The absolute meter-tonne-second system is a system of mechanical units. The meter is the base unit of length or distance, the tonne is chosen to be a unit of mass, and the second is the base unit of time. All other mechanical quantities, and specifically force, are expressed in terms of combinations of these three units. In absolute systems, Newton's second law is expressed as F = ma, and since acceleration is the second derivative of a distance with respect to time, if the unit of length or distance is denoted by L, the unit of mass by M, and the unit of time by T, the unit of force becomes a derived unit of dimensions MLT−2, in this case tonne·metersecond2, which is known as the sthène.