No one wants to deal with that many interactions. Instead, physics treats the ball as one, and that’s mostly fine. But to make sure others understand what you’re doing when modeling your interactions, you need to define a “system”. Perhaps, for simplicity, the system determines that it is the ball itself. In that case, we can only deal with the momentum of the ball and the forces due to external interactions, and ignore any interactions between those atoms. I could even forget about the interaction between the ball’s fuzzy surface and the rubbery part inside.
It is also possible to have a system that consists of multiple objects. Imagine a tennis ball attached to a soccer ball with a string. If we want to use a system consisting of both balls, we only look at forces due to external interactions. Do not include the force exerted by the string on either ball.
For the momentum of this system, we use the total mass, which is the sum of the masses of the balls, and the velocity of the center of mass of the system. A soccer ball has more mass, so this center of gravity is closer to the soccer ball and farther along the string from the tennis ball.
Courtesy of Rhett Allan
guess what? Human beings are also made of things, and people also have a center of gravity. However, human physics can be disrupted because humans can change shape. Different parts such as arms and legs can be placed in different positions. However, a rough approximation is that the center of gravity of a standing person is somewhere between the navel and the spine. For a seated person, bending your legs will bring your center of gravity slightly closer to your chest.
bruce lee and the target system
From a physics standpoint, any punch can be complicated. So let’s keep it as simple as possible by considering a 1 inch punch for a system consisting of 1 puncher and 1 punch. There’s a famous video of Bruce Lee punching martial artist Joe Louis at an exhibition, so let’s call them Bruce and Joe respectively.
With this system, forces due to internal interactions can be neglected. Yes, that means you don’t actually need to look up the power of a 1 inch punch. This is an interaction between two of her objects (Bruce and Joe) within the same system.
