No one wants to deal with that many interactions. Instead, in physics, we treat the ball as a thing—and that’s mostly okay. However, to ensure that others understand what we are doing when modeling interactions, we must define our “system”. Perhaps, for the sake of ease, we decided the system was just a ball. If so, we only deal with the momentum of the ball and any forces due to external interactions, and we can ignore all those atom-atom interactions. We can even forget about the interaction between the ball’s matte surface and its rubber interior.
It is also possible to have a system consisting of many objects. Imagine a tennis ball tied with a string to a soccer ball. If I wanted to use a system that included both balls, I would consider only the forces due to external interactions. I will not include the force the rope exerts on either ball.
For the momentum of this system, I will 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 more mass, this center of mass will be closer to it along the string and further away from the tennis ball.
Courtesy of Rhett Alllain
Guess? Man is also made of matter, and man also has a center of mass. But human physics can get messy, as they can change shape. Different parts, like arms and legs, can be positioned differently. However, a good rough estimate is that a standing person’s center of gravity is somewhere between their navel and spine. For a person in a sitting position, their bent legs will move their center of gravity a little closer to their chest.
Bruce Lee system plus goal
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 punch and one punch. Let’s call them Bruce and Joe, because there are a famous video Bruce Lee punching boxer Joe Lewis at an exhibition.
With this system, we can ignore any forces due to internal interactions. Well, that means we don’t really have to consider the force from the 1-inch punch. It is the interaction between two objects in the same system (Bruce and Joe).
