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Unformatted text preview: W A Relation between Open Systems and Closed Systems This only works for some interactions (e.g. gravitational, springs, electrical, and molecular) but not for friction, air resistance, or other “dissipation” interactions. Suppose the earth exerts a force on an object (this is the force of gravity) and we take the positive ydirection to be up (this is the usual choice), then the “weight” of the object is F by earth on object = (m object g) This force on the object points straight down (g = 9.8 N/kg is the gravitational field of the earth) So W to object = F by earth on object ! y object = m object g ! y object Open system: Energy is transferred from the earth to increase the kinetic energy of the object ! KE = W so ! KE = m g ! y Closed system: Loss of potential energy is equal to gain in kinetic energy ! KE + ! PE = 0 so ! KE + m g ! y = 0 (same answer) KE object " ! KE object > 0 PE objectearth # ! PE objectearth < 0 KE object " ! KE object > 0 W...
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This note was uploaded on 10/22/2010 for the course PHY 7A 56192 taught by Professor ? during the Summer '09 term at UC Davis.
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