax is a wedge. Many car jacks are screws.

35 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. Balance Wheelbarrow Shovel

36 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 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 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 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 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 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 pulley is a rope wrapped around a grooved wheel. The two main types of pulleys are called fixed pulleys & movable pulleys. 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.

43 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 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. s.org/scienceforkids/phys ics/machines/pulley.htm

45 A compound machine is a system of 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 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. 4.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.

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49 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 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.

51 Energy has many forms. In some way, all of these forms are related to each other. Energy can be converted from one form to another.

52 Mechanical energy is the energy of the movement or the position of an object. The mechanical energy of a moving object is also called its kinetic energy. The mechanical energy of an object due to its position is also called potential energy. A windmill is a compound machine that uses mechanical energy to do work.

53 All matter is made up of particles called atoms. A great amount of energy is stored in an atom's core, or nucleus. This energy, called nuclear energy, is released when an atomic nucleus breaks apart, or when a new nucleus forms. The sun has great amounts of nuclear, heat, and electromagnetic energy.

54 Chemical bonds between atoms hold substances together. Breaking these bonds releases large amounts of energy. Chemical energy is the energy stored in the bonds between atoms. Energy released from gasoline drives the piston that moves the car wheels.

55 When electrons flow through conducting material, a moving charge is formed. This moving charge is electricity, a form of electromagnetic energy. electric/how.asp

56 When a substance becomes warmer, its particles move faster. The fast-moving particles of air collide with the slower moving water particles, increasing their motion. Energy flows, as heat, from the warm air to the cool water.

57 How does energy change from one form to another as you light the match? When you move the match head against the striking strip, particles in the match head begin to move faster. As this happens, mechanical energy changes to heat energy. The heat energy causes a chemical change to begin. The match head begins to change color and release stored chemical energy. The chemical energy changes into heat energy and light energy.

58 As energy changes from one form to another, it is never created or destroyed. This fact has been observed to be true in so many different situations that it has become a scientific law. According to the Law of Conservation of Mass and Energy, the amount of mass and energy in the universe is always the same. Albert Einstein predicted the relationship between matter and energy. He expressed it in mathematical form as:

59 1.Mechanical energy – mixer; nuclear energy – reactors in a nuclear power plant; chemical energy – combustion of gasoline; electromagnetic energy – electricity; heat – energy used tin cooking. 2.a. chemical energy stored in the gasoline is converted to heat and mechanical energy in the engine. b. chemical energy released from food by the body is converted into heat energy used in running. 3. Energy changes from one form to another, it is never created or destroyed.


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