Questions tagged [measurements]

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2answers
1k views

Different precision for masses of moon and earth online

The Google search page for "mass of moon" shows a value with nine significant digits of precision. The corresponding page for "mass of earth" shows a value with only four significant digits. (The ...
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2answers
95 views

How would someone determine the speed of sound in water experimentally?

I'm in highschool so I'm looking for a highschool-level lab idea.
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1answer
18 views

how to understand sensitivity of a system

I am reading a book about measurement. It introduces a concept of sensitivity of the system: the output of the system when the input undergoes a small change. Assuming the system can be described with ...
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4answers
244 views

Measuring very small temperature differences

Can one use a thermometer with $\pm$5 mK accuracy to measure a temperature difference of 2 mK (the measurement is near 100 mK temperature on a sample on an ADR)? Using the same thermometer, I am ...
2
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2answers
43 views

Replacing '< x' by a defined value [closed]

I do have a dataset containing nearly a decade of particulate matter measurements. At the lower precision limit of the measuring instrument, there are values given like ...
31
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3answers
2k views

Why does interaction with a Stern-Gerlach machine, or any other laboratory apparatus, not cause a measurement?

Consider a Stern-Gerlach machine that measures the $z$-component of the spin of an electron. Suppose our electron's initial state is an equal superposition of $$|\text{spin up}, \text{going right} \...
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2answers
87 views

Is measurement of coordinates possible near strong gravity?

We know that Schwarzchild metric describes an asymptotically flat spacetime. This means that far away from the event horizon we can safely interpret the $r$ coordinate as distance from the center. ...
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1answer
41 views

Handling systematic error that varies between measurements

I have a block of glass. The glass is tilted at some angle $\alpha$ with respect to a fixed axis. I rotate the glass and $\alpha$ changes. The way I am rotating the glass is by placing it onto a stick ...
3
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3answers
244 views

Measurement and transition rates

We can use time dependant perturbation theory (specifically Fermi's Golden Rule) to calculate the transition rate (probability of transition per unit time) from one energy eigenstate to another. ...
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1answer
702 views

Precision and Accuracy course review [closed]

In the following course review question I need to choose all the answers that are true about precision and accuracy 1)Definitions of accuracy and precision depend on the type of measurement you are ...
3
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1answer
158 views

A question about error analysis, please help?

So in class we are doing an experiment on the period of a pendulum where you measure the period of oscillation with a stopwatch. Now the way the lab manual shows how to do the error analysis is by ...
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0answers
74 views

Non-linear bell's inequality?

Consider the Bell setting, where Alice and Bob get inputs $x,y$ and they perform measurements $\{M^x_a\}_a,\{N^y_b\}_b$ respectively on their shared pure state $\Psi_{AB}$ to obtain outputs $a,b$. ...
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3answers
488 views

What is precision?

I’ve learnt that accuracy is how close to the true value a measurement is (i.e. low discrepancy between the experimental and theoretical values). Precision, on the other hand, is the reproducibility ...
2
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0answers
221 views

How can we measure light quality, quantity and duration in the field?

I am interested in measuring the light availability for coffee plants under a range of different adaptation practices (shade, cover crops etc). We want to ensure that the measurement can be automated (...
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3answers
60 views

When analyzing data from an experiment, is each $x_i$ a random variable?

When we perform one experiment we have to measure several quantities and take some number of measurements for each of them. Those quantities can be things like positions, instants of time, and so ...
3
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1answer
281 views

Why do we divide the standard deviation by $\sqrt{n}$? [duplicate]

I've been studying experimental physics on the book "The art of experimental physics" and on the chapter about error analysis there's something that has been bothering me. The author says: Now that ...
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4answers
4k views

How to measure the inner diameter of thin tube (0.5mm to 2mm)

Does anybody know what method or equipment could be used to measure the inner diameter of capillary tubes? They would be in the region of 0.5-2mm in diameter. Edit: the tube is made of plastic and I ...
0
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1answer
706 views

Problems measuring viscosity of water with a Falling Ball Viscometer

I am measuring the viscosity in water using a falling ball viscometer. In my system, I believe that exists two forces: The force difference between the weight and buoyancy of the sphere: $F_g = m'g =...
4
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3answers
428 views

Definition of temperature: “absolute” temperature and linear relations between temperature and thermometric properties

I'm confused about the definition of temperature, and I would like to make some points clearer since I did not find an explanation. Firstly I have heard of three adjectives regarding temperature (...
3
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2answers
656 views

Precision way to measure liquid volume? (avoiding capillary forces error)

Capillary forces is an enemy of every volume measurement for a rigid body or liquid. However, for rigid bodies there's that nice method used by jewelers: take a glass with water, measure its weight, ...
2
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0answers
308 views

17 MeV Anomaly in Beryllium Nuclear Decays - is it just a bump in sensitivity?

edit: @annav has noted an informative blog post summarizing the theoretical (but not experimental error) issues related to this measurement. There is some excitement about a bump in an electron-...
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0answers
96 views

Indistinguishability of Mixed and Pure States in Complex Systems

Consider the following two states: $\left|\psi\right\rangle = \frac{1}{\sqrt{2}} \left|\psi_1\right\rangle\otimes\left|\chi_1\right\rangle+\frac{1}{\sqrt{2}}\left|\psi_2\right\rangle\otimes\left|\...
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0answers
149 views

Is an interaction-free measurement an irreversible process?

From what I've understood from reading different online sources including PSE*, measurements in quantum mechanics are generally argued to be irreversible (at least, when a macroscopic measuring device ...
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2answers
377 views

Why physicists care about significant figures?

I came across the concept of significant figures through physics books not math books, surprisingly. I know the rules of them and they are trivial but why physicists care much about them? Why the ...
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2answers
318 views

How does a Geiger counter, such as RD1706, converts counts per minute to micro Sievert per hour?

The RADEX RD1706 contains two SBM20-1 type Geiger–Müller counters. I understand how the counts per minute value can be computed. But how is the micro Sievert per hour value obtained? And what is its ...
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0answers
251 views

What is actually happening when a magnetoresistive magnetometer degausses itself?

I'm trying to understand if anisotropic magnetoresistive devices themselves (not other passive ferromagnetic components) can become magnetized, and if so, how the application of a DC 1.1 gauss field ...
2
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1answer
157 views

Measuring gravity using (only) pendulums and rulers

Kater's pendulum was used to measure the local force of gravity. However, the procedure was to time it by using a standard pendulum clock! To use, the pendulum was hung from a bracket on a wall, ...
2
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1answer
91 views

Is the use of term Measurement appropriate in quantum physics [closed]

It may not sound like a physics question, but I think, it is. When we measure spin of an electron in a certain direction, we are actually, setting it up in that direction. That does not sound like ...
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0answers
209 views

States after measurement

I am learing about quantum mechanics, and I am quite unclear about the the role of measurement and the density operator. Let $\mathcal H$ be a Hilbert space, $\psi(t) \in \mathcal H$, $O$ be a ...
3
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1answer
141 views

The Validity of a Heuristic Explanation of Black Hole Complementarity?

In one of the messenger lectures at Cornell in 2013, Leonard Susskind gave a heuristic argument for black hole complementarity. Suppose Alice is stationed far away from a Schwarzschild black hole and ...
3
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2answers
215 views

On Newton's discovery of the universal law of gravitation

I'm reading Peter Collier's book on relativity and was on the chapter where he describes how Newton figured out the law for universal gravitation. This is the cannonball referred to in the following ...
2
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1answer
172 views

Are there any industry usage for gas thermometer? [closed]

I've been searching on this for a while now. Are there any specific advantages and disadvantages for using gas thermometer? Are there any specific industry usage for this kind of thermometer?
2
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1answer
781 views

What are the tools used by Newton to measure motion?

I was wondering how did Sir Newton measured motion of objects (non celestial ones). What was his toolskit at the time?
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3answers
3k views

Projection operators in quantum mechanics

I'm reading a book on quantum mechanics and a topic in there was not well explained. I couldn't find an answer on the web. Say $\psi(x)$ is a position wave function of a one dimensional free particle,...
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0answers
340 views

How to determine the three-velocity measured by a single observer?

In this question the issue of extracting the three-velocity of a test particle in some frame arose. The answer provided by @PaulT and in this paper (arXiv) seems to me to give rise to ambiguity when ...
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1answer
86 views

Entangled qubit measurement

I came across this while reading lecture notes and I have no idea how they got the $M_0$ and $M_1$. The way I see it, the matrix $U$ is a block matrix: $$ \left[ \begin{array}{ c c } P & ...
10
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1answer
176 views

Have we measured the rate at which Jupiter is shrinking?

On most websites that I looked at, it says Jupiter is shrinking 2cm/year (for example wikipedia and Quora). However neither of these website say whether this rate is measurable (the latter says it is ...
12
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5answers
342 views

Does a measurement always require the exchange of energy?

Can you measure something about something without adding or removing energy from it? This comment has got me wondering if there is a way to measure a property or a quantity of something without some ...
3
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1answer
151 views

How to read numbers reported in this LIGO paper?

I am not sure how to read the numbers in this excerpt from the abstract of GW151226: Observation of Gravitational Waves from a 22-Solar-Mass Binary Black Hole Coalescence: Are the +/- small numbers ...
2
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1answer
743 views

How to measure relative phases of quantum states

If I have a large number of identical systems in identical quantum state $\Psi$ and an observable $A$ whose eigenstates are $\alpha_n$: $$ \Psi = \sum_n c_n \alpha_n $$ I can get absolute values ...
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1answer
52 views

How the data or models given in physics or mathematics problems can be obtained in real life? [closed]

When ever there is physics or mathematics problem. we are already given a mathematical model. Can't we make own models from given diagrams or data collected from the problem? sometimes angle is given ...
3
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1answer
91 views

What are the effects of compositional changes in a thermocouple's junction?

We use thermocouples (type B) to measure the temperature of metals (Fe, Ag, Pd, etc) in a furnace under pressure. We weld two thermocouple wires together using an arc welder. This junction then comes ...
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2answers
4k views

Error Analysis involving random errors

The question goes like this: In an experiment, the time period of an oscillating object in five successive measurements is found to be $0.52$s, $0.56$s, $0.57$s, $0.54$s, $0.59$s. The least count ...
1
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1answer
295 views

How to find the covariance matrix after a partial homodyne measurement?

The Gaussian state of two modes, with quadrature operators $X_1,P_1,X_2,P_2$, is given by a displacement vector $d$ and covariance matrix $\sigma = \begin{bmatrix} Var(X_1,X1) & Var(X_1,P_1) &...
3
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1answer
556 views

How can I measure the amplitude of a light wave?

Suppose I have a light wave and I want to measure its amplitude, or check to see if it has an amplitude of a certain value: how would one go about doing this?
4
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2answers
225 views

Combining two data points with different uncertainties

I have two separate algorithms (call them "A1" and "A2") which reconstruct the $(x, y)$-position of an event in a particle detector. I can test both of these algorithms on simulated events from a very ...
0
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0answers
44 views

Does the definition of the metre have to take into account spacetime curvature?

The definition of the metre is: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second Does this have to be modified in a ...
1
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1answer
95 views

Systematic+Random Uncertainty for velocity measurement

I have a question regarding systematic and random uncertainties. I have to measure the mean value of a velocity measurement in flow field at a point. I've recorded say 1000 samples in time, which I ...
3
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1answer
821 views

How to measure the mass and balance point of a human forearm?

I've been trying to find this out for about a month now. I'm usually met with humorous but not really helpful comments about chopping my own arm off. I'm preferably looking to do this on a budget ...
0
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4answers
487 views

Is there an official list of independent units of measurements?

When I say 'independent units', I mean those which cannot be broken down anymore, and simultaneously forms the basis for any more, complex measurements. For example, height, length, and width can all ...