18. We place the reference position for evaluating gravitational potential energy at the relaxed position of the spring. We use xfor the spring's compression, measured positively downwards (so x> 0 means it is compressed). (a) With x= 0.190 m, Eq. 7-26 gives 217.22 J7.2 J2sWkx=−≈−for the work done by the spring force. Using Newton's third law, we see that the work done on the spring is 7.2 J. (b) As noted above,
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