Work

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1. Work and Power

2. Work is another concept that has a special meaning in science that is different from your everyday concept. In everyday use, work is associated with a task to be accomplished or the time to be spent in performing task. However, the amount of work you do has nothing to do with how tired you feel in performing a certain task. A force only does when the forces causes an object to move . If the object does not move, no work is done .

3. There are two important considerations to remember about work: 1. Something must move whenever work is done. The movement must be in the same direction as the direction of the force.

4. Work is the product of the magnitude of the displacement and the component of the force in the direction of the displacement. Thus, W = Fd or W – Fd cos Where, F – magnitude of force (N) d – magnitude of displacement (m) W – magnitude of work done by the object (J) – angle between force and displacement  

5. Example 1. When a box is pushed at a horizontal distance of 0.5 m by a horizontal force of 10 N, how much work is done? Given: F = 10N d = 0.5 m W = F x d = 10 N x 0. 5 m = 5 Nm (since Nm = J) = 5 J The work done on the box is 5 J.

6. Example 2. Suppose you pull your schoolbag with a force of 30 N parallel to the ground to your classroom 20 m away, how much work have you done? Given : F = 30 N d = 20 m W = Fd = 30N ( 20m) = 600Nm = 600J

7. Example 3. A girl pushes a box at a horizontal distance of 0.7 m away. When the work is done on the box is 8 Joules, how much is the horizontal force? Given: d = 0.7 m W = 8 J f = ? Since W = fd and (Nm = J), so we have f = = = 11. 43 N The horizontal force is 11.43 N.  

8. Power Different people perform work at a different rate. Some work is done faster than others. The quantity that describes how much work is done in a period of time is called power . It is the rate of doing work. Therefore, if you climb the stairs in the shortest period of time, you are the most powerful among your classmates. According to the definition, power P is expressed as : P = (Equation 1)  

9. where W is the work done and t is the time it takes for the work to be done. Using the definition of work, you have: P = (Equation 2) The unit of power is joule/second. It is simply called watt (W). This unit is named after the Scottish inventor James Watt (1736-1819). Another unit of power commonly used especially in electric appliances is the horsepower ( hp ), which is equal to 746 W.  

10. Since work is measured in joule (J) and time in seconds (s), power is expressed in watts (W). This is in honor of James Watt, the inventor of steam engine. 1 Watt (W) = 1 Nm/s 1 joule per second (J/s) = 1 W 1 horse power (h/p) = 746 W 1,000 W = 1 kilowatt (kW) 1 kW = 1.34 hp

11. Example 1. Rose has a weight of 470 N. What is her power output in watts and in horsepower when she climbs a stairs of 4-m high in just 3.5 seconds? Given: w = 470 N d = 4.0 m t = 3.5 s Solution: Since Rose need a force at least equal to her weight, her power output would be: P = = = = 537.14285714 Nm/s = 537W Expressing in horsepower, you have: 537 W x = 0.42 hp