An accelerometer is a measuring device for specific external force. Specific external force is the sum total of external forces acting on an object divided by the mass. Accelerometers do not measure internal forces such as gravity. An accelerometer sitting still on a table top will read one g of specific force due to the table pushing up on the accelerometer equal to its weight. If dropped in a vacuum it will read zero. For computation of position, the Inertial Navigation System adds in the estimate of the local gravity acceleration to the signal coming off of the accelerometers. Accelerometers may be part of an Inertial Navigation System, used to detect and measure vibrations, or for measuring acceleration due to gravity (inclination). An accelerometer inherently measures its own motion (locomotion), in contrast to a device based on remote sensing.