Simple Nature - Light and Matter

c / Left: In a frame of referencethat speeds up with the truck, thebowling ball appears to changeits state of motion for no reason.Right: In an inertial frame of reference,which the surface of theearth approximately is, the bowlingball stands still, which makessense because there is nothingthat would cause it to change itsstate of motion.The principle of Galilean relativity states that inertial framesexist, and that the same laws of physics apply in all inertial framesof reference, regardless of one frame’s straight-line, constant-speedmotion relative to another. 4Another way of putting it is that all inertial frames are createdequal. We can say whether one inertial frame is in motion or at restrelative to another, but there is no privileged “rest frame.” Thereis no experiment that comes out any different in laboratories indifferent inertial frames, so there is no experiment that could tell uswhich inertial frame is really, truly at rest.The speed of sound example 3⊲ The speed of sound in air is only 340 m/s, so unless you liveat a near-polar latitude, you’re moving at greater than the speedof sound right now due to the Earth’s rotation. In that case, whydon’t we experience exciting phenomena like sonic booms all thetime? ⊲ It might seem as though you’re unprepared to deal withthis question right now, since the only law of physics you knowis conservation of mass, and conservation of mass doesn’t tellyou anything obviously useful about the speed of sound or sonicbooms. Galilean relativity, however, is a blanket statement aboutall the laws of physics, so in a situation like this, it may let youpredict the results of the laws of physics without actually knowingwhat all the laws are! If the laws of physics predict a certain valuefor the speed of sound, then they had better predict the speedof the sound relative to the air, not their speed relative to some4 The principle of Galilean relativity is extended on page 190.64 Chapter 1 Conservation of Mass