LECTURE 6 RELATIVISTIC MOMENTUM AND ENERGY PHYS 420-SPRING 2006 Dennis Papadopoulos

Fig. 2-1, p. 42

Fig. 2-1a, p. 42

Fig. 2-1b, p. 42

What is energy? The capability to do WORK ->W What is work ?

Where did the 1 on the right hand side come from? It's the starting value of the integral. Now all of the terms in this equation are energies. When  > 1, we have non-zero kinetic energy. So, if we think of  m o c 2 as the total energy of body, and write  m o c 2 = m o c 2 +K then (   1)m o c 2 is the kinetic energy, and m o c 2 is an energy that a body has when v = 0 and  = 1. Remember the proper time and proper length. These were the time and length of a body measured in its own frame. So we could write E=E o +K where E 0 = mc 2 is the proper energy of a body - the energy that it has, even when it is not moving.

Fig. 2-2, p. 42

FISSION One case of the fission of 236 U. The net mass of the initial neutron plus the 235 U nucleus is 219,883 MeV/c 2. The net mass of the fission products (two neutrons, a 95 Mo nucleus and a 139 La nucleus) is 219,675 MeV/c 2 - smaller because of the stronger binding of the Mo and La nuclei. The "missing mass'' of 208 MeV/c 2 goes into the kinetic energy of the fragments (mainly the neutrons), which of course adds up to 208 MeV.

PAIR PRODUCTION Electron-positron PAIR PRODUCTION by gamma rays (above) and by electrons (below). The positron (e + ) is the ANTIPARTICLE of the electron (e - ). The gamma ray (  ) must have an energy of at least MeV [twice the rest mass energy of an electron] and the pair production must take place near a heavy nucleus (Z) which absorbs the momentum of the .