A wide variety of sensors are installed in a smartphone, which are not only necessary for functionality and handling, but also make the smartphone an excellent measuring device. Acceleration sensors provide the relevant information so that the smartphone can recognize whether it is held vertically or horizontally. We will now take a look at how this works.

1. Consider a weight hanging from a spring. How does the deflection of the spring change when we move the spring in different directions? (For simplicity, assume that the spring can only be enlarged in one dimension.)
2. How can we make a mechanical accelerometer using three springs and weights. In other words, an experimental apparatus with which we can measure in which direction we accelerate and how strongly.
3. On the left you can see a strongly enlarged picture of an acceleration sensor. Explain how it works. Hint: The accelerometer consists of capacitors, and a spring mechanism. How does the capacitance of the capacitor behave when the sensor is accelerated?
4. Now explain how a smartphone can detect how you are holding your smartphone with the help of acceleration sensors.

1. If a weight is suspended from a spring and we accelerate the spring in the y-direction to the spring direction, the deflection of the spring increases or decreases depending on the direction in which we accelerate. If we know the mass of the weight and the spring constant, we can calculate the acceleration. Accelerations in x and z direction do not change the deflection of the spring in this idealized example.
2. If we use three mutually orthogonal spring weight systems which are firmly connected to a body, the acceleration of the body can be determined in this way.
3. In the picture of the acceleration sensor, we recognize capacitors whose plates are connected by a spring system. If the sensor is accelerated, the capacitance of the capacitor changes. As with the weight on the spring, we can calculate the change in capacitance by knowing the moving mass, the spring constant, the distance between the capacitor plates and the capacitance of the capacitor in the non-accelerated case.
4. The smartphone has three acceleration sensors that are placed orthogonally to each other. This makes it possible to find out in which direction the acceleration due to gravity is pointing and to determine the screen direction accordingly.