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Chapter 3 Energy: Notes - Chapter 3 Energy Work 3.1 Work...

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Chapter 3: Energy Work 3.1 Work done on the object is defined as the products of the applied force and the parallel distance through which the force acts: work = force * distance W = Fd Mechanical work is the products of a force and the distance an object moves as a result of the force. There are two important considerations to remember about this definition: (1) something must move whenever work is done, and (2) movement must be in the same direction as the direction of the force. Ex: moving a book to a higher shelve in a book case. Applying vertically upward force equal to the weight of the book. The work done on the book can be calculated by multiplying the weight of the book by the distance it was moved. Units of Work In the metric system a force is measured in newtons (N) and distance is measured in meters (m) so the unit of work is: W = Fd W = (newton)(meter) W = (N)(m) Newton-meter is a unit of work. This unit name is called a joule (J) 1 joule = 1 newton meter The units for a newton are kg*m/s² and the unit for a meter is m . Therefor the units for a joule are kg*m²/s² . In the English system force is measured in pounds and distance in feet. Ft*lb does not have its own name like the N*m does. Power 3.2 Lifting of weight (force exerted) the vertical height of the stairs (distance through which the force is exerted). A person who weighs 120 lb and climbs a stairway with a vertical distance of 10 ft will do (120lb)(10ft) or 1,200 ft*lb of work. The amount of work will not change if the person were to run up the stairs because the same amount of work is accomplished. The same amount of energy is used, but at a greater rate when running. The rate that energy is transformed or the rate that the work is done is called power . Power is measured as work per unit of time, power = work time p = W t
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Considering just the work and time factors a 120lb person who ran up the 10ft stairs in 4 seconds would have a power rating of P = W = (120 lb)(10 ft) = 300 ft*lb t 4 s s When the steam engine was created by James Watt a new measurement of horsepower was created. Horsepower has a power rating of 550 ft*lb/s. To convert a power rating in English units to horsepower simply divide the power rating by 550 ft*lb/s/hp. Example: the 120 lb person has a power rating of 400 ft*lb/s / 550 ft*lb/s/hp, or 0.7hp The metric system measures power in joules per second. The unit J/s, has a name. It is called a watt (W). The watt is used with metric prefixes for large numbers: 1,000 W = 1 kilowatt (kW) and 1,000,000 W = 1 megawatt (MW). 746 W is equal to 1 horsepower. One kilowatt is equal to about 1 1/3 horsepower.
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