81. (a) From Eq. 22-38 (and the facts that ±±ii=1⋅and ji=0⋅), the potential energy is ()( )3026ˆˆˆ3.00i4.00j 1.24 10C m4000 N C i1.49 10J.UpE−−⎡⎤⎡⎤=− ⋅ =−+×⋅⋅⎣⎦⎣⎦=−×GG(b) From Eq. 22-34 (and the facts that ii0×=and ±± ±ji=k×−), the torque is GGGτ=×=+×⋅ ××⋅−−pE300124 104000198 103026..
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