Teacher: Aurora Comis

1. Work & Energy Teacher: Aurora Comis

3. Communication (What language do we need working with the content? What Physics language will learners communicate during the lesson?) • Simple Present (A Force is….) • Present Perfect (We have measured ….) • Imperative (Let us consider…..) • Content (Subject matter - What are my objectives? What are the learning outcomes?) • Definition of Physics Work • Definition of Energy • Forms of Energy • Laws of Conservation The 4 C’s • Cognition (What thinking skills are demanded to the learners?) • Repeating procedures • Ordering steps • Checking results • Handling formulas • Defining concepts • Making hypotheses, interpreting, judging and evaluating to solve problems • Culture (What are the cultural implications of the topic?) • Some historical background • Practical and technological implications

5. Work When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. There are three key ingredients to work - force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement. There are several good examples of work that can be observed in everyday life - a horse pulling a plow through the field, a father pushing a grocery cart down the aisle of a grocery store, a freshman lifting a backpack full of books upon her shoulder, a weightlifter lifting a barbell above his head, an Olympian launching the shot-put, etc. In each case described here there is a force exerted upon an object to cause that object to be displaced.

6. Is it Work? • 1. A teacher applies a force to a wall and becomes exhausted. Answer: No This isn’t an example of work. The wall isn’t displaced. A force MUST cause a displacement in order for work to be done. • 2. A book falls off a table and free falls to the ground. Answer: Yes This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i.e., "fall").

7. 3. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. Answer: No This isn’t an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). Yet the force does not cause the displacement. To cause a displacement, there must be a component of force in the direction of the displacement. • 4. A rocket accelerates through space. Answer: Yes This is an example of work. There is a force (the expelled gases push on the rocket) which causes the rocket to be displaced through space.

8. What is Work? Hopefully you were given the right answer, but chances are fifty-fifty you were shrugged off. Not because the right answer is so difficult to know, but rather because the right answer is so difficult to explain, or at least difficult to explain in a way that can be grasped quickly. I think this is mostly due to the fact that the word work has two meanings: the ordinary one of everyday life and the technical one of physics. Work done is a scalar quantity and its SI unit is the Joule. James Prescott Joule (1818–1889) was a wealthy English brewer who dabbled in various aspects of Science and Economics. Joule realized that mechanical work, heat, and electric energy were all somehow interconvertible. Infact: Heat can do work. Work can make heat. Work can make electricity, Electricity can do work, Electricity can make heat. Heat can make electricity. Energy is a versatile actor.

9. Work definition A change in the position, speed, state, or form of matter. Also, the energy expended when an object's speed is increased or when it moves against an opposing force. The amount of work done is calculated by multiplying the force exerted on an object by the distance through which the object is moved, as long as the force is constant and motion is in a straight line in the direction of the force. Technically, Work is the "force-displacement product" W = FΔs cos α Where: F is the Force exerted on the object Δs is the distance through which the object is moved cosα depends by the inclination of the force to the plane in which the object is located

10. W = FΔs cos α I understand that for many of you this is a meaningless definition. Actually, quite the contrary. This definition is so compact it's like poetry. It says as much as it can in as few words as possible. It's so compact that explaining it in ordinary language makes the half dozen words of the technical definition expand to nearly a hundred words of so-called "natural language". Let me explain what work is though a series of mental images. Whenever an example is presented, remember that work is done whenever a force causes a displacement. Imagine that a physics teacher is standing motionless before a class of students. Since he isn't exerting any forces that will displace anything outside of his body he isn't doing any work. Obviously. But doing this for any length of time will certainly drain him of energy just as if he had pushed papers across his desk all day (an example where a force does result in a displacement). Surely, you could now convince him that his definition of work must be wrong. Maybe a lesser teacher would cave under the pressure, but not a physics teacher.

11. Most certainly, a physics teacher or any other person standing is doing work, but the work being done isn't easily visible. Inside the body the heart is pumping blood, the digestive system is grinding away on breakfast, receptors are driving molecules across cell membranes. We do work even as we sleep. Forces causing displacement are happening everywhere under our skins. The human body is a busy place. If a system as a whole exerts a force on its surroundings and a displacement occurs, the work done is called external work. A physics teacher pushing papers across his desk is doing external work. A physics teacher standing motionless is not doing any significant external work. If a part of a system exerts a force on another part of the same system and a displacement occurs, the work done is called internal work. A physics teacher thinking deeply is doing internal work. A physics teacher doing anything — or nothing for that matter — is doing internal work. In mechanics, when we say work has been done we are usually referring to external work.

12. Work done can also be positive or negative. When 0 ≤ α< 90, work done is positive. Work done by a force is positive if the applied force has a component in the direction of the displacement. When a body is falling down, the force of gravitation is acting in the downward direction. The displacement is also in the downward direction. Thus the work done by the gravitational force on the body is positive. Consider the same body being lifted in the upward direction. In this case, the force of gravity is acting in the downward direction. But, the displacement of the body is in the upward direction. Since the angle between the force and displacement is 180, the work done by the gravitational force on the body is negative.

13. Work’s examples Consider the simplest possible case of Work done. A force F is acting on an object. The object has a displacement S in the direction of the force. Then the Work done is the product of force and displacement. W = FΔs What will happen in the case when the applied Force isn’t in the direction of displacement but rather at an angle to it? In such a case we will consider the component of force in the direction of displacement. This component will be effective in doing work as shown. W = FΔs cos α

14. When the applied Force is perpendicular to the direction of displacement, the Work done is zero. W = 0

15. Energy Definition: Energy is the capacity of a physical system to perform work. Energy exists in several forms such as heat, kinetic or mechanical energy, light, potential energy, electrical, or other forms. Everything we do involves energy.  Getting up, going to school, and doing chores require energy.  In fact, everything that happens in the universe, from the eruption of volcanoes, to the sprouting of seed, to the moving of people, takes energy.  When we turn on a motor, drive a car, cook on a stove or switch on a light, we are using energy.

16. Energy is everywhere and abundant, yet it has no mass and can't be touched. However, you can see and feel the effects energy has on many materials. Energy can produce motion, heat, or light. Energy cannot be created or destroyed.  However, it can be changed from one form into another. Changing energy back and forth from one form or state to another is how we control it for our use. Do you remember what is the mass of an object? The Mass is a property of matter related to inertia. As the mass of an object increases, so does its inertia. Mass can be thought of as the quantity of matter in an object. Mass is not the same as weight, which depends on the gravitational forces acting on an object that has mass.

18. Energy Around Us We use the concept of Energy to help us describe how and why things behave the way they do. We talk about solar Energy, nuclear Energy, electrical Energy, chemical Energy, etc. If you apply a Force to an object, you may change its energy. That Energy must be used to do work, or accelerate, an object. Energy is not something you can hold or touch. It is just another means of helping us to understand the world around us. Energy is called a scalar because there is no direction to Energy (as opposed to vectors). Scientists measure energy in units called joules.

19. Law of Conservation of Energy Energy is neither created nor destroyed, but can change from one form into another. In an isolated system (that haven’t exchanges with the outside)the total Energy remains constant, though Energy may transform into another form. Two billiard balls colliding, for example, may come to rest, with the resulting Energy becoming sound and perhaps a bit of heat at the point of collision.

20. Forms of Energy Energy comes in six forms:  chemical energy (Energy stored within the bonds between molecules. The most common examples of chemical energy are fuels such as gasoline, coal, and natural gas), electrical energy, radiant energy (The form of energy related to the movement of light), electromagnetic waves, or particles, mechanical energy (The energy a substance or system has because of its motion), nuclear energy (The energy released when the nuclei of atoms are split or fused) and thermal energy (Also known as heat energy; the energy of moving or vibrating molecules).  These six forms of energy are all related.  Each form can be converted or changed into the other forms.  For example, when wood burns, its chemical energy changes into thermal (heat) energy and radiant (light) energy. Not all energy conversions are as simple as burning wood. 

21. Energy Transformations The reason we transform energy from one form to another is because different end uses require. For example, sometimes we want electrical energy to power light bulbs that change electricity into electromagnetic radiation, some of which is visible as light.  At other times we want the kinetic energy of a rotating shaft to provide mechanical energy to propel an automobile, so we transform chemical energy from fuel into thermal energy in a car engine, and then into the kinetic energy of the rotating crankshaft that provides power to turn the wheels.

22. Potential & Kinetic Energy All mechanical Energy can be in one of two states:  Potential Energy or Kinetic Energy. Kinetic and potential Energies are found in all objects. Energy can be transferred from potential to kinetic and between objects

23. Potential Energy Potential Energy (PE) is energy that is "stored" because of the position and/or arrangement of the object. To be used, potential Energy must be converted into one of the six forms of kinetic Energy. PE=mgh Where: m=mass g=the acceleration of gravity h=height A car on top of a hill, is an example of Potential energy Gravitational Potential Energy is the energy possessed by a body because of its elevation (height) relative to a lower elevation, that is, the energy that could be obtained by letting it fall to a lower elevation. 

24. Kinetic Energy If an object is moving, it is said to have Kinetic Energy (KE). The amount of Kinetic Energy that all moving objects have depends on their speed and mass. When a car brakes its Kinetic Energy is changed into heat energy. It is equal to one half the product of the mass of a body and the square of its velocity.

25. Energy can be changed from one form to another.  For example, as water falls over a waterfall, its gravitational potential energy is first transformed into kinetic energy, then into thermal energy when it hits the ground.  Or the kinetic energy of the water stream could be transformed into the rotational kinetic energy of a turbine shaft, then into electricity by turning the shaft of a generator, then into thermal energy by passing the electricity through a resistor, raising its temperature.  Finally, heat can be transferred from the resistor to the surrounding air, to warm a house.

26. Energy cannot be created or destroyed, but it can be transformed.  • That's really what we mean when we say we are "using" energy.  The law of conservation of energy means that when energy is being used, it is not being used up.  Instead, it is being changed from one form into another.  • A car engine burns fuel, converting the fuel's chemical energy into mechanical energy to make the car move.  • Windmills change the wind's energy into mechanical energy to turn turbines, which then produce electricity.  • Solar cells change sunlight (radiant energy) into electrical energy. • Energy may change form many times on its way to doing work in your home, such as toasting bread.  In the process, some energy is converted into unwanted thermal energy or waste heat.  In fact, it is impossible to convert one form of energy into another without wasting some energy.In many energy conversions, more energy is wasted than is used for work.  For example, automobile engines typically waste more than two-thirds of the total energy used, primarily through heat. • On the other hand, electrical motors convert electrical energy into mechanical energy with only about 10 percent of the energy wasted via heat.  Energy efficiency means converting energy from one form to another with the least possible waste.law of nature is called the law of conservation of energy.

27. Energy Control Systems People have always looked for ways to control energy for their own use.  For example, the steam engine allowed full control of a powerful energy source:   expanding steam.  People could control the direction, amount of power, and the duration of the energy output.Since the invention of the steam engine, people have developed many new methods of controlling energy.  We control energy for transportation, heating, and cooling. • Every control system has three parts: • The original source of energy. • All the conversions the energy goes through, including the transmission (moving) of energy from one place to another. • The eventual use of the energy.

28. One example of an Energy Control System is the generation, transmission, and use of electricity.  In a coal-fired power plant, the original source of energy is the coal. From the source, the energy goes through the following conversions: - Chemical energy in coal is changed to thermal energy by burning. - Thermal energy boils water, which increases the speed of the molecules in the water and produces steam (mechanical energy). - Mechanical energy of the steam turns the blades in the turbine (mechanical energy), which generates electricity (electrical energy). The electricity is then transmitted along power lines to where it is needed, such as in a home.  There the electricity may be used to power motors, operate appliances such as radios and television sets, and provide light and heat.

29. Energy pathwaysDepending on the sources and uses, energy can take many different pathways through an energy control system.

30. Vocabulary

31. Websites • http://www.energyeducation.tx.gov/energy/section_1/topics/index.html • http://www.physics4kids.com/files/motion_energy.html • http://www.bbc.co.uk/schools/gcsebitesize/ • http://www.physicsclassroom.com/Class/