Toss your keys along the floor, they’ll skitter along for a bit as friction does negative work on them, reducing their kinetic. Quickly, the keys come to rest, and the kinetic energy you gave them is gone. But in this case they can’t turn around (as they did when you picked them up), gather speed from the same frictional force that acted to slow them, and jump back into your hand with the same kinetic energy. Once the motion stops, the friction stops, and it can not restore the kinetic energy of the keys as gravity is able to. | 66 Unit Ten ENERGY READING PASSAGE Friction Internal energy and Heat Toss your keys along the floor they ll skitter along for a bit as friction does negative work on them reducing their kinetic. Quickly the keys come to rest and the kinetic energy you gave them is gone. But in this case they can t turn around as they did when you picked them up gather speed from the same frictional force that acted to slow them and jump back into your hand with the same kinetic energy. Once the motion stops the friction stops and it can not restore the kinetic energy of the keys as gravity is able to do. Frictional forces don t store energy in the form of potential energy. But all the kinetic energy that you gave the keys when you tossed them doesn t just disappear. The keys skid across the surface of the floor scraping and catching. The affected surface molecules of the keys and the floor are pushed through some tiny distance giving them extra kinetic energy. Molecules throughout any solid object bounce around in all directions even though they are held in place by their bonds with the molecules around them. When molecules of the keys and the floor strike each other however they bounce around even faster. The affected molecules slam into the nearest neighbors in all directions and these too move a bit faster. All the organized kinetic energy the keys had just before they hit the floor disappears and most of it scatters aimlessly among the molecules. That chaotic energy now is part of the energy stored in the matter. We call that energy the internal energy of the keys and the floor. The internal energy in matter can be thought of as the sum of the kinetic and potential energies of all of its molecules including as we see energy called chemical gravitational and nuclear and energy associated with the presence of mass itself. When the molecules of matter move faster the matter becomes warmer that is its temperature increases. But all the keys original kinetic energy doesn t just go .
