Specific force

Specific force is defined as the force per unit mass.

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

Specific force (also called g-force) is measured in meters/second² (m·s^-2) which is the units for acceleration. Specific forces are not coordinate-accelerations, but rather proper accelerations, which are accelerations 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 per mass is the cause of, and a measure of, the body's proper acceleration. For bodies constrained from accelerating, the specific force will not equal the proper acceleration.

g-force acceleration is the same as 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 along 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. , not "standard" acceleration relative to a coordinate system.

Hydraulics

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

• F_s = y²/2 + (q²)/gy

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

See also

• Acceleration
• Proper acceleration