# Tag Info

Accepted

### How can one detect a device (phone) falling using its 3-axis accelerometer?

TL;DR: Accelerometer records the $x,y,z$ projections of acceleration $\vec{a_r} = \vec{a} - \vec{g}$, where $\vec{a}$ is the total acceleration of the object. When the object is free-falling, the ...
• 1,978

### What's the difference between proper acceleration and coordinate acceleration?

Why does proper acceleration does not depend on a coordinate system, and why coordinate acceleration does? Essentially, this defines the difference. Coordinate acceleration depends on the ...
• 59.8k
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### Is there any sensor that creates a visual image of magnetic fields?

You maybe interested in looking here: Magnetic field gel based viewing film or for more advanced applications the ferrolens (commercial product named Ferrocell). However these optical magnetic sensors ...
• 4,170

### Why do the sensitivities of magnetometers and gravimiters have such strange units?

These figures represent the root mean square background measurement noise per unit bandwidth, in other words if you had an ideal bandpass filter placed after the instrument whose bandwidth is B [Hz] ...
• 19.8k
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### The change of mechanical into electromagnetic waves and vice versa

Yes, a microphone is a device that takes mechanical vibrations from sound waves and turns them into electrical signals. A loudspeaker takes electrical signals and creates mechanical vibrations of a ...
• 13.7k

### A logic question on binary variables

Assuming that each sensor sees only the light from the other night light, and assuming that each night lights is bright enough to reliably trigger the other's sensor, then you have discovered a ...
• 15.2k
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### Does the size of CCD/CMOS of a telescope affects the magnification?

The magnification of a telescope is the ratio of the focal length of telescope over the focal length of the eyepiece. At least for a telescope that you want to look through with your eye. The ...
• 411
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### Can a syringe be used as a hanging scale if needed?

Yes you are basically right, and I will add a historical note. When Robert Boyle (1627-1691) did his experiments which led to what we now call Boyle's law, he compared air to a spring and wrote of "...
• 59.6k
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### Is it possible to "override" the Earth magnetic field locally?

As long as you know the magnitude and direction of the Earth's field at that spot, it's very possible! Suppose you measure the magnitude of the field as $B_{earth}$ in tesla (T), and you set up a ...
• 35.6k

### The change of mechanical into electromagnetic waves and vice versa

Amending the previous answer, which refers more to air pressure, there is also the piezoelectric effect of some materials that can be used to transform mechanical waves and pressure from various other ...
• 4,170

### What is the negative pressure?

what is meant by negative pressure? A pressure that is below zero gauge pressure, i.e, below 1 atm. To translate it into a positive value, you need to use units of absolute pressure. In general those ...
• 73.7k
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### How can I determine how much energy a sensor pixel can withstand before exploding?

This isn't trivial. Silicon sensors can generally handle power well above saturation without damage. Usually, the limit is temperature. So, what you need to do is a heat transfer calculation involving ...
• 21.8k

### How does smartphone VR really work?

Phones and other VR systems typically have a 6 axis Inertial Measurement Unit (IMU). These devices can sense accelerations in all 3 translational axes and all 3 rotational axes. Software (or ...
• 50.2k
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### light point and photosensor

In order to see something, that "something" must either emit or reflect photons into your eye (or detector). If photons leave a light source in some direction, but are seen/detected "in the air", then ...
• 10.6k
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### Ficticious forces influences what an accelerometer exactly measures?

I've spent some time thinking about my question and I think that i've arrived to the answer. Accelerometers measure only the acceleration of non-fictitious external forces To reach this conclusion I ...
• 41

### Why can't I use a gyroscope/accelerometer only as a compass?

In theory you could. A mems gyroscope - assuming you could hold it flat and still enough for long enough - can sense geographic North. But you need a low noise sensor and some clever filtering ...
• 30.8k

### Inductive sensor - How does it work?

When a tooth is near the core of the coil, the magnetic flux through the core will greatly increase ,because the tooth (and the wheel) helps lowering the reluctance of the magnetic circuit generated ...
• 1,586
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### What plastic-like materials allow IR to pass through them?

If we look at you device (GP2Y0A51SK0F) specification file at Sharp website we find the following section (p. 6): In case that protection is set in front of the emitter and detector portion, the ...
• 15.9k

### When the barometer drops, my well level seems to rise - how?

The porous rock just outside the well bore has a certain pressure in it. That changes only slowly. That pressure pushes the water up, and the air pressure pushes the water down. They reach ...
• 14.6k

### How is measurement uncertainty defined / calculated (of a sensor or measurement)?

The uncertainty specified by a manufacturer is not evaluated by considering a single sensor but by considering the spread across the whole production, determined both from sampling the production and ...
• 4,925

### Is Daredevil's superpower conceptually possible?

Bats famously use echolocation to hunt for insects, but bats are sensitive to ultrasound. Human hearing tops out at frequencies around 20 kHz (or lower for middle-aged humans), for which the ...
• 91.6k

### How does a sampling rate of 1ns equate to 15cm for an optical instrument?

We can easily check this by calculating how far light travels in 1 ns. $$x = c \cdot t = 299792458\,\text{m}/\text{s} \cdot 10^{-9}\,\text{s} \approx 0.3\,\text{m} = 30\,\text{cm}$$ By how a LIDAR ...
• 10.3k
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### What is a simple model of telescope sensitivity?

This question seems to be about photodetectors, not telescopes. If $N$ photons fall on the active area of a pixel then $\epsilon_Q N$ photoelectrons will be produced. where $\epsilon_Q$ is the quantum ...
• 14.4k
1 vote
Accepted

### Why photodiode measures random phase superpositions on continuous-wave (10kHz) white light?

I have changed the cable (connecting between signal generator and the laser driver) from to Now the output is expected, and does not depend on orientation orposition of the photodiode.
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1 vote

### Unique optical map of a uniform metal object

Exposure to a mildly corrosive environment will indeed etch into that surface a pattern of chemical attack which will be random and hence unique for every part. The amount of attack varies from one ...
• 95.6k
1 vote
Accepted

### Measuring pressure or weight from an uneven weight distributed surface

As I am not familiar with force sensing resistors (FRS) but after watching a couple of Utube videos I do wonder about their accuracy and the proper way to use them. In particular, the effect of not ...
• 73.7k
1 vote
Accepted

### Phone accelerometer - formula to calculate angular gravity

As the angle of the device is adjusted to ~45° in the Z-Y plane (stationary, screen angled back) the Z-plane reads ~7g and the Y-plane reads ~7g for a total of ~14g. The device is sending you the ...
• 41.2k
1 vote

### Calculating Velocity from Acceleration (Accelerometer)

This should be a comment but I wanted to include a graph. It would help to have details of the motion that you are observing. Have you checked that the accelerations along the two other ...
• 97.9k

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