Workshop 3 Web Highlights "Energy" Scientists often use the concept of energy to think about how objects and systems behave. They would say that the moving car possesses kinetic energy and the stretched rubber band possesses potential energy. Rather than talk about the forces involved, as we have done in these workshops, we could describe the students' rubber-band car experiments in terms of energy. The potential energy in the stretched rubber band is changed into the kinetic energy of the car's motion. Friction dissipates the car's energy and brings it to rest. http://www.physicsclassroom.com/Class/energy/U5L1b.html http://www.physicsclassroom.com/Class/energy/U5L1c.html "Forces Affecting Motion" As Sallie and Katy indicate, there are many forces acting on the rubber-band-powered cars. Since the car has mass, its weight is that force that acts downward toward the center of the Earth. The floor is also pushing up with an equal and opposite or normal force. We could even consider the weight of the atmosphere acting on the car. Rather than clutter our diagrams with all the forces acting, we chose to focus on the forces that act along the line of the car's motion. "Friction" Leonardo da Vinci was the first to study friction systematically. He developed some ideas about this force that scientists still use. Da Vinci distinguished between sliding and rolling friction and also studied the effects of lubrication. Regardless of the type, friction always opposes motion. Friction is the resistance to the sliding, rolling, or flowing motion of a substance due to its contact with another substance. We sometimes see the term "static friction," which refers to the greater force required to start an object sliding than that needed to keep it sliding along a surface. Rolling friction is caused primarily by the small indentations formed as one surface rolls over another. Actually friction is what makes an object roll down on an incline. If friction were not acting, the object would slide down the incline. Air resistance is really a type of fluid friction, since both liquids and gases are considered fluids. For more on friction, visit the following Web sites: http://www.le.ac.uk/education/centres/sci/selfstudy/fforce.html