Questions tagged [measurements]

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1answer
59 views

How to average the loudness of a audio file from its amplitude?

I would like to average the loudness of different sounds so that their means could be used as a dependent variable in a statistical regression, but I have some troubles to understand how loudness ...
2
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2answers
81 views

Is there any experiment that doesn't have a finite countable number of outcomes?

There are a lot of random variables in physics that are, in principle, continuous and unbounded (canonical examples: time, position along each direction). In practice, it is my understanding that the ...
2
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1answer
47 views

Is an atomic clock's measurement principle independent of what a light clock provides

Special Relativity has the light clock which basically counts how often a photon goes forth and back along a fixed distance. Our most accurate clock, defining the second (at least as of today), is an ...
2
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1answer
92 views

Which program to analyse and graph laboratory measurements?

I'm interested in which program would you recommend for drawing graphs with x and y errors. Also to be able to analyse data and then graph it. My ideal graph would look like this: Until now I was ...
1
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1answer
91 views

How to check if a thing I have is exactly of 1 kg or not?

The definition of a kilogram is as far I know, Kilogram: The mass of a cylinder made of platinum-iridium alloy kept at International Bureau of Weights and Measures is defined as 1 kg. But it is ...
2
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1answer
81 views

How is the Length of a Meter Physically Measured

I have two parts to this question. First, I understand that the meter is defined as the distance light travels in 1/299,792,458 seconds. But how is this distance actually measured? The second is ...
1
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1answer
146 views

Accurate measurement of total lung capacity / vital capacity

I'm currently working on a practical problem: How to measure your total lung capacity and/or your vital capacity accurately. Any suggestions or experience? One of the methods that can be found with a ...
-1
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1answer
306 views

Fractional uncertainty question

Q. A ball falls freely from rest with an acceleration g. The variation with time t of its displacement s is given by s = 1/2 gt^2. The percentage uncertainty in the value of t is ±3% and that in the ...
0
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1answer
131 views

Smallest uncertainty ever achieved in position measurement in QM?

The Heisenberg uncertainty principle states that $$\Delta x\Delta y\geq\hbar/2$$ Since the magnitude of $\hbar$ is $10^{-34}$ we could measure both $x$ and $p$ with an uncertainty magnitude of $10^{-...
2
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1answer
49 views

Ultra low level light detection: What measurement principles are used?

I was reading up on how Avalanche Photodiodes(APD) are used to detect very low light levels, some are even used in "photon-counting" experiments (in Geiger mode). However, a survey of some commercial ...
2
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1answer
60 views

Is it possible to measure the number of spacetime dimensions?

My question is pretty straightforward to be stated but I don't know whether the answer is as easily reported. It is remarkable and very interesting that Physics work in (almost) any number of ...
1
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1answer
142 views

What is the relation between a measurement and an observable?

Observables are represented by Hermitian operators. First of all, it's a little strange (to me) that some measurable physical quantity is represented by a transformation (or linear map), given that I ...
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0answers
80 views

How much of Sun's mass is “regular” matter? How precisely do we know Sun's composition?

This is a question which spun of this Can the Sun / Earth have a dark matter core? I have argued that it is possible to pretty accurately estimate how much mass of Sun is in the regular matter, so ...
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3answers
201 views

Superposition of position eigenstates after measurement of position?

In a discussion with someone recently, the person made the following statement: Even when we observe a particle, it is still in superposition of position states within the resolution of the ...
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3answers
108 views

How come 84000 has 2 Significant Digits and 84000.0 has 6 Significant Digits?

Isn't there a decimal point after every whole number. So shouldn't 84000 also have 6 Significant Digits?
2
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0answers
43 views

Question about statement of uncertainty

I have a doubt about uncertainty of a measurement. I have a sample of a specific material which is my reference material and it is calibrated and NIST traceable. This sentence is in its data-sheet: ...
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2answers
68 views

A doubt related to Significant Digits

Could someone please explain this statement to me "Reporting the result of measurement that includes more digits than significant digits is superfluous and also misleading since it would give a ...
0
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2answers
325 views

How the last digit in significant figures is considered doubtful?

If a reading of a length on meter rod is 44.6cm with least count of 1mm And last point of the length is exactly on 44.6 not in between of 44.6 or 44.7 Then how is it doubtful?
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0answers
101 views

Causality in Quantum mechanics

If we consider a quantum preparation procedure on a system followed by a projection measurement as constituting two sequential events in space-time (which seems plausible ,given that the Copenhagen ...
1
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2answers
191 views

Radioactive Dating Accuracy After 60,000 years [duplicate]

Why is radioactive or Carbon 14 dating only accurate for 60,000 years? In theory, there should always be at least SOME Carbon 14 in everything that was living at any given time. This is because no ...
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0answers
114 views

How is thrust per unit power measured?

Typically, thrust efficiency is measured in specific impulse, as thrust per fuel consumption rate: $$I_\text{sp} = \frac{F_\text{thrust}}{\dot{m}_\text{fuel}} \quad \text{or} \quad I_\text{sp} = \...
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3answers
141 views

How to define approximate meter using primitive to no tools? [closed]

Imagine you've lost somewhere in the wild due to some catastrophic event, and don't have any measurement tools with you. How do you find approximate meter, millimeter, etc. with materials like sticks ...
2
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2answers
119 views

Simple question about propagation of error/uncertainty

I'm stuck on a seemingly simple question about propagation of error. Say we repeatedly measure the speed of a particle, and we estimate the uncertainty in the measured speed to be 10 percent. What is ...
1
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1answer
43 views

Several thought experiments on the difference between direct and indirect measurements

I have read several books regarding basic concepts in physics but I'm still puzzled by own lack of knowledge. I shall not give definitions, because I presume we are all familiar with them. I do not ...
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4answers
816 views

How do I find the instantaneous velocity in real life? [closed]

I am learning about Instantaneous speed and velocity in Khan Academy. I understand the concept and calculation, but how do I know the different instantaneous speed at different points in real life?
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1answer
275 views

Propagation of errors

I was going through a problem in finding error in measurement of focal length, if object and image distances are given with appropriate errors. I found two methods to find the error at different ...
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0answers
33 views

Measurement of a real apparatus

Do we know the approximate form of measurement operators for a realistic measuring apparatus? For instance, consider a cloud/bubble/spark chamber. What operator $\hat X$ can be considered to measure ...
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0answers
112 views

How to measure the Thermal Noise in a conductor?

Johnson Noise in a conductor is very small: between 10-9 to 10^−12 V How to measure it using an oscilloscope and amplifier? If it were a resistor that would be easy, but conductors have much smaller ...
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1answer
3k views

Uncertainty in the average of two measurements (with their respective uncertainty)

I know that generally the uncertainty in the mean of a sample should be equal to: $\frac{V_{max} - V_{min}}{2} $ where $V_{max}$ is the maximum value and $V_{min}$ the minimum value of the sample ...
3
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2answers
122 views

Human body as a measuring instrument

One can easily find descriptions of human eyes and ears used for measuring phenomenons. Finding what resolution do eyes have or what frequencies can human hear is no trouble. It is also seen in scales ...
2
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1answer
70 views

How did they measure -20 db sound?

Current world record for quietest room is Microsoft anechoic chamber that was measured at -20 db.How did they measure it? I mean,lowest noise measurement microphone I found is 1 inch special low ...
1
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1answer
43 views

Measurement of decay rates and statistics

During my nuclear physics lab I was measuring decay rates of Fe59. The average was around 20 decays per second. But I did two series - in one I was measuring number of decays per 90 seconds and in the ...
5
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3answers
316 views

How do you physically measure voltage and current?

Short Version: How exactly do (modern) voltage/current meters measure the numbers they do, from first principles? References much appreciated. We measure length with a ruler, time with a clock, and ...
3
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2answers
233 views

Are there places in space completely devoid of light?

I'd be interested to know how far from the sun you'd have to be for it to feel 100% dark, as the human eye sees it. But also, are there places in space so far removed that there is nothing visible ...
2
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2answers
81 views

What experimental bounds do we have on big $G$?

I know that there has been a large amount of controversy surrounding the exact value of the gravitational constant $G$, but I know that there is not a substantial difference in the measured value. So ...
2
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1answer
68 views

Generalised representation of a 2x2 Positive Operator-Valued Measure

Let {$E_{i}$} be a set of 2x2 POVM operators, satisfying $\sum_{i}E_{i}=\mathbb{I_{2x2}}$. We know that a general 2x2 Hermitian matrix (say, $H$) can be represented by $$ H = \left[{\begin{array}{...
1
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1answer
65 views

Measure mass of an object only using gravitational force

Most if not all of our everyday methods of determining mass / weight work on the principle of normal force. The push provided to you by the surface which allows to sit/stand/lie down over it.The ...
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2answers
79 views

Problem of measuring barrier potential by a voltmeter

I have read in my book that The barrier potential of a diode junction cannot be measured by a voltmeter only. It says that the thermal equilibrium is disrupted. Besides , the contact potential is also ...
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5answers
976 views

What do clocks measure?

I recently asked a question about the measurement of time, and it has become apparent that I'm really asking a set of related questions, the premises of which need to be shored up and articulated ...
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1answer
17 views

Error regions in a simplistic measurement

Suppose I measure two certain parameters, $a$ and $b$, in an object $O$ for two instances. My measurements are Instance 1----> $a=1.4$, $b=1.4$ Instance 2---->$a=1.5$, $b=1.4$ Can, I say, hence for ...
1
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1answer
74 views

Probability of obtaining general quantum measurement outcome

The Fundamental Theorem of Quantum Measurement is stated as follows: Every set of operators $\{ A_n \}$ $n =1,...,N$ that satisfies $\sum_n A_n^{\dagger}A_n = I$ describes a possible measurement on a ...
1
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1answer
54 views

Three levels system and measurement

A three levels system has the hamiltonian $$\mathcal{H}=\begin{pmatrix}E_0+\alpha&&\\&E_0&i\beta\\&-i\beta&E_0\end{pmatrix},$$ with $\alpha,\beta\in\mathbb{R}$, $\alpha>\...
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0answers
51 views

Estimate the body surface area [duplicate]

This is a question back when I was in the third year of my (junior) middle school. However I still do not know an elegant answer to it. The question is composed of a single line. Measure the body ...
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1answer
246 views

Are ‘fundamental measures’ a thing?

The question I want to ask is: What measures are needed to describe the physical world and what are the fundamental ones of those, in the proper sense of the word fundamental? But that might be too ...
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0answers
38 views

Is there a formal definition of what a significant figure really is? [duplicate]

What does it mean, formally, when we say that a digit in a number is a significant figure?
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1answer
37 views

How do uncertainties in digital measurements work?

Suppose I stand on an electronic weighing scale, and it reports my weight as 74.5 kilos. Usually a lot of books would say that the last digit is uncertain, and that the uncertainty is 0.1kg, and that ...
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1answer
99 views

Does the symbol $K$ mean anything besides kelvin in any context of this scientific paper on the second page on the chart? [closed]

I was looking at this scientific paper(https://arxiv.org/abs/1205.1684) and I saw them use the symbol "K" for a nemerical value, but due to the context of the paper I wasn't sure if they were talking ...
2
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1answer
342 views

How to measure a static electric field?

I looked up google but didn't find any design for measuring electric field that doesn't vary with time. My own idea is to use two parallel plates (like a capacitor but without the dielectric). In an ...
0
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1answer
63 views

How is the energy of a particle measured in a Cherenkov detector [closed]

I don't understand how is the energy measured in a cherenkov detector. How is the number of photons radiated related to the energy of the particle or the measure of energy has nothing to do with the ...
0
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1answer
48 views

When does a particle in a quantum state interact with matter?

Lets have a particle in vacuum. If we do not observe, the Quantum Theory & Copenhagen interpretation says that the particle is nowhere and at the same time it is somewhere ( i.e the quantum state $...