Why does a stationary phone's accelerometer still not read zero?
手机没动，加速度计为什么还不显示零？
手机安静地放在桌上，里面的加速度计却不一定显示零。
A phone rests quietly on a table, yet its accelerometer does not necessarily read zero.
没有动，为什么还有读数呢？
Why is there a reading when it is not moving?
关键是，这种读数不只与手机的运动有关，还包括重力的影响。
The key is that this reading does not only concern the phone's motion; it also includes the influence of gravity.
加速度计是一种感应器，可以把作用在它上的力变成数据。
An accelerometer is a sensor that can turn forces acting on it into data.
在安卓系统的说明中，它给出的数值把重力也算了进去。
In Android's documentation, the values it reports include gravity.
重力是地球对东西的吸引，手机放着不动，也不代表这种影响消失了。
Gravity is Earth's attraction on objects; a stationary phone does not mean this influence has disappeared.
因此，读数不是零，并不能单独证明手机正在移动。
Thus, a nonzero reading alone cannot establish that the phone is moving.
这也不是说，桌上的手机正在越跑越快。
Nor does it mean that the phone on the table is moving faster and faster.
要看手机运动带来的变化，就需要把重力这一部分与其他部分分开。
To examine changes due to the phone's motion, the gravity component needs to be separated from the others.
不过，重力的信息本身也有用：它可以帮助系统判断手机相对地面的姿势。
Gravity information is itself useful: it can help the system determine the phone's orientation relative to the ground.
感应器可以提供三个方向的数据，而不是只给一个总数。
The sensor can provide data along three directions, rather than only one overall value.
不同手机处理这些数据的办法可以不同，所以不能把一个说明当成所有设备的做法。
Different phones may process these data differently, so one description cannot represent every device's implementation.
这个例子有趣的地方是：数字在告诉我们什么，要看感应器把哪些影响算进了读数。
The interesting point is that what a number tells us depends on which influences the sensor includes in its reading.
