Nobody wants to deal with so many interactions. Instead, in physics, we treat the ball as one thing—and that’s basically fine. However, in order to make sure that other people understand what we are doing when we model an interaction, we must define our “system”. Perhaps, for simplicity, we will decide that the system is just the ball itself. If so, we are only dealing with the ball’s momentum and any forces caused by external interactions, and we can ignore all of these atom-atom interactions. We could even forget about the interaction between the fuzzy surface of the ball and its inner rubber part.
It is also possible to have a system consisting of more than one object. Imagine a tennis ball attached to a soccer ball with a string. If I wanted to use a system consisting of both balls, then I would consider only forces due to external interactions. I would not include the force with which the string acts on any of the balls.
For the momentum of this system, I would use its total mass, which is the sum of the masses of the balls and the velocity of the center of mass of the system. Since the soccer ball has a lot of mass, this center of mass will be closer to it along the string and further away from the tennis ball.
Guess what? People are also made of matter, and a person also has a center of mass. But people’s physics can get confusing as they can change shape. Different parts, such as arms and legs, may be positioned differently. However, a good ballpark estimate is that the center of mass for a standing person is somewhere between the navel and the spine. For a person in a seated position, their bent legs will move their center of mass a little closer to their chest.
Bruce Lee system plus purpose
From the point of view of physics, any impact can be difficult. So let’s keep things as simple as possible by considering a 1-inch punch for a one-punch and one-punch system. Let’s call them Bruce and Joe, respectively, because famous video Bruce Lee beats martial artist Joe Lewis at an exhibition.
With this system, we can ignore any forces due to internal interactions. Yes, this means that we don’t really need to look at the force of a 1 inch impact. It is an interaction between two entities in the same system (Bruce and Joe).