|MadSci Network: Physics|
This is a rather philosophical question. Galileo and later Newton proposed as a physical law that a body will continue in motion, or will remain still, until a force changes its state. This is the way Nature works. Asking why is a very tricky question and can be answered in many ways. For example, suppose that observer "1" sees that an object moves in a vacuum, so that it never bumps into something. That would be perpetual motion, isn't it? Now suppose that observer "2" moves at exactly the same speed as the object and moves along it. This observer would say: "This object is not moving. It is permanently at rest. You are the ones who are moving in the opposite direction, and that's why you think the object is moving perpetually." Who is right? Physics says that both observers are equivalent. Nevertheless the laws of physics that observers "1" and "2" can determine about the object must be the same. Thus perpetual motion is compatible with physical laws, and even necessary if they are to make sense. I recommend that you read "The character of physical law" by Richard Feynman for further insight into how physical laws are made. Of course in real life, we never have a perfect vacuum. If I throw something, it will stop at some point because the force of friction with the air or floor will act against its movement. The explanation is that its energy is being transformed into heat due to that friction, and hence it is lost. Of course it can bounce off a wall and keep moving a little longer if it is made of some elastic material that looses little energy when it is deformed. What then keeps atoms moving? Atoms can be very elastic so to speak, or they can absorb energy very efficiently depending on the kind of collision they suffer. An atom that moves in a perfect vacuum won't stop moving. If, on the other hand, the atom moves within a container with lots of atoms, when it collides with an atom of the wall of the container or with a free atom within the container, it can recoil with the same energy, or it can pass its energy to the other atom and slow down, or it can receive more energy from the other atom and speed up. If there are enough atoms around, some atoms will speed up and some will slow down in every collision, so that the average energy, and the total energy of the container stay the same. We call this motion "heat", and as you know, a well insulated container will not cool down or heat up easily, like a thermos bottle, which means that atoms are exchanging energy all the time, but the energy of the whole stays about the same. It is possible to reduce the motion of atoms by cooling the container and it is possible to speed them up by reheating the container. If we could prevent the container from cooling down, the atoms in it would be loosing or gaining energy in every collision, but on average their total energy content would be the same. Their perpetual motion would be due to the fact that the energy in the container would never be lost, but then we would have the perfect thermos bottle, and this is impossible. We always have some kind of heat leak. I hope this is a satisfactory answer. Vladimir Escalante Ramirez
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