A spring demonstrates some basic engineering concepts:
Do some work, compress a spring –
Strong muscles are essential.
The energy you give the thing
Has lots of new potential.
The rate at which you do the work
(A megajoule per hour?),
Provided that you do not shirk,
Will demonstrate your power.
Now make the spring propel some rocks.
This could be quite frenetic:
The energy the spring unlocks
Is dangerously kinetic.