Read about units | 43 Discussions | Page 1

  1. F

    I The new definition of kg and the mass-energy equivalence

    Hi, today I stumbled upon a 2016 article in Scientific American about the (then) possibility of re-defining the kilogram through Planck's constant. The article is really a very quick review of the topic. At some point the author states the following "So for years, physicists have chased an...
  2. R

    Moment of Inertia and units

    Calculating moment of inertia and translating it between units, I've become confused. The example is a mass of 1kg at 2000mm from the pivot. The force is applied at 1000mm from the pivot. Basics as far as i'm aware: Moment of inertia = mass * Distance to center of rotation^2 Torque = Moment...
  3. Poetria

    Air pressure - units

    Homework Statement [/B] dp/dh = -.13*p h in kilometers I should convert it into meters 2. The attempt at a solution I thought it would be enough to multiply -0.13 by 1000. :( dp/dh = -130*p but it is wrong.
  4. Q

    I How do we justify "Natural" Units

    How is it that when using "natural" units we drop the units themselves. I understand that you can arbitrarily change the magnitude of a parameter by choosing a new unit. For example Oliver R. Smoot is exactly 1 smoot tall. However, in natural units with [c]=[h/(2π)]=1 the "smoot" part is...
  5. J

    B When Difference between Kelvin and Celsius are negligible

    This question is more for practicality than anything else, and I realize it may be partly subjective. At what temperatures can the 273 degree difference between Kelvin and Celsius scales be ignored? I'm thinking for examples regarding stars and very high temperatures. Surely once you get to 15...
  6. enter

    Imperial units

    America is mostly leading in science, especially in astronomy, yet they still use imperial units. (I am not talking about the great people doing the science.) Everyone else is using SI and the metric system however they insist on using miles, yards, pounds, feet, etc., and continue on being a...
  7. P

    I SI definition of second: what day/year is the reference year

    The SI definition of a second was originally based on 1/86 400 of a day - but the earth's rotation is a) unsteady and b) changing so it was changed to a defined number of cycles of an atomic clock. Was there a specific day for which the atomic clock was calibrated? Put another way, why were 9...
  8. J

    B Unit conversion meters newtons years seconds

    Hi, I need to convert a value of 12*10-16 m7 N-3 y-1 to the units of kPa-3 m2 s-1. Can anyone help me? where: m = meters, N = Newton, y = year, s = seconds. Thanks!
  9. Jezza

    I The Planck units, just a fad?

    As a physics undergrad, a set of natural units is nothing strange to me, we use them all the time. Having said this, I've never used Planck units. Does any area of research use them on a typical day in the office? There also seems to be this idea that I hear from time to time, perpetuated by...
  10. scottdave

    Insights Make Units Work for You - Comments

    Greg Bernhardt submitted a new PF Insights post Make Units Work for You Continue reading the Original PF Insights Post.
  11. D

    SI vs USC Units in Engineering

    It is well known that most of the world use the SI system of units while the USA and a couple of other countries continue to use US Customary units (ft-lb-s = FPS or in-lb-s = IPS). The question of which is better is very much a matter of personal preference, but I'd like to avoid that issue...
  12. A

    B Kepler's Third Law confusion

    I am confused about the units used in Kepler's 3rd law. Is T supposed to be in years or days? Is R supposed to be in kilometers or meters? Is there ever an instance where one combination of units is preferable over another (for example, if you want to use the answers from Kepler's third law to...
  13. F

    I Why am I getting two different results in emu and SI unit?

    I am computing force between two magnetic poles each of one unit pole (in emu) and situated one centimeter apart. In electromagnetic units: ##F_{dyne}=\dfrac{p^2}{r_{cm}^2}=\dfrac{1^2}{1^2}=1 dyne## where ##p## is pole strength in emu In SI units: ##F_{N}=k_A \dfrac{P^2}{r_m^2}=10^{-7}...
  14. DrClaude

    Dimensionless units

    The topic has been discussed before at PF, in particular @haruspex's Insight Can angles be assigned a dimension? Nature has now published an editorial on the subject: SI units need reform to avoid confusion
  15. Erenjaeger

    B Unsure of astrophysics unit

    what is this unit, ⊕ ?? was reading an astrophysics book and came across it. "Gas Giant planets (>10 M⊕)" Im guessing that its mega something?
  16. Nim

    I Why Is There No SI Unit For Information?

    Why is the quantity "information" missing from the SI system? Also, if they did add it, do you think it would be added as a base unit or dimensionless derived unit?
  17. T

    B Temperature Conversions

    Why is that whenever we convert a temperature, say ##^\circ C## to ##^\circ F##, we use ##T_F = \frac{9}{5}T_C+32##, but whenever it involves a rate, say ##^\circ C/m##, we only multiply by the slope of the previously given formula? I would say it's because of the latter being a rate so...
  18. B

    I Hamiltonian for spin-1/2 particle in B-field: units issue

    Take a spin-1/2 particle of mass ##m## and charge ##e## and place it in a magnetic field in the ##z## direction so that ##\mathbf B=B\mathbf e_z##. The corresponding Hamiltonian is $$\hat H=\frac{eB}{mc}\hat S_z.$$ This must have units of joules overall, and since the eigenvalues of ##\hat S_z##...
  19. L

    How do I use this equation? Estimating half-life of a decay

    How should I go about using equation 8.18? Link can be found below. In the book, an example is used where Th-220 --> C-12 + Po-208 with a Q value of 32.1 MeV is used, and it is said to yield t1/2 = 2.3x106 but for the life of me I cannot reproduce this result. This is what I did: Going from...
  20. L

    Pressure variation with sound waves equation

    Homework Statement P(average) for a speaker is 10 W. Gamma is 1.4 (ratio of specific heats), molar mass is 28.8 g/mol, air temperature is 50F, and pressure is 1atm. Find Pmax at 100 I have this equation that gives Intensity = (Pmax^2)/(2*Rho*v) where rho is density, and v is speed of sound...
  21. R

    How do I convert cubic picometer to cubic centimeter?

    Part of the question says: An unit cell has a cell edge of 288 pm. I got to the part where volume (288 pm)^3 = (288 x 10^-10 cm)^3 I have no idea how that equals 2.39 x 10^-23 cm^3 Only the Bold part ^
  22. Z

    How to find the units for Ksp

    Homework Statement I have found, through experimentation, the ksp of calcium hydroxide to be 3.0 x10^-7 (which i know has a pretty large percent error). However, i am conflicted as to what the units should be because logically, plugging the units into the ksp equation should give you units of...
  23. P

    Units of the given potential box

    Homework Statement A car (particle) with mass m = 1 t drives into a barrier with a height of 1 m and a thickness of 3 m. The kinetic energy shall be sufficient to get classically over a barrier with half of the height. Derive an equation for the tunnelling probability. What is the probability...
  24. Nader AbdlGhani

    I Why Planck's Constant Has Dimensions and a Unit?

    Despite being a constant, It has both dimensions and a unit, can someone kindly explain why ?
  25. N

    Correct formula for induced voltage with a stationary coil?

    I'm making an AC generator with a stationary coil and rotating magnets. What would be the correct formula to find induced voltage at a certain rotation speed (along with the units)? All sources I've found are either non-applicable or do not explain what units to use. Thank you for the help.
  26. Oannes

    Finding Surface Area in square feet with Volume & Thickness

    Homework Statement How large a surface area in units of square feet will 1 gallon of paint cover if we apply a coat of paint that is 0.1cm thick? Homework Equations Since Volume is L * W * H and we can assume the object is square besides the height which in this case will be the thickness. So...
  27. entity314

    Hysteresis Coefficent Units

    I'm working with hysteresis losses in transformers at the moment, and I was wanting to know if there are units for the coefficient. Normally, I'd just rearrange the equation for units, below, h=ηB1.6f η=hf-1B-1.6 so it would have units JsT-1.6 However, given the power of 1.6, I'm not sure it...
  28. C

    B For oscillations, why do we use angles in waves and oscillat

    For example, the term angular frequency, it units is radian per second. For phase, it is also measured in radians or degrees, why is that? Why is the math the same when you use angles to describe oscillations?
  29. S

    What are s.g. units ??

    Hi all, I am currently trying to understand a paper which presents a model for sand production and wellbore collapse in the oil&gas industry. The paper refers to stresses and pressure in "s.g." instead of Pa or other pressure/stress units... values tend to range from 0.4 s.g. for cohesive rock...
  30. O

    I How long can you be exposed to gamma radiation for

    So my question is, how long can you be exposed to a gamma radiation for? Specifically, I've been working with a sealed cobalt-60 source for a final presentation. Now my professor noted to me that these sources we're using aren't radioactive enough to cause us any harm. However, I've been...