Ch6_HallR

=Lesson 1: Basic Terminology and Concepts= toc

1/12/12: Lesson A Summary, Method 1
__Topic Sentence: Work occurs when a force causes a displacement,__ __and can be found by force•displacement•cosine(theta), where theta is the angle between the force and the displacement vectors.__ //A1: Definition and Mathematics of Work://
 * A: Work:**
 * Work: When a force causes displacement
 * [[image:http://www.physicsclassroom.com/Class/energy/u5l1a1.gif width="196" height="18" align="left"]]


 * where **F** is the force, **d** is the displacement, and the angle ( **theta** ) is defined as the angle between the force and the displacement vector.
 * The angle measure is defined as the angle between the force and the displacement.
 * [[image:http://www.physicsclassroom.com/Class/energy/u5l1a2.gif width="348" height="264" align="left"]]

> **The Joule is the unit of work.****1 Joule = 1 Newton * 1 meter****1 J = 1 N * m**
 * When a force is exerted on an object at an angle to the horizontal, only a part of the force contributes to (or causes) a horizontal displacement.
 * The cosine theta in the work equation relates to the //cause// factor - it //selects// the portion of the force that actually causes a displacement.
 * Forces that act in the direction opposite the objects motion to slow it down are called negative work.
 * The //negative// of negative work refers to the numerical value that results when values of F, d and theta are substituted into the work equation.
 * Since the force vector is directly opposite the displacement vector, theta is 180 degrees (The cosine(180 degrees) is -1 and so a negative value results for the amount of work done upon the object).
 * The standard metric unit of work s the **Joule** (abbreviated **J** ); one Joule is equivalent to one Newton of force causing a displacement of one meter.
 * [[image:http://www.physicsclassroom.com/Class/energy/u5l1a6.gif width="377" height="89" align="center"]]

//A2: Calculating the Amount of Work Done by Forces://
 * When a force acts to cause an object to be displaced, three quantities must be known in order to calculate the work: force, displacement and the angle between the force and the displacement.
 * The work is subsequently calculated as force•displacement•cosine(theta) where //theta// is the angle between the force and the displacement vectors.

=Lesson 2: Work, Energy, and Power=

1/30/12: Lesson A Summary, Method 1
__Topic Sentence: Forces can be categorized as internal or external forces.__ =** "Why Do My Parents Tell Me to Turn off the Lights? **" Activity=
 * A: Internal vs. External Forces**
 * All types of forces could be categorized as contact forces or as action-at-a-distance forces.
 * Whether a force was categorized as an action-at-a-distance force was dependent upon whether or not that type of force could exist even when the objects were not physically touching.
 * The force of gravity, electrical forces, and magnetic forces were examples of forces that could exist between two objects even when they are not physically touching.
 * Forces can also be grouped by whether or not their presence is capable of changing an object's total mechanical energy.
 * There are certain types of forces, that when present and when involved in doing work on objects will change the total mechanical energy of the object, and there are other types of forces that can never change the total mechanical energy of an object, but rather can only transform the energy of an object from potential energy to kinetic energy (or vice versa).
 * The two categories of forces are referred to as internal forces and external forces.
 * External forces include the applied force, normal force, tension force, friction force, and air resistance force.
 * Internal forces include the gravity forces, magnetic force, electrical force, and spring force.
 * When net work is done upon an object by an external force, the total mechanical energy (KE +PE) of that object is changed.
 * f the work is //positive work//, then the object will gain energy, and if the work is //negative work//, then the object will lose energy.
 * This energy will either be potential, kinetic, or both.
 * The work that is done will be __equal__ to the change in mechanical energy of the object.
 * Because external forces are capable of changing the total mechanical energy of an object, they are nonconservative forces.
 * When the only type of force doing net work upon an object is an internal force, the total mechanical energy of that object remains constant, but changes form.
 * Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are conservative forces.
 * If positive work is done on an object by an external force, then the object gains mechanical energy, and if the force and the displacement are in the opposite direction, then negative work is done on the object; the object subsequently loses mechanical energy.
 * Data:**
 * Sample Calculations:**

6 a.) Total amount due: $254.03 b.) Total kWh consumed that month: 1369 kWh
 * Questions:**

7 a.) Average cost per kWh: $0.19 c.) They are different because the cost per kWh I used for the calculations of my own usage does not take into account the increasing costs for the different tiers of amount of kWh used.

8. Using the cost for the first 60 kWh used ($0.178 per kWh), I cost my parents $4.04, while if I use the average cost of kWh of $0.19 per kWh, I cost them $4.31.

I believe that this is a very normal cost of electricity. Electricity is needed in life, and yet is still relatively cheap when placed against the other fees and payments associated with life. However, my calculations above only take into account 10 of my commonly used appliances. In reality, there are many other electrical appliances that I also use, so the cost could be as much as twice as much. For example, central heating, air conditioning, and my family's hot tub would also add to the extensive cost. Nonetheless, for size of my family and house, I believe that the bill is still rather reasonable. While I do not believe that I need to change anything, if I was going to, I would think that it would be most important to turn off the lights when leaving a room, unplugging appliances when not using them, and be smarter about the use of such appliances when they are needed.
 * Evaluation and Homework:**