No one wants to deal with that many interactions. Instead, in physics we treat the ball as one thing—and that’s mostly fine. However, to make sure that other people understand what we are doing when we model an interaction, we have to define our “system.” Perhaps, to make it easy, we decide the system is just the ball itself. If so, we deal only with the ball’s momentum and any forces due to external interactions, and we can ignore all those atom-atom interactions. We could even forget about the interaction between the ball’s fuzzy surface and its interior rubber part.
It’s also possible to have a system that consists of more than one object. Imagine a tennis ball attached by a string to a soccer ball. If I want to use a system consisting of both balls, then I would only look at forces due to external interactions. I wouldn’t include the force the string exerts on either ball.
For the momentum of this system, I would use its total mass, which is the sum of the mass of the balls, and the velocity of the center of the system’s mass. Since the soccer ball has a larger mass, this center of mass would be closer to it along the string and farther from the tennis ball.
Courtesy of Rhett Allain
Guess what? Humans are also made of stuff, and a person also has a center of mass. But the physics of humans can get messy, since they can change shape. Different parts, like arms and legs, can be positioned differently. However, a good rough estimation is that the center of mass for a standing person is somewhere between their belly button and their spine. For a person in a sitting position, their bent legs will move their center of mass a little closer to their chest.
The System of Bruce Lee Plus the Target
From a physics perspective, any punch can be complicated. So let’s make it as simple as possible by considering the 1-inch punch for a system consisting of one puncher and one punchee. Let’s call them Bruce and Joe, respectively, since there’s a famous video of Bruce Lee punching martial artist Joe Lewis at an exhibition.
With this system we can ignore any forces due to internal interactions. Yes, that means that we don’t actually have to look at the force from the 1-inch punch. It’s an interaction between two objects in the same system (Bruce and Joe).