POWERPOWER

Indice Pagina 2 POWER In physics, Power is an important quantity. First I will give you a pratical example and then I will define it. Suppose we have two people, named Luigi and Maria, that do the same work, that is carryng 10 boxes from one place to another (the distance is the same). Let’s suppose that Luigi carries two boxes at a time, while Maria, being a girl, carries only one box at a time. It is evident that Luigi will do the same amount of work as Maria in less time. Another example Climbing the stairs quickly (picture 1) takes the same physical work as when you go to a normal pace (picture 2). In both cases, in fact, the muscles of the legs must develop a force that contrasts the gravity for the difference in level. What changes is the speed with which you perform the work.

Indice Pagina 3 Now we will give the definition: POWER is the physical quantity that takes into consideration both work and time; That is It is called Power the physical quantity that considers the work done and the time needed. In symbols: If we measure the work in Joule and the time in seconds, Power is measured in Watt or in its multiples as Kilowatts (KW): 1 KW = 10 3 Watt = Watt The watt (W) is the SI unit of power: it is equal to one joule (J) of work performed per second (s) Another unit of misure of the power, we don’t use anymore, is the horse power (in symbo H P) 1 HP = 735 Watt = 0,735 KWatt

Indice Pagina 4 Example What is the Power, if a force produces a Work of 60 J in a time of 10 sec? In symbols: Example Consider two engines A and B, the former produced work of 40 J in a time of 10 seconds, the latter a work of 20 J in a time of 5 seconds. What can you say about the power of the two engines? a) P A > P B b) P A = P B c) P A < P B Solution: whereas so, the answer is: P A = P B

Indice Pagina 5 Typical power outputs Washing machine motor 250 W Athlete 400 W Large jet engine75∙10 6 W

Indice Pagina 6 Some information about James Watt’s life James Watt was born on 19 January 1736 in Greenock, in Scotland. Both his parents were Presbyterians: his father was a shipwright, while his mother, Agnes Muirhead, came from a distinguished family. Initially Watt was mostly schooled at home by his mother but later he attended Greenock Grammar School. He exhibited great manual dexterity, engineering skills and an aptitude for mathematics, while Latin and Greek failed to interest him.Greenock Presbyteriansshipwrightschooled at homemathematicsLatinGreek In 1764, Watt married Margaret Miller, with whom he had five children. His wife died and in 1776 he was married again, to Ann MacGregor, with whom he had two other children. He died on 19 August 1819 at his home Heathfield. Watt began to be interested in steam engines and in order to reduce the waste of energy he invented a regulator ( it is still called James Watts regulator) to maintain uniform the speed of the steam engine and found a way to transform the alternating rectilinear motion of the piston in the continuous rotary motion. The Watt, SI unit of Power; takes the name after him.Watt