y ∠U=U-U1=-W=-F.d mg 2. The potential energy of gravity For the ball-Earth system, We take upward direction to be y positive direction △=((2)-U()-(mg)dhy=mg(y2-y1) J(2=U(D+mg(2-yi), dependent on U(y) The physically important quantity is AU, not U(2)orU) If We set U(=0)=0(the reference zero point of U is at O) We have u()=mgy (129) Ch 12 Energy II: Potential energyCh.12 Energy II: Potential energy 2. The potential energy of gravity For the ball-Earth system, we take upward direction to be y positive direction ( ) ( ) ( ) ( ) 2 1 2 1 2 1 U U y U y m g dy m g y y y y  = − = − − = −  The physically important quantity is , ΔU not or . ( ) 2 U y ( ) 1 U y U(y1 = 0) = 0 U( y) = mgy (12-9) If We set We have (the reference zero point of U is at O) y2 y1 y  ΔU =U −U = −W = − F ds f i   mg ( ) ( ) ( ) 2 1 2 1 U y =U y +mg y − y , dependent on ( ) 1 U y