Effective mass (solid-state physics) from both sides gives where is the th element of , is the th element of , and are the th and th elements of , respectively, and is the total energy of the particle according to the Planck–Einstein relation. The index is contracted by the use of Einstein notation (there is an implicit summation over ). Since Newton's second law uses the inertial mass (not the gravitational mass), we can identify the inverse of this mass in the equation above as the tensor This tensor expresses the change in group velocity due to a change in crystal momentum. Its
Biomechanics of sprint running ground forces is also very important. Weyand et al. (2000) came up with the following equation for determining sprint velocity: where formula_2 is the sprint velocity (m/s), formula_3the step frequency (1/s), formula_4 the average force applied to the ground (N), formula_5 the body weight (N), and formula_6 the contact length (m). In short, sprint velocity is reliant on three main factors: step frequency (how many steps you can take per second), average vertical force applied to the ground, and contact length (distance your center of mass translates over the course of one contact period). The formula was tested by having