OpenStax College Physics for AP® Courses, Chapter 4, Problem 30 (Test Prep for AP® Courses)

This is College Physics Answers with Shaun Dychko. When this ball hits the ground, the question is, what fundamental force explains the fact that it rebounds back up? Or why does the ground exert a force on this ball when it hits the ground? We can determine the answer by a process of elimination if you want. We can certainly say that gravity is not the reason because gravity is strictly an attractive force between bodies, and between a small object like the ball and the ground, gravity, it's big enough that it makes the ball fall certainly, because the ground is a huge thing called the earth. But the fact that the gravity is always attractive means that you should expect the ball to stay touching the ground, but instead we see that the ball bounces back up. That cannot be explained by gravity because gravity only attracts. That's gravity taken off the list of possibilities. Then there's the strong nuclear force. Well, the name gives a bit of a clue that it can't be the answer because it's a nuclear force and which is to say that it acts on a distance scale of a nucleus. It only works for things that are super, super, super, super close, and only works on nucleons within a nucleus. It's also strictly an attractive force. Those are two reasons why the strong nuclear force cannot explain this repulsion of the ball from the ground. There's also this weak nuclear force but that's a bit of a strange one and it's also attractive only and it is on short distance scales as well. That's not going to be it, which leaves us with only one remaining possibility, that's the electromagnetic force. That is the last remaining fundamental force that we could consider. Just based on process of elimination, this one has to be it. But we can also check to make sure it is logical. Can the electromagnetic force be repulsive? Yes, it can. Because if you have equal charges close to each other or even farther from each other, it doesn't matter the distance. And that's important because it does act on large distances. Then it can be repulsive, so that works. That is in fact what's happening here. We have atoms within the ball, getting really close to atoms within the floor. As the squishing tries to push those atoms closer together, the like charges of them repel each other. It's mostly the electrons that are concerned here because the the protons are buried inside the nucleus, but it's the electrons that are on the outside and on a shell of the atom that are interacting with each other. The electrons are are repelling each other from adjacent atoms and that is the explanation for this bouncing.