<P> * Including contribution from resistance to gravity . † Directed 40 degrees from horizontal . </P> <P> An accelerometer, in its simplest form, is a damped mass on the end of a spring, with some way of measuring how far the mass has moved on the spring in a particular direction, called an' axis' . </P> <P> Accelerometers are often calibrated to measure g - force along one or more axes . If a stationary, single - axis accelerometer is oriented so that its measuring axis is horizontal, its output will be 0 g, and it will continue to be 0 g if mounted in an automobile traveling at a constant velocity on a level road . When the driver presses on the brake or gas pedal, the accelerometer will register positive or negative acceleration . </P> <P> If the accelerometer is rotated by 90 ° so that it is vertical, it will read + 1 g upwards even though stationary . In that situation, the accelerometer is subject to two forces: the gravitational force and the ground reaction force of the surface it is resting on . Only the latter force can be measured by the accelerometer, due to mechanical interaction between the accelerometer and the ground . The reading is the acceleration the instrument would have if it were exclusively subject to that force . </P>

When does a body experience acceleration due to gravity