Book excerpt: Forces and motion are at work all around you—when you kick a ball, ride a bike, or drop a book. But what exactly are forces? And how do they make things move? For thousands of years, scientists have been testing hypotheses about forces and motion and learning from the results of their experiments. These lead to more questions: Why do you speed up when you're biking down a hill? Why are heavier objects harder to pull? Why doesn't the moon float off into space? The answers may surprise you! Find out how forces and motion are at play with everything on Earth and beyond.