All Questions
Tagged with units measurements
43 questions
3
votes
1
answer
103
views
Why is amount of substance considered a fundamental quantity? [duplicate]
The unit of amount of substance is mole. 1 mole is defined as the amount of substance that contains Avogadro Number (6.022 x 10^23) no. of entities in a substance. It is a fundamental unit.
However, ...
- 33
0
votes
2
answers
86
views
Can we measure $c=1$?
Usually the speed of light is measured using the S.I. units, yielding a result in $m/s$.
In geometric/Planck/Natural units it is known that we take $c=1$.
Clearly this is just a redefinition of the ...
- 1,854
0
votes
1
answer
53
views
Does changing units affect Significant Figures
Does changing the unit of a physical quantity has no influence whatsoever on the number of significant figures?
Suppose the mass of a particle is given as 0.060kg
0.060 has 2 significant figures as ...
- 3
1
vote
2
answers
133
views
How were angles measured in ancient times?
What is one degree (angle measurement)? I know that we calculate one second using atomic clock, one kilogram using Planck's constant, one meter using speed of light but how do we define one degree? If ...
- 11
2
votes
1
answer
118
views
The Principle of Homogeneity of dimensions states that you can add,subtract quantities with same dimensions but we cannot add a constant with an angle
Both a constant and a plane and solid angle are dimensionless ie they have the same dimensions , so according to principle of homogeneity should you not be able to equate them ?
But it would be absurd ...
- 23
0
votes
1
answer
90
views
Mass, time and length are measured with 5% accuracy each, what will be the total accuracy in torque? [closed]
I don't understand the solution of the above question, the solution i know is :-
Accuracy in torque = [M%] + [2 × L%] + [2 × T%]
Which will be equal to 25%
I dont understand why 2 was multiplied to L% ...
- 3
0
votes
5
answers
639
views
Is the magnitude of a vector equal to its length
Suppose a body has a velocity $V =2\hat i$ metres per second. The magnitude of velocity is 2 meters per second . We can draw a vector of length 2 cm to represent this velocity.
According to the below ...
- 1,340
1
vote
1
answer
193
views
Halliday Resnick and Krane chapter 1, problem 2 part C. Can't understand instructor's solution [closed]
I understand how to solve this question but I don't understand the solution in the instructors manual for part c.
The question is:
The average distance of the Sun from the Earth is 390 times
the ...
- 139
1
vote
1
answer
80
views
Couldn't understand an example used in explaining fundamental and derived quantities
In my book under topic of fundamental and derived quantities, there is an example which is supposed to explain this concept and it goes like this,
As a simple example, if a unit of length is defined, ...
- 157
0
votes
1
answer
41
views
Minimum information required to measure your local physical environment
In Andy Weir's "Project Hail Mary" protagonist Ryland Grace wakes up in an environment and with a few physics experiments timing falling objects he relatively quickly determines that he is ...
- 187
2
votes
1
answer
178
views
Good book/resource/chapters on the foundations and science of accuracy, precision, and measurement?
I guess measurement and accuracy is a science, and thus, I assume there's foundations and universal theory that can be learned from scratch, independently of international conventions used, just like ...
0
votes
1
answer
637
views
Fractional error in time period
The length of a pendulum is measured using a meter scale which has 2000 divisions. If the measured value L is 50cm, the accuracy in the determination of g is 1.1% and the time taken for 100 ...
0
votes
1
answer
165
views
How does an accuracy specified in vol% translates to ppm?
I need to setup some CO2 concentration measurement. The datasheet of the sensor is:
Range: 0 to 25 vol%
Accuracy < 0.5 vol% + 3% of measured value
vol% is the percentage by volume. So the range ...
- 103
0
votes
1
answer
90
views
How to define "unit spatial length" in general relativity?
Suppose I have a metric with time-independent $g_{\mu\nu}$. Suppose I have a small measuring rod at some spatial position and I define it to be of unit length. Will it still have unit length when it ...
- 127
-1
votes
2
answers
415
views
How do scientists derive equations and formulas? [closed]
How do scientists derive equations?
For example, how do they know that:
Work done = Force × Displacement
And why is it always multiplication and not anything else? Why not addition?
- 167
0
votes
1
answer
61
views
Non-hypothetical problem of using rulers to measure lengths
Consider two factories on earth's Equator producing rulers of the same length. One factory produces rulers made of wood and the other one produces rulers made of aluminium. Both types of rulers are ...
- 1,911
0
votes
1
answer
40
views
Generalised Unit Conversion
I would like to know the general method of converting between units. For example converting Joules to $\frac{g\cdot cm^2}{s^2}$
I assume it's possible because the SI unit of energy is $\frac{kg\cdot ...
user279797
0
votes
1
answer
95
views
Why does a right-circular cylinder helps reduce surface area of the former International Prototype of Kilograms
I read on Wikipedia that right-circular cylinder shape helps reduce surface area of the former IPK, but could not find an explanation as to why. So how does such shape helps reduce its surface area? ...
- 81
1
vote
3
answers
435
views
Units of observables in quantum mechanics
Observables in quantum mechanics are described by Hermitian operators $\hat A: V \to V$, where $V$ is the Hilbert space of states.
Examples include the $x$-coordinate operator $\hat x$, the $x$-...
- 448
-1
votes
1
answer
77
views
Adding or subtracting mean absolute error
In an experiment to find the length of a rod, n observations were made. ${a_1,\;a_2,\;.\;.\;.\;a_n}$. The mean (${a_{mean}}$) of those observations will give a value very close to the true length of ...
- 359
1
vote
0
answers
89
views
How to Measure Electric-Field Probe Sensitivity?
I am doing some reading on current methods for detecting electric fields. A figure of merit for the systems is, naturally, the minimum strength of field that can be measured, in units of Volts per ...
- 1,576
1
vote
1
answer
120
views
Are there any physical quantities that are loglog?
Most units, e.g. for length, time, mass, etc, are linear in relation to the underlying property.
But there are some special quantities, where the units can vary several orders of magnitude within the ...
- 111
1
vote
1
answer
1k
views
Profile of a platinum-iridium meter bar
The former meter standard, platinum-iridium meter bar, had a specific cross section somewhat resembling mixed variant of letters "X" and "H" with serifs (Image source):
What was the reasoning behind ...
- 353
1
vote
2
answers
329
views
What does "as small as a fraction of an angstrom" mean?
I was reading my school textbook in which the following statement was given:
The resolution of such an electron microscope is limited finally by the fact that electrons can also behave as waves! (...
1
vote
1
answer
643
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 ...
1
vote
0
answers
221
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} = \...
- 189
-1
votes
1
answer
473
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 ...
- 119
1
vote
2
answers
82
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 ...
- 113
1
vote
1
answer
11k
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 ...
- 1,157
0
votes
1
answer
99
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 ...
- 1,024
0
votes
2
answers
6k
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 ...
- 419
1
vote
1
answer
511
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 ...
- 297
0
votes
1
answer
168
views
Using photometric quantities for determination of intensity of laser light
I would like to measure the intensity of a laser beam after it has passed through a translucent material. However, the sensor that I am using returns values in lux, which reflects the subjective ...
- 249
0
votes
1
answer
88
views
How could the BOE (barrel of oil equivalent) be measured?
The barrel of oil equivalent was given as an example of gigajoule scale energy.
"The barrel of oil equivalent (BOE) is a unit of energy based on the approximate energy released by burning one barrel (...
- 459
0
votes
2
answers
121
views
How are the SI units "generalised"?
How exactly are the SI units generalised from their definitions?
E.g. the kilogram is a weight of an object of cylindrical form, with diameter and height of about 39 mm, and is made of an alloy of ...
- 459
-1
votes
3
answers
105
views
Why are measurements standardized the way they are?
Using meters as a base length, squaring or cubing lengths smaller than 0.67m makes the square term larger than the cubed term. This fact causes certain properties of physics (how rain needs to form?) ...
- 21
2
votes
2
answers
294
views
How do we deal with irrationals in Physics? [duplicate]
This question is really very basic: how do we deal with irrationals in Physics? If for instance, in some meaningful calculation we get a length of $\pi$ meters, or a force of $\pi$ Newtons, how should ...
- 37.4k
1
vote
1
answer
2k
views
Photometer: measured Irradiance $L$ converted to photon rate
I am conducting an experiment in which the power meter reading of $410\,nm$ narrow bandpass stimulus is noted to be 30 $\frac{\mu W}{cm^2}$ at a distance of 1 inch away from the light source.
I wish ...
- 11
1
vote
4
answers
12k
views
Magnitude refers to number or number with units?
This question is about terminology for physical quantities.
When we talk about magnitude (while talking about scalars and vectors) do we refer to just number or Number along with units?
example: If ...
- 7,463
2
votes
0
answers
468
views
Galaxies and projected radius vs arcmins
I have two sets of data (globular clusters), one for the Andromeda Galaxy and one for the Milky Way.
I want to compare the distribution of globular clusters between the two galaxies according to ...
- 190
0
votes
1
answer
253
views
$\mu$T in to $\frac{W}{m^2}$ (for interpreting EMF readings)
I am considering purchasing an EMF reader, to collect data about what is being thrown off of power lines and various other sources in the house to reach some conclusions.
An issue is, the meter can ...
0
votes
1
answer
5k
views
Question Based On Units And Measurements Involving Lengths [duplicate]
The below is the sixth question of the very first chapter from halliday and resnicks fundamentals of physics text,which i'am not able to comprehend.
Harward Bridge,which connects MIT with its ...
- 472
-1
votes
1
answer
26k
views
How light years is measured? [closed]
How are light years are measured? I remember the distance between earth and moon are measured by the delay in light which travels and comes back. But how are light years calculated?
- 107