Little changed over the following two millennia. Eventually, Isaac Newton (1643–1727) laid down fundamental physical laws involving mass, force, and momentum, and his second law of motion made it possible to derive the basic energy units. Using modern scientific units, 1 joule is the force of 1 newton—that is, the mass of 1 kilogram accelerated by 1 m/s2 acting over a distance of 1 meter.27 But this definition refers only to kinetic (mechanical) energy, and it certainly does not provide an intuitive understanding of energy in all of its forms.
