Mass (say) 1 kilogram. Then we can measure the mass of any other object in the universe by colliding it with the reference object and measuring the accelerations. Additionally, mass relates a body's momentum p to its linear velocity v: and the body's kinetic energy "K" to its velocity: The primary difficulty with Mach's definition of mass is that it fails to take into account the potential energy (or binding energy) needed to bring two masses sufficiently close to one another to perform the measurement of mass. This is most vividly demonstrated by comparing the mass of the proton in the
Magnetic reconnection where formula_18 is the mass density of the plasma. Solving for the outflow velocity then gives where formula_20 is the Alfvén velocity. With the above relations, the dimensionless reconnection rate formula_21 can then be written in two forms, the first in terms of formula_22 using the result earlier derived from Ohm's law, the second in terms of formula_23 from the conservation of mass as Since the dimensionless Lundquist number formula_25 is given by the two different expressions of formula_21 are multiplied by each other and then square-rooted, giving a simple relation between the reconnection rate formula_21 and the Lundquist number