5.18 (a) johnston and gilman also observed that, at a constant temperature, the dislocation velocity obeyed a power law. assuming that the dislocation velocity exponent, m, is 16.5 and that the stress, d, for a velocity of 1 cm/s is 5.30 mpa, determine the stress in mpa needed to obtain a velocity that is 5 times greater. (b) also give your answer in psi.