Ch6_malleys

=Work, Energy, and Power= toc

Definition and Mathematics of 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. Mathematically, work can be expressed by the following equation. 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. **To Do Work, Forces Must //Cause// Displacements** A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!The horizontal component is found by multiplying the force F by the cosine of the angle between F and d. In this sense, the cosine theta in the work equation relates to the //cause// factor - it //selects// the portion of the force that actually causes a displacement. **The Meaning of Theta** When determining the measure of the angle in the work equation, it is important to recognize that the angle has a precise definition - it is the angle between the force and the displacement vector. **The Meaning of Negative Work** On occasion, a force acts upon a moving object to hinder a displacement. These situations involve what is commonly 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. **Units of Work** **The Joule is the unit of work.****1 Joule = 1 Newton * 1 meter****1 J = 1 N * m**

Calculating the Amount of Work Done by Forces
The work is subsequently calculated as force•displacement•cosine(theta) where //theta// is the angle between the force and the displacement vectors.

=The Work-Energy Relationship=

Internal vs. External Forces
Forces can be categorized as internal forces or 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. The importance of categorizing a force as being either internal or external is related to the ability of that type of force to change an object's total mechanical energy when it does work upon an object. 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. Under such circumstances, 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 sometimes referred to as **nonconservative forces**. When the only type of force doing net work upon an object is an internal force, 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, yet the total amount of mechanical is conserved. Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are sometimes referred to as **conservative forces**.

For the first activity, I got 4/5 correct, and I was immediately able to recognize my mistake on the one that I got wrong. For the second activity, I got all of the answers correct! Either way, it was nice to see the explanation there, just in case I was a little confused on something.

=Power Costs Activity = The cost of electricity doesn't seem very high to me. Especially considering the amount we use it, I think it comes at a very reasonable price. On these 10 objects, I cost my mom about $0.38 a day; this adds up to about $11.40 a month. I don't think that number is unreasonable, either. Of course, there are always things that I could do to save my mom some money - turn off the lights when I'm not in the room, unplug things when I'm not using them, etc. But all things considered, I think the amount we pay for electricity is fair.