The Work Done by an Applied force
Now suppose that we displace the block along the 
axis while continuing to apply a force 
to it. During the displacement, our applied force does work 
on the block while the spring force does work 
. By Eq. 7-10, the change 
in the kinetic energy of the block due to these two energy transfers is
(7-27)
in which Kf is the kinetic energy at the end of the displacement and Kt is that at the start of the displacement. If the block is stationary before and after the displacement, then Kf and Kt are both zero and Eq. 7-27 reduces to
(7-28)
If a block that is attached to a spring is stationary before and after a displacement, then the work done on it by the applied force displacing it is the negative of the work done on it by the spring force.
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