IOS understands iphone accelerometer data in free fall

Question

Why is it that when I lower my phone about 4 feet on the pillow and draw the amount of user acceleration, I get peak values ​​of 1.5 g, then 1 g, then 2.5 g, then 1 g, then 2.5 g, then 0g per 1 second of time frame in free fall with 10 samples. Ideally, it should accelerate at a constant speed down due to gravity minus air resistance, so why does the acceleration rise and fall during free fall? What causes this noise?

Answer 1

The most important concept that you need to understand before reading my message is that the acceleration relative to the ground (not the accelerometer reading) due to gravity will always be 1.0 g. However, this acceleration can be reduced / enhanced due to external factors, such as air resistance, braking force, applied force, etc.

It is important to understand the difference between the actual acceleration and the accelerometer reading before you notice the variation between my answer and others. I answered your question from the point of view of acceleration, because your graph does not seem to reflect the readings of a crude accelerometer, but rather acceleration relative to the ground. To clarify:

• Accelerometer value = abs (acceleration WRT ground - 1 g)

Thus:

• When the acceleration is 0g (the object is at rest), the accelerometer reads 1g.
• When the acceleration is 1g (the object is in free fall), the accelerometer reads 0g.

The following is the likely situation for each WRT overclocking location that you placed:

1.5g: when removing the iPhone, you probably accidentally applied a small force of ~ 0.5 g, causing an acceleration of 1.5 g (1.0 g due to gravity + 0.5 g).

1.0g: as soon as it is in the actual free fall, it reads ~ 1.0g (acceleration due to gravity). This is the acceleration that he should hypothetically read all the time when he is in free fall, neglecting the resistance of the air.

2.5g: When it hits the pillow, it has an acceleration of up ~ 2.5g, because it stops after having received speed from gravity.

0.0g: after it has stopped, it has 0.0g of acceleration, because it does not accelerate. Acceleration due to gravity was neutralized by normal forced exposure to the pillow.

Answer 2

The accelerometer measures force in relation to the iPhone case. When he sits on the table, gravity forces the accelerometer chip at the bottom of the iPhone with 1G force. When in free fall, both the accelerometer chip and the iPhone case will experience the same downward force due to 1G gravity. But since both the chip and the case will accelerate down at the same speed in free fall, there is no chip power on the iPhone case, they will both fall together, so you get very close to reading G.

So, 1 G while you hold it in the air. 0 Gs in free fall. Many G during the deepest contact part of the rebound. 0 Gs during the rise in the air of a part of the rebound rebound. Many G during the next contact with the pillow, etc. Return to 1 G when sitting on the pillow.

This is a slight simplification, since the measurement actually refers to one part of the chip to another, but the argument still works, but more on the nanoscale.

Answer 3

Einstein discovered something obvious (but hidden in sight). That is: what we experience, because gravity is actually acceleration. When on earth, and not moving, we actually accelerate upward at 1G (one Earth's gravity). Thus, the earth does expand outward from the center.

Now a funny thing happens to matter (or a large mass similar to the Earth): the space and time around the mass are actually destroyed at the same speed as the expansion. Therefore, the clock works faster when you move away from the center of the earth or from the surface of the earth. This has been verified in countless experiments and must be considered for the proper operation of GPS satellites (for example).

Now a real accelerometer will always measure 1G on the ground. When you release it, the accelerometer goes into "free fall", which means that it is now moving at a constant speed (not accelerating) and, thus, reads: 0G. Instead of accelerating the accelerometer, the earth accelerates to meet a falling object, because the earth accelerated upward within one second from the weightless gravity experienced by the accelerometer. Thus, the instantaneous acceleration when hitting a pillow is much higher than 1G. It’s a good idea to use a pillow like spring to gently introduce Earth’s new instantaneous speed onto an instantly weightless accelerometer.

Instant acceleration for any tool is very destructive ... and the iPhone is very expensive.

Answer 4

Does the phone rotate? Air exposure will vary depending on the “profile” presented in the direction of acceleration (down), which means that the acceleration will change. Sample 0g is either a spot where the location of the sensor in the phone turned out to be spinning / rotating downward at the same speed as gravity and / or the speed of impact on the phone (unlikely to fall by 4 inches).

Answer 5

Usually, when the phone is sitting on the table, the acceleration is 1G. With free fall, the acceleration is 0G. I have never played with the iPhone’s accelerometer to find out if the support logic supports “fakes” 0G when stationary and “fakes” 1G when dropped, but if that happens, it will inevitably be imperfect.

What you should see (using a non-fugged accelerometer) in free fall starts with 1G, quickly drops to 0G when falling, quickly increases to a few G when it hits the ground, and then returns to 1G after the rebound stops, perhaps after several cycles up and down.