Ch6_FavaA

=Lesson 2 - The Work-Energy Theorem= toc Internal vs. External Forces 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). Forces can be categorized as internal forces or external forces. For our purposes, we will simply say that **external forces** include the applied force, normal force, tension force, friction force, and air resistance force. And for our purposes, the **internal forces** include the gravity forces, magnetic force, electrical force, and spring force.

When the only type of force doing net work upon an object is an internal force (for example, gravitational and spring forces), the __ [|total mechanical energy (KE + PE)] __ of that object remains constant. In such cases, the object's energy changes form. This is referred to as energy conservation. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical is conserved. They are sometimes referred to as **conservative forces** .When net work is done upon an object by an external force, the __ [|total mechanical energy (KE + PE)] __ of that object is changed. If the work is//positive work//, then the object will gain energy. If the work is //negative work//, then the object will lose energy. The gain or loss in energy can be in the form of __ [|potential energy] __, __ [|kinetic energy] __, or both. Under such circumstances, the work that is done will be __equal__ to the change in mechanical energy of the object. They are sometimes referred to as **nonconservative forces**.

//KE to PE or PE to KE Questions//: 1-5 correct

When work is done by external forces (nonconservative forces), the total mechanical energy of the object is altered. The work that is done can be positive work or negative work depending on whether the force doing the work is directed opposite the object's motion or in the same direction as the object's motion. If the force and the displacement are in the same direction, then positive work is done on the object. If positive work is done on an object by an external force, then the object gains mechanical energy. 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.

//+ or - and PE or KE Questions//: 2 wrong; 1,3,4,5,6, correct

Note that in the five situations described above, a horizontal force can never change the potential energy of an object; horizontal forces cannot cause vertical displacements.

=Power Activity=


 * What does this cost your parents?
 * $3.08
 * Look at your household electric bill. Find two numbers:


 * 1) the total dollar amount due
 * $178.30
 * 1) kWh consumed that month.
 * 1064


 * Costs of Electricity.
 * 1) Calculate the average cost per kWh by dividing the dollar amount by the kWh consumed. This is approximately what each kWh costs your family. (Note: this figure includes fees and taxes, too). The cost will probably be between $0.10-$0.20 per kWh – depending on how much electricity you use. Electricity costs more if you exceed a certain usage.
 * [[image:Screen_shot_2012-02-27_at_1.54.28_PM.png]]
 * 1) You can also look this value up on the bill itself.
 * $0.19
 * 1) Compare the two values. Why are they different?
 * They are different because the first includes all taxes/fees and Rockland Electric's profit, whereas the second is just the sheer cost of the electricity.
 * Multiply the kWh you use per day by the cost per kWh to determine what you cost your parents.
 * [[image:Screen_shot_2012-02-27_at_1.54.32_PM.png]]


 * __ Evaluate __**
 * 1) Does the cost of electricity seem like a lot to you? Discuss this with your friends.
 * No, it doesn’t seem like a lot to me and is around the same, even lower, than most of my friends.
 * 1) Keep in mind that this cost only reflects ten appliances. What do you think your total cost actually is? Are there major appliances that you may have excluded from your list of ten that would make a big difference?
 * I don’t think there are any other appliances that would really add all that much to my total kWh usage, therefore I wouldn’t predict that I cost all that much more for my parents per day.
 * 1) If you think your consumption is a lot – what can you change? Do you need/should change?
 * Although I don’t think my consumption is that much, I could change it by not keeping my phone and laptop plugged in all night because it is still pretty wasteful since it only takes about an hour for each to fully charge.

This activity actually really surprised me. I had never previously looked at one of my parent’s electric bills, and I assumed electric was far more expensive than it actually is. Though, I can also see where electric bills could start to add up if it is wasted instead of conserved. Some people may leave unnecessary lights, televisions, radios, etc. on all day and night. Although this may not amount to all that much money in the end, it is still so wasteful and can’t possibly be good for the environment.