A particle of rest mass m is moving with a velocity v\(\hat{k}\), with respect to an inertial frame S. The energy of the particle as measured by an observer S', who is moving with a uniform velocity u\(\hat{i}\) with respect to S (in terms of Y_u = 1/\(\sqrt{1-u^2/c^2}\) and Y_v = 1/\(\sqrt{1-v^2/c^2}\) is