By Glen Phelan
Forces: A World of Pushes and Pulls guides students as they explore how forces affect the motion of objects. Students discover that an object at rest can move only when a force acts on it. They learn that a moving object continues moving at the same speed and in the same direction unless the combination of all forces acting on it—the net force—is greater than zero. Students learn that the size of the force on an object and the mass of the object affect the rate at which the object moves. At the end of each two-page spread, a brief statement called The Bottom Line reinforces students’ understanding by summing up the key ideas about forces covered in those pages.