Specific force

Specific force is defined as the non-gravitational force per unit mass.

\mbox{Specific Force} = \frac{\mathrm{Force_{non-gravitational}}}{{\mathrm{Mass}}}

Specific force (also called g-force and mass-specific force) is measured in meters/second² (m·s-2) which is the units for acceleration. Thus, specific force is not actually a force, but a type of acceleration. However, the (mass-)specific force is not a coordinate-acceleration, but rather a proper acceleration, which is the acceleration relative to free-fall. Forces, specific forces, and proper accelerations are the same in all reference frames, but coordinate accelerations are frame-dependent. For free bodies, the specific force is the cause of, and a measure of, the body's proper acceleration.

The g-force acceleration is the same as the specific force. The acceleration of an object free falling towards the earth depends on the reference frame (it disappears in the free-fall frame, also called the inertial frame), but any g-force "acceleration" will be present in all frames. This specific force is zero for freely-falling objects, since gravity acting alone does not produce g-forces or specific forces.

Accelerometers on the surface of the Earth measure a constant 9.8 m/s^2 even when they are not accelerating (that is, when they do not undergo coordinate acceleration). This is because accelerometers measure the proper acceleration produced by the g-force exerted by the ground (gravity acting alone never produces g-force or specific force). Accelerometers measure specific force (proper acceleration), which is the acceleration relative to free-fall,[1], not the "standard" acceleration that is relative to a coordinate system.

Hydraulics

In open channel hydraulics, specific force (F_s) has a different meaning:

F_s = \frac{y^2}{2} %2B \frac{q^2}{gy}

where q is the discharge per unit width (q = Q/B) and y is the flow depth.

See also

References

  1. ^ www.strapdownassociates.com/Accels%20Measure.pdf