Questions tagged [measurements]

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1answer
108 views

Projection on an axis

Can we say that the coordinates in 3-space corresponding to the Bloch sphere, by taking expectation values of the Pauli spin operators is the same as the projections onto the different axis? \begin{...
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2answers
2k views

Total and static pressure: which one is measured?

Which pressure is measure using measuring device in a pipe flow. My first intuition that it is the static pressure. This is confirmed by this Wikipedia article (link) The concepts of total ...
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1answer
47 views

Calculations involving quantum measurement operators

In a quantum measurement theory text, the following is stated. Please advise on the query regarding the calculation below. Consider the following measurement operator: $$\hat{A}(\alpha) = \bigg(\frac{...
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1answer
119 views

The physics of Harrison's marine chronometers

I have read some of the history of John Harrison's marine chronometers and have found it fascinating. I would like to deepen my understanding of Harrison's amazing inventions by learning more about ...
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2answers
45 views

Change in state upon localization of particle into a region (x, x+a )

Suppose I momentarily, with hundred percent certainty, assure that a particle is in range $(x,x+a)$, by using some sort of detector. If initial state was ket $\alpha$, what is the final state? Is ...
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1answer
104 views

How to construct a 12-micron thin layer of powder+epoxy?

Okay, this is more of an engineering tactic question perhaps, but any insight into any possible process will be very, very appreciated. So I'm mixing a powder with epoxy, making a pasty, fluid semi-...
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1answer
219 views

True white collimated light / nobody uses this?

Based on a previous question is this the difraction limit of our microscope? , I got the idea to use a true white parallel back light to illuminate an object for measurements (target accuracy is 1 ...
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1answer
428 views

How to estimate the frequency of the sample rate?

I'm studying for my class of physics laboratory and I need help with something: Let's say I need to deduce the constant of elasticity of a spring and I will do it using a dual-range force sensor, ...
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4answers
687 views

Definition of mass in Newtonian mechanics

I believe that for any physical quantity, to know what significance it has in any situation or that it's different from other quantities, we have to first start by assigning a value to it, or some ...
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1answer
56 views

Quantum Uncertainty in Measurement of Fundamental Quantities

Fundamental constants (such as electron rest mass or charge, which are stated with high precision and, I think, are believed to be certain), I believe are verified by measurements that involve some ...
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2answers
123 views

What happens if you make an observation of the spins of two entangled, far apart electrons at exactly the same time? [closed]

If we consider the spins of two, far apart, entangled electrons, what will happen if we make a measurement on both spins at exactly the same time (let's assume time is not discrete)? I see it as a ...
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1answer
131 views

understanding resolving power in QM

Consider the definition of resolving power of two states in quantum mechanics as being the absolute value of the difference between the probabilities of two states following the measurement averaged ...
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5answers
307 views

A non-metaphysical interpretation of measurement in quantum mechanics

In quantum mechanics, measurement plays a fundamental role and to me, its role is usually described in a rather odd way. The Copenhagen interpretation states thus When that device makes a ...
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2answers
45 views

Optimization problem and measurements

This is more of math problem, but my doubt is about the measurement units of the final answer so I figured I'd post it here. Problem: A lighthouse is located on a small island 3 km away from a ...
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1answer
210 views

If two commuting observables do not share eigenspaces, can't one measurement affect the other due to an extra wave function collapse?

When we learn quantum mechanics, we are told that the only way to extract information from a system is to conduct measurements. We are told that if two observables commute then performing one ...
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1answer
84 views

Is EMF/EMR additive (two devices produce twice the EMR)?

I'm interested in electromagnetic radiation radiated from home appliances. Wanted to know if I have two phones (for arguments sake assume that they are perfectly identical) and they both are in sync ...
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0answers
57 views

Measurement phrase: “determined to 1 cm precisely”, “centimeter-precision” [closed]

I'm not perfectly sure about the stated phrase (or similar ones since I had to translate it). Does it mean +-1 cm, or +-0.5 cm? I guess many people use it in the first way but only the latter will ...
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2answers
3k views

What are the problems with using spirit level experiment to prove the flat-earth theory?

Full disclosure: I do not believe the Earth is flat. However, a guy recently posted a YouTube video that purports to prove that the Earth is flat due to there having been no changes in the position ...
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3answers
4k views

How is temperature defined, and measured?

In questions like this one, temperatures of millions of degrees (Celsius, Kelvin, it doesn't really matter at that point) are mentioned. But, what does it mean exactly? What is measured, and how? As ...
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1answer
9k views

Dimensions of electric charge

I was studying dimensional analysis, which is a technique used in conversion of units, checking the homogeneity of equations and also sometimes deriving unknown equations, if we can guess the factors ...
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2answers
97 views

How do I measure loudness level of a device in environment?

I want to measure the individual loudness level of a device in environment. Fistly, I measure loudness level when the device is ON (95.5dB) Then I measure loudness level when the device is OFF (48....
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1answer
312 views

How to deal with outliers in experimental physics?

In experimental physics, is it generally considered a good practice to eliminate outliers? I was taught that, when doing a linear regression, I should prune all the data that doesn't fit within 3 ...
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1answer
102 views

Assumptions in the proofs for the optimality of Grover's Search Algorithm

I was trying to understand this paper in which it is proved that Grover's search algorithm is optimal. On page 4, beginning of section 2 of the paper the author says the following In the proof I ...
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1answer
88 views

Changing length of one second

This is not exactly a question about physics, but, I was wondering, what would be a clever new definition of second, which is currently defined as the duration of 9 192 631 770 periods of the ...
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0answers
390 views

Working principle of a DC clamp-meter

I am trying to understand how a DC clamp meter works and found this image: (Source: http://www.kew-ltd.co.jp/en/support/mame/detail.php?id=60) My first guess of how this works is that somehow Ampere'...
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1answer
139 views

A quantum entanglement experiment

Suppose we have an entangled electron-positron pair, such that total spin is zero. Separate them, and let Alice measure the spin (along $z$) of the electron, obtaining, say, $+\frac{1}{2}$. All the ...
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1answer
70 views

Why do physical units have different prefixes than the ones used for counting? [closed]

For counting, we use thousand, million, billion, trillion etc. For physical quantities, thousand becomes Kilo, million is Mega and (short scale) billion is Giga. In both these systems, the consecutive ...
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1answer
211 views

Walter Lewin vids-why ± 0.5 cm uncertainty why not ± 0.1?

here is the link to walter lewin video lecture please jump to 13:13 https://www.youtube.com/watch?v=q9IWoQ199_o 8.01x - Lect 2 - 1D Kinematics - Speed, Velocity, Acceleration i thought that all ...
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1answer
477 views

Determine length of optical fiber [closed]

Say I have 1 km of the optical fiber on a reel. But I only need 200 m of it and I don't have a right optical source or say I don't want to lose time coupling the light into the fiber. I also don't ...
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1answer
42 views

Pressure: Measurement vs Definition?

I'm a bit confused about the relation between how we measure pressure and how it is defined. Consider a gas inside a container, if we connect a manometer and read the height difference, we say that ...
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1answer
31 views

Question about extra zeros in measurement

I most often encounter data values in problems which have some meaningless looking zeros after the decimal point at the end. There is no non-zero number to the right of the zeros. For example, the ...
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1answer
32 views

How to get second-order equation from 2 first-order equations with the same dependent variable?

I have a current sensor thats returns an output in voltage (Ts) dependent on the current that passes throught the inputs (I) and the voltage feed input (Ta). I built a table and measured the Ts, while ...
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2answers
2k views

Significant Digits for different units [closed]

I read, From (iv), 12.3 has three significant figures. And from (v) we can infer that 12.30 has four significant figures. So let's say it's meters, then 12.30 m = 1230 cm = 12300 mm But 1230 and ...
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1answer
2k views

How to find the least count of Vernier Calipers when Vernier Scale Divisions are greater than Main Scale Divisions?

When Vernier Scale Divisions are smaller than Main Scale Divisions, the least count is given by $1 \text{MSD} - 1 \text{VSD}$. But what if say, $3 \text{VSD}=5 \text{MSD}$ ? Do we write the least ...
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1answer
466 views

Why does radiometric dating tell us the ages of rocks?

I understand well how radiometric dating works in theory. In practice, I'm not sure how one would calibrate this for rocks. I've always been told that we can determine rocks' formation time via ...
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1answer
11k views

How can a beam balance measure mass?

In Newtonian physics, mass is the amount of matter in an object. So, how can a beam balance measure the amount of matter in an object (which is the mass of the object).
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3answers
457 views

Why are 'units' of measure discrete? [closed]

Why do we measure quantities like length, time, mass in terms of a discrete quantifiable unit (for eg: length in metres or planck lengths, if one scales down to that level) ? Is it possible to ...
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2answers
378 views

How do hygrometers work?

How does this hygrometer work? Please be clear. I didn't understand this past on hygrometers in a physics textbook. Can someone elaborate?
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1answer
555 views

Uncertainty calculation

Here's an exam question that confused me: Time for bar to swing through 10 oscillations as measured by a stop clock = 15.7 s Determine the percentage uncertainty in the time t suggested by the ...
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1answer
1k views

How to account for acceleration due to gravity when reading from accelerometer?

I'm obtaining readings from an accelerometer, and only care about using these readings to measure the total amount of motion which is occurring. However in order to interpret these readings, I need to ...
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1answer
60 views

Measuring Vibration

I want to measure the vibration in an environment so that I can isolate some equipment. What is the standard way of measuring this? Basically, I want to mitigate the effects of a nearby train. More ...
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0answers
325 views

What is the difference between Differential cross section, Rutherford scattering cross section and coulomb collision cross section?

I've been reading about cross sections and then I kind of got confused. What exactly is the difference between elastic scattering cross section, Differential cross section, Rutherford scattering cross ...
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129 views

what is the nuclear cross section dependent on?

I understand that the nuclear cross section determines the probability of fusion taking place. When I was talking to my friend he said that the cross section doesn't depend on anything but the ...
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1answer
160 views

A unit that is not coherent?

"Derived units are defined as products of powers of the base units. When the product of powers includes no numerical factor other than one, the derived units are called coherent derived units" I know ...
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1answer
44 views

Assigning distributions to the result of measurements

In my data analysis course we were taught that whenever we perform a measurement, the measured value is to be interpreted as the mean of the distribution for the true value. And the error in ...
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1answer
1k views

Why is bomb strength measured in tons?

For atomic bombs especially, strength is often measured in tons, kilotons, megatons, gigatons. A ton is a measure of weight (perhaps also force?), and the bomb definitely doesn't weigh in the ...
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1answer
126 views

finding period of periodic signal with varying amplitude

I have some data from experiment as shown in the figure. It looks roughly sinusoidal, but with an increasing amplitude, probably due to attenuation of the signal at different measured position. What ...
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5answers
3k views

Can someone explain uncertainty in measurement?

I really cannot get how a measuring instrument with the smallest division of let's say 20 has an estimated uncertainty of half that value. Firstly, why half, and secondly, why the smallest division? ...
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10answers
8k views

Easy to perform quantitative experiments at home [closed]

What are some easy to perform physics experiments that can be done at home (with not too much special equipment) and that allow to actually measure/plot data and draw conclusions from it? My son is ...
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5answers
3k views

Experimentally measure velocity/momentum of a particle in quantum mechanics

In the context of quantum mechanics one cannot measure the velocity of a particle by measuring its position at two quick instants of time and dividing by the time interval. That is, $$ v = \frac{x_2 - ...