Elastic Potential Energy
We next consider the block-spring system shown in Fig. 8-3, with the block moving on the end of a spring of spring constant 
. As the block moves from point 
to point 
, the spring force 
does work on the block. To find the corresponding change in the elastic potential energy of the block-spring system, we substitute 
for 
in Eq. 8-6. We then have
or 
(8-10)
To associate a potential energy value 
with the block at position x, we choose the reference configuration to be when the spring is at its relaxed length and the block is at 
. Then the elastic potential energy 
is 0, and Eq. 8-10 becomes
which gives us
(elastic potential energy). (8-11)
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