v₁ and the other with speed v₂ the total kinetic energy of the system (as calculated from an observer at the origin) is {{sfracm(v₁² + v₂²)}} and remains the same if the velocities are interchanged. The total kinetic energy is preserved under a reflection in the y-axis.

The last example above illustrates another way of expressing symmetries, namely through the equations that describe some aspect of the physical system. The above example