Consider the rock-on-a-string accelerometer in Figure 4.30 ....

Key Concepts

Effective Gravity

Effective gravity is the combination of the actual gravitational acceleration and any additional accelerations due to the motion of the system. In an accelerating vehicle, the apparent direction of 'down' is tilted from the true vertical because the sensor experiences both gravitational acceleration and the inertial acceleration from the motion.

Net Acceleration Vector and Angle Calculation

The net acceleration vector is obtained by vectorially adding the gravitational acceleration and any other applied accelerations, such as horizontal acceleration. The orientation of the accelerometer, or the angle it makes with the vertical, is determined using trigonometric relationships (for instance, the arctan function applied to the ratio of the horizontal acceleration to gravitational acceleration).

Accelerometer

An accelerometer is a device that measures acceleration forces, typically using a mass attached to a spring or string. It detects changes in motion by tracking the displacement or angle of the mass relative to the device's frame, which provides a measure of acceleration when compared to the known force of gravity.

Non-Inertial Reference Frames

When measurements are taken inside an accelerating system, like an airplane, the frame of reference is non-inertial. In such frames, fictitious forces arise that combine with gravitational forces, and the sensor readings reflect this effective acceleration rather than just the gravitational pull.

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