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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 y-direction 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 object-earth # ! PE object-earth < 0 KE object " ! KE object > 0 W...
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- Summer '09