<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> <P> A three - axis accelerometer will output zero ‐ g on all three axes if it is dropped or otherwise put into a ballistic trajectory (also known as an inertial trajectory), so that it experiences "free fall," as do astronauts in orbit (astronauts experience small tidal accelerations called microgravity, which are neglected for the sake of discussion here). Some amusement park rides can provide several seconds at near - zero g . Riding NASA's "Vomit Comet" provides near - zero g for about 25 seconds at a time . </P>

How many g's do astronauts experience during takeoff