iOS 理解 iPhone 自由落体加速度计数据
为什么当我将手机从大约 4 英尺高处掉落到枕头上并绘制用户加速度的大小时，我得到的峰值为 1.5g，然后是 1g 2.5g，然后 1g，然后 2.5g，然后 0g，在 1 秒时间范围内自由落体，有 10 个样本。理想情况下，由于重力减去空气阻力，它应该以恒定的速率向下加速，那么为什么在自由落体时加速度会上下变化呢？是什么导致了这种噪音？
Why is it that when i drop my phone from about 4 feet onto a pillow, and plot the magnitude of the user acceleration i get peak values of 1.5g then 1g then 2.5g then 1g then 2.5g then 0g in a 1 second time frame in free fall with 10 samples. Ideally it should be accelerating at a constant rate downwards due to gravity minus the air resistance, so why does the acceleration go up and down while in free fall? What is causing this noise?
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在阅读我的文章之前需要理解的最重要的概念是，由于重力而产生的相对于地面的加速度（不是加速度计读数）始终为 1.0g。然而，这种加速度可能会因空气阻力、制动力、施加力等外部因素而减弱/增强。
在注意到我的答案与其他答案之间的差异之前，重要的是要认识到实际加速度与加速度计读数之间的差异。我已经回答了您关于加速度的问题，因为您的图表似乎并不反映原始加速度计读数，而是反映相对于地面的加速度。澄清一下：
因此：
以下是您发布的每个 WRT 地面加速度的可能情况：
1.5g：当 iPhone 掉落时，您可能意外地施加了约 0.5g 的小力，导致 1.5g 的加速度（1.0g 由于重力 + 0.5g 应用）。
1.0g：一旦实际自由落体，读数为~1.0g（重力加速度）。这是假设它在自由落体的整个过程中应该读取的加速度，忽略空气阻力。
2.5g：当它撞击枕头时，它的向上加速度约为 2.5g，因为它在重力作用下获得速度后停止。
0.0g：停止后，由于没有加速，因此具有0.0g的加速度。重力加速度已被枕头施加的法向力抵消。
The most important concept to understand before reading my post is that acceleration relative to the ground (not accelerometer reading) due to gravity will always be 1.0g. However, this acceleration can be diminished/enhanced by external factors such as air resistance, stopping force, applied force, etc.
It is important to recognize the difference between actual acceleration and the accelerometer reading before noticing the variation between my answer and the others. I have answered your question in terms of acceleration because your graph does not seem to reflect raw accelerometer readings, but rather acceleration relative to the ground. To clarify:
Therefore:
Here is a probable situation for each acceleration WRT ground that you posted:
1.5g: When dropping the iPhone, you probably accidentally applied a small force of ~0.5g, causing an acceleration of 1.5g (1.0g due to gravity + 0.5g applied).
1.0g: Once it is in actual free fall, it reads ~1.0g (acceleration due to gravity). This is the acceleration it should hypothetically be reading the entire time it is in free fall, neglecting air resistance.
2.5g: When it hits the pillow, it has an upwards acceleration of ~2.5g because it is stopping after having gained speed from gravity.
0.0g: After it has stopped, it has 0.0g of acceleration because it isn't accelerating. Acceleration due to gravity has been neutralized by the normal forced exerted by pillow.
爱因斯坦发现了一些显而易见的东西（但隐藏在众目睽睽之下）。也就是说：我们体验到的重力实际上是加速度。当在地面上且不动时，我们实际上以 1G（一个地球重力）向上加速。所以地球实际上是从中心向外膨胀的。
现在，物质（或像地球这样的大质量）发生了一件有趣的事情：质量周围的空间和时间实际上以与膨胀完全相同的速度塌陷。这就是为什么当你远离地球中心或地球表面时时钟走得更快的原因。这已经在无数实验中得到验证，并且必须考虑到这一点，以便 GPS 卫星正常运行（例如）。
现在，真正的加速度计将始终在地面上测量 1G。当您释放它时，加速度计进入“自由落体”状态，这意味着它现在以恒定速度移动（不加速），因此读数：0G。地面不是加速计加速，而是向上加速以满足下落物体，因为在加速计经历的一秒失重自由落体过程中地面已经向上加速。因此，撞击枕头时的瞬时加速度远高于1G。使用枕头作为弹簧，轻轻地将新的地球瞬时速度引入瞬时失重加速度计是一个好主意。
瞬时加速对任何乐器来说都是非常具有破坏性的……而且 iPhone 非常昂贵。
Einstein discovered something obvious (but hidden in plain sight). That is: what we experience as gravity is actually acceleration. When on the ground, and not moving, we are actually accelerating upward at 1G (one earth gravity). So the earth is actually expanding outward from the center.
Now a funny thing happens with matter (or a large mass like the Earth): space and time around the mass actually collapses at the exact same rate as the expansion. That's why clocks run faster when you move away from the center of the earth or off the Earth's surface. This has been verified in countless experiments and must be taken into consideration in order for GPS satellites to function correctly (for example).
Now a true accelerometer will always measure 1G on the ground. When you released it, the accelerometer goes into "free fall" which means it is now moving at a constant velocity (not accelerating)and thus reads: 0G. Instead of the accelerometer accelerating, the ground accelerates up to meet the falling object because the ground has accelerated upward during the one second of weightless free-fall experienced by the accelerometer. The instantaneous acceleration when striking the pillow is therefore much higher than 1G. Using the pillow as a spring to gently introduce the new instantaneous velocity of the earth to the momentarily weightless accelerometer was a good idea.
Instantaneous acceleration to any instrument is very destructive... and iPhones very expensive.
加速度计正在测量相对于 iPhone 外壳的力。当你坐在桌子上不动时，重力会以 1G 的力将加速计芯片压入 iPhone 外壳的底部。当自由落体时，加速度计芯片和 iPhone 外壳都会因 1G 重力而受到相同的向下力。但是，由于在自由落体过程中，芯片和外壳都会以相同的速度向下加速，因此芯片对 iPhone 外壳没有作用力，它们都会一起落下，因此你会得到非常接近的读数到零 G。
所以当你把它举在空中时，它是 1 G。 0 G 自由落体。在弹跳的最深接触部分期间有很多G。 0 Gs 在空中反弹部分的反弹。下次与枕头接触时会产生许多G。当坐在枕头上不动时回到1G。
这是一个轻微的简化，因为测量实际上是芯片的一个部分相对于另一个部分的测量，但这个论点仍然有效，但更多的是在纳米尺度上。
The accelerometer is measuring force with respect to the iPhone case. When sitting still on the table, gravity is forcing the accelerometer chip into the bottom of the iPhone case with a force of 1G. When in free fall, both the accelerometer chip and the iPhone case will undergo the same downward force due to gravity of 1G. But then since, in free fall, both the chip and the case will be accelerating downward at the identical rate, there is no force of the chip against the iPhone case, they'll both be falling together, so you get a reading very close to zero G.
So 1 G while you hold it still up in the air. 0 Gs in free fall. Many G's during the deepest contact part of the bounce. 0 Gs during the up in the air part of the bounce rebound. Many G's during the next contact with the pillow, etc. Back to 1 G when sitting still on the pillow.
This is a slight simplification, as the measurement is actually of one part of the chip against another, but the argument still works, but more on a nano-scale.
手机在转吗？空气阻力会根据加速度方向（向下）呈现的“轮廓”而变化，这意味着加速度会变化。 0g 样本要么是手机中传感器的位置碰巧以与重力相同的速率向下旋转/旋转的点，要么是手机达到终端速度（不可能是 4 英尺高的坠落）。
Is the phone spinning? Air resistance will vary depending on the "profile" presented in the direction of acceleration (down), which means acceleration will vary. The 0g sample is either a spot where the location of the sensor in the phone happened to be rotating/spinning downwards at the same rate as gravity, and/or the phone hit terminal velocity (not likely on a 4' fall).
正常情况下，当手机放在桌子上时，加速度为1G。自由落体时加速度为 0G。我从来没有玩过iPhone的加速度计，不知道支撑逻辑是否在静止时“假”0G，在跌落时“假”1G，但如果是的话，它也不可避免地是不完美的。
在自由落体中，您应该看到（从未经捏造的加速度计），从 1G 开始，在下降时迅速减小到 0G，当撞击“地面”时迅速增加到几个 G，然后在反弹停止后返回到 1G，也许经过几次上下循环后。
Normally, when the phone is sitting on the table, the acceleration is 1G. In a free-fall the acceleration is 0G. I've never played with the iPhone accelerometer to know whether the support logic "fakes" 0G when stationary and "fakes" 1G when falling, but if it does it would inevitably be imperfect.
What you should see (from an un-fudged accelerometer) in a free-fall is starting at 1G, decreasing rapidly to 0G while falling, increasing rapidly to several Gs when it hits the "ground", then returning to 1G after rebounding stops, perhaps after several cycles up and down.