8 25 1 reference frames and the classical Galilean relativity = n=n =a a=a, F na=n( I'=F Descriptions of what happens as a result of the laws of mechanics may different from one inertial reference frame to another, R but the laws o mechanics are the l same 825.2 the need for change and the postulates of the special theory 1. Why we need relativity theory troubles with our ideas about time The pion T or n created in a high-energy particle accelerator is a very unstable particle. Its lifetime at rest is 26.0 ns; when it moves at a speed of v=0.913c, an average distance of D=17.4 m are observed before decaying in the laboratory. We can calculate the lifetime in this case by D/v=63.7 ns, it is much larger than the lifetime at rest Such an effect cannot be explained by Newtonian physics.7 F ma m a F a a m m = = ′ ′ = ′ = ′ = ′ r r r r r r z z y y x x a a a a a a ′ = ′ = ′ = Descriptions of what happens as a result of the laws of mechanics may different from one inertial reference frame to another, but the laws of mechanics are the same. §25.1 reference frames and the classical Galilean relativity §25.2 the need for change and the postulates of the special theory 1. Why we need relativity theory? 1troubles with our ideas about time The pion π+ or π- created in a high-energy particle accelerator is a very unstable particle. Its lifetime at rest is 26.0 ns; when it moves at a speed of v= 0.913c, an average distance of D=17.4 m are observed before decaying in the laboratory. We can calculate the lifetime in this case by D/v=63.7 ns, it is much larger than the lifetime at rest. Such an effect cannot be explained by Newtoneian physics!