The Physics Of The Heavy Bag

I know that for every action, there is an equal and opposite reaction. I also understand that an object at rest tends to stay at rest unless acted upon by an outside force. I am that action. I am that outside force. I know this bag of sand and rags never did a thing to me, but I feel the need to pummel it as if it had. It's exercise.
When I was in high school, I didn't have a punching bag. I used to hit walls. And doors. And lockers. I punched a whole lot of lockers. The thing about lockers: not much give. They're made of metal, and though I left my mark on more than a few of them, my knuckles always fared worse. That was okay with me. There was a certain amount of bravado and macho posturing that such demonstrations of force provided. I may not have been much of a tough guy, but those lockers cowered when they heard my approaching footsteps.
Or so I like to think. My parents bought me a punching bag. My dad put it up in the garage. It was a speed bag, the kind that boxers use to practice their speed and rhythm. Most of my youthful angst could not be channeled into the constructive enterprise of learning a skill. That wasn't what I was after, exactly. What I had in mind was not the steady rat-a-tat-tat, but great big thudding blows, like Rocky in the meat locker. I wanted to hammer something.
Little by little, that feeling left me. It became clear, over the years, that all those people who said that hitting inanimate objects was a waste of my time and energy were correct. Disciplining all those doors and lockers just got me sore hands. Nothing else.
And so years passed and I got more comfortable in my skin. I let the angst go. I became a grown-up. I became a grown-up with a wife who had a heavy bag on her Amazon wish list. She used to practice martial arts: she knows Kung Fu. For Valentine's Day, I bought her a heavy bag, and we hung it in the garage. It only took me a couple weeks to wear out the pair of gloves we bought to share. I got my own, and I have been working on wearing them out as well. These are sturdier, and have stood up to the pounding I'm giving.
Only now I'm not trying to prove anything. I'm not punching through anything. I'm exercising. I'm thinking about physics.