What is Mass: Definition and 1000 Discussions

Mass is both a property of a physical body and a measure of its resistance to acceleration (rate of change of velocity with respect to time) when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.
The SI base unit of mass is the kilogram (kg). In physics, mass is not the same as weight, even though mass is often determined by measuring the object's weight using a spring scale, rather than balance scale comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity, but it would still have the same mass. This is because weight is a force, while mass is the property that (along with gravity) determines the strength of this force.

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  1. L

    A Lyman-Series: calculating the nuclear mass of an isotope

    Assuming I have a given wavelength λ of the hydrogen spectrum and I want to calculate the nuclear mass of the isotope, is my approach correct? Rydberg-formula: $$\frac{1}{\lambda} = R_M \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right)$$ with $$R_M = \frac{R_\infty}{1 + \frac{m_e}{M}}$$...
  2. I_Try_Math

    Calculating torque on a pendulum

    Let ##m_{r}=1## kg be the mass of the rod and ##m_{s}=0.5## kg be the mass of the sphere. ## \tau = -rFsin\theta ## ## = -r([m_{r}+m_{s}]g)sin\theta ## ## =-1.3(1.5)(9.8)sin30 ## ## \tau = -9.6 ## My book's answer key disagrees and my initial thoughts are that maybe the mass in my...
  3. I_Try_Math

    What mass must be placed on the cord to keep the pulley from rotating?

    I suppose to keep the pulley from rotating the net torque has to be zero? Let ## F_{r} ## be the force that the 5 kg mass on the ramp exerts on the pulley and ## F_{d} ## be the force exerted straight down by the other mass on the pulley. Let ## r = 0.3 ## m be the outer radius of the pulley...
  4. I

    B The inverse-square law: Gravitational force on two falling marbles

    Imagine making a hole in the ground, about a mile deep, with a large and square diameter. In the middle of the hole, there is a hollow and narrow tube with all air sucked out. Next to one of the walls, so close that it's touching, there is another hollow tube without air inside. Two identical...
  5. R

    I Deriving e=mc^2, how is it possible?

    I was recently very surprised when I had a looked up relativistic kinetic energy. All sources gave the kinetic energy as the difference between total energy and rest energy, in some or other variant of the formula ##E_k=(\gamma−1)mc^2##. I didn't really understand at first. It seemed overly...
  6. Rick16

    I How can the volume of a star be determined?

    Until very recently I thought that according to general relativity masses curve spacetime as a whole, i.e. the complete abstract 4-dimensional spacetime. It came as a complete surprise to me to learn that you can take away the time-component and are left with a curved 3-dimensional space, which...
  7. Quantum55151

    Mass vs. Weight

    In the 2nd question, I definitely think it's D, because the force the rock exerts on your toe is equal in magnitude to the normal force the toe exerts on the rock which in turn is equal to the rock's weight, which is related to m by the constant g. In the 1st question, I am not as certain, but I...
  8. B

    I Does mass draw in space time?

    I have read widely about spacetime but not deeply. I am not good with high mathematics. I am aware of and comfortable with the concept: “matter tells spacetime how to curve, and curved spacetime tells matter how to move.” But not much deeper. This is a thought experiment. Is there an equation...
  9. D

    I What would a calculus author have to say on ##\int r^2dm##?

    So I've been searching around for rigorous explanations for things like ##dx## in physics, I'm not looking to fully commit myself to reading the relevant literature at the moment but just want to know what I'll have to do in order to understand. Perhaps I'll make a separate thread about that...
  10. A

    Potential Energy Problem: Pulling a Chain up onto a Table

    So, the first thing that came to mind when I was trying to figure out how to set this up is that it will be a dU problem. After trying to figure out how to set it up to no avail, I took a look at how they solved it in the solutions manual. It's making absolutely no sense to me... They state...
  11. bellum

    How can I identify the molecular fomula, based on an m/z of 350.7897?

    TL;DR Summary: I have the aforementioned m/z and a mass spec graph, my lecturer completely glossed over how to find the molecular formula so I'm stumped. I have a mass spec with 4 peaks and the m/z value, how can I find the molecular formula.
  12. ergospherical

    I What temperature would 1 keV neutrinos have today?

    Suppose there was a 4th generation of neutrino (X, say) with mass m ~ 1 keV. The other three neutrino generations decouple at T ~ 1 MeV and are not heated during ##e^{\pm}## annihilation (whereas the plasma is heated, leading to a bookwork ##T_{\nu}/T_{\gamma}## factor due to conservation of...
  13. hamishmidd

    Finding a formula for displacement of a mass on a spring using v.

    I have tried to answer this using the relevant equations I am provided on my formula sheet, however I get stuck pretty close to the end. I start with 1/2mv^2=1/2kx^2 at the equilibrium position, and kx=mg, x=mg/k. This gets me to v^2=mg^2/k, but I don't know where to go from there. The potential...
  14. R

    B How Much Mass Required to Demonstrate Gravity?

    If one were to demonstrate gravity independent of earth's constant gravity, how could it be done? Assuming this would need to be done in space, what minimum proportions of mass would be required to demonstrate gravitational pull to a human's naked eye?
  15. kirito

    what the relation between the amplitude magnitude and mass at resonance?

    should I be expecting a higher amplitude at resonance for a mass that's heavier to an extent form another where each is attached to a spring vertically , I assumed that's true since the heavier mass will stretch the spring more meaning when moving like a sin or cos wave the amplitude...
  16. Ranku

    I Determining stellar mass independent of its gravity

    What are the independent and accurate ways of determining stellar mass apart from its gravitational effect?
  17. sandeepts

    B Mass defect and electron transition

    According to STR: E=MC^2. When an electron and proton are independent( without influence of any kind of fields, especially electrostatic fields )their rest masses are Me and Mp. When they combine to form Hydrogen atoms they emit photons. So, some energy loss in the form of photons. So, now...
  18. stephen193

    I Where can I find information about this nucleon mass coincidence?

    I can't possibly be the first to notice this numerical coincidence, but my search skills are inadequate to find information about it, and I was hoping others could point me toward papers on the subject. Neutron mass, mN: 939565420.52 (54) Proton mass, mP: 938272088.16 (29)...
  19. M

    B Energy required to bring 1 kg 1000 meter above the surface of the Earth

    How can i calculate the Energy required to bring 1 kg 1000 meter above the surface of the earth ?
  20. Kostik

    A Variation of CMFR mass density ##\rho## along a world line

    Dirac ("GTR" p. 47) makes an interesting observation immediately after obtaining Einstein's field equations with the simple energy-momentum tensor ##T^{\mu\nu}=\rho v^\mu v^\nu##. (##v^\mu## is the four-velocity.) First, the conservation of matter ##\left( \rho v^\mu \sqrt{-g}...
  21. rosedog09

    Drag Forces and Terminal Velocity

    Teacher says Im wrong on both these questions. I have consulted with other teachers and they say im correct. What do you guys think? 3. As mass increases, so does terminal velocity. 5. Fnet = FDrag - Fg ma = FDrag - mg FDrag = 79(8) + 79(10) FDrag = 632 + 790 = 1422 N Teacher Comments 5...
  22. seonjunyoo

    B I wonder about a particle's energy when mass is obtained or lost

    According to Einstein's formula, energy is generated when mass is obtained or lost. What does losing or gaining mass mean for an atom or particle valence? It's simply because of the formula I don't want this kind of answer I want. What does it mean for an atom or particle to lose or gain mass...
  23. mister i

    B Percentage of known energy in the Universe

    It is said that the universe is made up of approximately 4.9% ordinary matter, 26.8% dark matter and 69.3% dark energy. Why isn't ordinary energy included in this "pie"? I suppose it is included within ordinary matter, but could it be calculated what % are particles with mass and what % is pure...
  24. S

    I Can there be slowly-falling accretion disks in black holes?

    Black holes accrete mass around them and it falls gradually up to the even horizon where mass is trapped by the black hole forever. However, the rate of mass falling from the accretion disk to the black hole ranges from being very fast to very long-lived, depending on various conditions...
  25. S

    Relativistic 2 -> 2 scattering with all equal masses

    Stuck on (c), part (i). Any ideas about what is the most elegant way to prove it, maybe using Mandelstam variables since this exercise is supposed to be about them?
  26. P

    A How the mass term of the Hamiltonian for a scalar fields transform?

    The Hamiltonian for a scalar field contains the term $$\int d^3x m^2 \phi(x) \phi(x)$$, does it changs to the following form? $$\int d^3x' {m'}^2 \phi'(x') \phi'(x')=\int d^3x' \gamma^2{m}^2 \phi(x) \phi(x)$$? As it is well known for a scalar field: $$\phi'(x')=\phi(x)$$ .
  27. I_Try_Math

    Vertical Mass Spring System | Analyzing work

    Is there a typo in this question? Supposing there was no friction, the block would fall until the force of the spring was equal to ##mg = 2 * 9.8 = 19.6##, taking the upward y direction as positive. Since ##F_{spring} = -200y## and ##19.6 = -200(-0.098)##, the block would fall 9.8 cm. It's not...
  28. keyzan

    Wave function of infinite potential well

    Hi guys i have this exercise: A particle of mass m, confined in the segment -a/2 < x < a/2 by a one-dimensional infinite potential well, is in a state represented by the wave function: 1. Determine the constant N from the normalization condition. To do this, I have to integral the square...
  29. M

    Why does mass not affect sliding speed down an inclined plane?

    I think that both kids experience the same acceleration (irrespective of mass) since the only force pushing them downwards is acceleration due to gravity, which is the same for both of them. Thus, since they start sliding down the hill at the same time (assumption), and are accelerating at the...
  30. bhobba

    B Is the Higgs Boson the Only Source of Mass?

    This is meant for our younger readers who have only seen popular accounts of where mass comes from. They often say it comes from the Higgs boson, which is sort of true. But it is deeper than that. Sabine has given a deeper popular account in the following video: As an aside, forget this...
  31. Ranku

    I Mass in an expanding or static spherical distribution of matter

    In a spherical distribution of matter - such as with clusters of galaxies - how to calculate how much mass there should be for it to not expand with the expanding universe - in other word, for it to be a bound, static system?
  32. D

    Problem Related to Photons with Mass

    Before boost we have Then using the Lorentz boost: I want to calculate: I tried multiplying the matrices together but I never get the stated answer which should be:
  33. M

    B "If a black hole has more mass then the event horizon shrinks" ???

    The title is a direct quote of this video by Dr. Becky Smethurst, an astrophysicist specializing in black hole research. This is a mistake, right? Supermassive black holes, for example, don't have tiny radii, compared to stellar mass BHs. Then there's the equation she presents seconds later...
  34. G

    I Do objects of different mass but same size/shape accelerate the same?

    Hi. I remember having learnt in school that if you'd like to verify that bodies of different mass accelerate the same in free fall, but don't have a vacuum available, the bodies should be of same size and shape (e.g. spheres). This made sense to me back then because drag should be the same...
  35. cxza

    Is the Final Velocity of a Fly Impacted by an Elephant's Mass?

    My solution was that the final velocity of the fly is equal to the mass of the elephant divided by the mass of the fly, and then multiplies by the delta in the elephant's velocity. My teacher said it was the wrong answer and that the calculations are presumably pretty long
  36. L

    Scarf modelled as a pulley system / F=ma exercise

    My teacher gave the above answer as a solution. However, I am not convinced that the proportion is really $$\frac{m_1}{m_1+m2}$$. If m2 << m1the proportion would be really big, right? But intuition tells me that it should be the opposite. Furthermore, if m2 >> m1, then one would expect the...
  37. T

    I Explanation of parallel axis theorem

    For a rotating system with mass m this theorem says that if it rotates about an axis distance x from but parallel to the axis through it's natural mass center (CM), then I moment of inertia is $$I=I_{CM}+mx^2$$ My thinking is if one move the axis x distance away from the axis through it's CM...
  38. T

    Speed of a mass falling into a star given the mass and radius of the star

    I tried the square root of ((2)(6.67*10^-11)(3.90E+30))/(5.70E+7) I got 1.55*10^-5 and that is wrong. Maybe I am using the wrong equation but this is the one of professor gave me and I don't know what I am doing wrong :-(
  39. Hak

    B How quickly does the Earth feel the effects of a halved Sun's mass?

    I have a doubt about gravitation. Suppose the mass of the Sun halves in an instant, after how long does the Earth ''notice'' it? That is, does the gravitational force also decrease instantaneously? Instinctively I would say yes, but I don't understand why it should be so. If, for example, we...
  40. B

    B Speed of separation of two masses

    Hello. Let's say we have two masses, each moving in 90 percentage of light speed in opposite direction. Then what will be the speed of the one mass according to an observer in the other mass?
  41. Username34

    How much impact mass does a boxing strike need to compensate for slower speed?

    I'm trying to wrap my head around the mass vs speed equation of mechanical power. If you double mass, you double power. If you double speed, you quadruple power... Fine, but what does that tell us about the relative importance when things aren't doubled? Let's say boxing Agent 1: has 3%...
  42. P

    I Can you increase the mass of a closed system with information?

    Let's assume that there is a closed box, with mass M. There are some random quantum processes inside it, say radioactive decay. Let's assume that we can manipulate the decay from the outside somehow, thus 'putting information' into the box. Can that affect its mass?
  43. Z

    A Higgs boson and its decay width

    hi, I noticed that with higher mass decay width also go higher - but for higgs boson its mass is higher that W and Z boson but its decay width is lower , why?
  44. BWV

    B Would a solid neutron mass be stable?

    not asking about neutron stars, where gravity holds neutrons together, rather would the strong force hold individual neutrons together in a solid mass, preventing their decay if one could somehow put them together in a lab?
  45. D

    I What was the four-momentum meant to include?

    Hello, was the four-momentum of relativity, Pν, supposed to include all mass and energy contributions from every field i.e. electromagnetic, strong, gravitational... Or is it just the momentum of what was known in Einstein's time?
  46. Ken G

    I Without degeneracy, when would Solar cores collapse?

    I believe I know the answer to this question, but it is still very informative to ask: Would iron stellar cores still collapse when they reached some mass without degeneracy (by which I mean, if electrons were not indistinguishable, so did not obey the Pauli exclusion principle on such large...
  47. a71cj34

    Same Charge, Same Mass? | Subatomic Particles

    I converted 3.1eV into J, substituted into E = mc^2. Since the energy is the same, I got the same answer for both: 5.52*10^-36 kg. This doesn't seem quite right- I doubt that a photon and an electron have the same mass. So, when two particles have the same charge, does that mean they have the...
  48. Ranku

    I Mass measurement of galaxies with non-gravitational means

    What are the non-gravitational ways of determining mass of galaxies, and how accurate are they?
  49. Lotto

    B How will object's velocity change when a frictional force increases?

    Let's say that the mass of the objest is suddenly bigger, so when I want to maintain the constant movement, my force must increase as well. But will the velocity have the same magnitude? I think that the velocity will be smaller, so if I want to have the same initial velocity, I must apply an...
  50. J

    B Force transition through spring

    We put object on weight ang get a mass. What would that mass be if we put a spring between object and weigt, so that the spring woul shrink to half its original size?
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