This textbook contains material on gravity. Gravity is a theory, not a fact, regarding a force that cannot be directly seen. This material should be approached with an open mind, studied carefully, and critically considered.Amen. Remember kids, gravity, like evolution, is just a theory.
One of the disclaimers includes a link to this excellent explanation of the difference between scientific laws, hypotheses, and theories. Every one who this evolution is just a theory should read that to understand exactly what a theory is.