(1) Two identical particles with rest mass 4 kg approach each other with equal but oppositely directed momenta. УА S u = 0.6c u = –0.6c m = 4 kg m = 4 kg (a) What is the momentum of the system made up of the two particles? (b) Is the rest mass of the system 8 kg? (2) What is the principle of equivalence? (3) Is the inertial mass equal to the gravitational mass?