My motivation for studying Hopf algebras is twofold:      I'm interested in representation theory and the algebraic structure of group rings.   I will also take a course on algebraic groups soon and I think it would be helpful to know about Hopf algebras for that.   So I'm looking for a good introductory book on Hopf algebras for self-study and especially for one that has many examples and applications to the two topics mentioned above.  (As for my background, I know abstract algebra,  commutative algebra and non-commutative algebra at a basic graduate level, I'm familiar with the basics on representation theory of (mostly finite) groups and category theory (including monoidal and symmetric monoidal, but not braided monoidal categories) and have some rudimentary knowledge of algebraic geometry. I know almost nothing about Lie algebras.)
I want to learn more about hopf algebras but I am having trouble finding a down to earth introduction to the subject with lots of motivation and examples. My algebra knowledge ranges from Dummit and Foote, Atiyah-Macdonald commutative algebra, and Hartshorne level algebraic geometry.  What are good places to look at for a hopf algebra beginner? My main motivation is for studying groupoid schemes from an algebraic point of view.
Take a sponge ball and compress it. The net force acting on the body is zero and the body isn't displaced.  So can we conclude that there is no work done on the ball?