# Tag Info

### Two equations for heat transfer: using mass and moles

From your first equation ($\Delta Q=C_v m \Delta T$), we see that $C_v$ must be the heat capacity per unit mass, known as the specific heat capacity. [As far as I know, $C_v$ isn't standard notation ...

### Why we take $k=1$ in $F=ma$?

I know that F is directly proportional to mass and acceleration but why in formula we take k=1 not other constant? This depends on your system of units. For SI units $$\Sigma \vec F = m \vec a$$ but ...
1 vote

### Why we take $k=1$ in $F=ma$?

The force was found to be proportional to the acceleration, not $ma$. This means that $F=ma$ with $m$ the constant of proportionality.
Accepted

### Lagrangian with negative mass

The sign of fermion mass is just a human convention. Let's say you have a Dirac Lagrangian with positive mass $m$: $$L = i\bar{\psi}\not{\partial}\psi - m\bar{\psi}\psi$$ If you redefine the fermion ...

### How $e/m$ ratio depend on voltage?

$\frac{1}{2} m v^{2} = e \cdot V$ Over here $e$ is electron charge, $m$ is mass of electron, $v$ is velocity of electron and $V$ is voltage. So, obviously your teacher did a mistake on his end. $e/m$ ...

### How small do you need to compress mass to make a black hole?

You can start with any constant positive density material, and accumulate a big enough ball, and it will become a black hole, without any need for compression. A black hole is a lot of material in a ...

### How much mass is needed to make the speed of light equal to 1 mile per second?

For photons, $E = h\nu$ where $\nu =$ frequency. Any change in photon energy does not change the photon velocity it changes the photon frequency. Inside the sun, the intense density means photons only ...

### How much mass is needed to make the speed of light equal to 1 mile per second?

It's not the mass that matters, it's the gravitational potential, which depends both on mass and on distance. Also, the Wikipedia quote is a bit misleading if taken out of context. The speed of a ...

### How much mass is needed to make the speed of light equal to 1 mile per second?

No amount of mass can change the speed of light. Mass corresponds to a change in the direction of the geodesics in which light can travel and along which light always travels at $c$, not a change in ...

### Is $F= ma$ applicable in only those cases where mass of the body remains constant?

$F=dp/dt=d/dt(m\cdot v)=dm/dt\cdot v + m\cdot dv/dt$ where the latter term is $m\cdot a$ So your snowball has something in common with rocket science, where the rocket reduces mass at whatever rate. ...

### A theory for calculating mass and charge of electron

String theorists have been unable to calculate the mass of the electron,though that was one of the initial goals when it was introduced in the 1970's. The last time I looked the 'phenomenology' ...

### Is the unit $m^2$ for area size ambiguous?

The force generated by a field originating from a point source (e.g., gravity or charge) is equally strong at all points equidistant from that source. All points equidistant from a given point ...
1 vote

### Is the fine structure constant a function of electron mass?

I know you worked hard on this and want a more useful answer, so sorry to be negative: the definition of the fine structure constant, as a constant, is just the low energy limit. It will not change ...

### Is 'rest mass' same for every observer/perspective?

Rest mass of a body is the mass measured when the body is at rest w.r.t. you. It is always the same. Just make sure that the object's whose rest mass you want to measure is at rest w.r.t. you. The ...

### Is the unit $m^2$ for area size ambiguous?

You are confusing the unit with the physical quantity. The unit with which you measure an area should be identified with the unit with which you use to add the squared distance components in ...

### Minimum size of black hole

Stars having mass as big as 25 solar masses are prone to collapse to form black holes, during the process a part of the star spills out and most of the part collapse to form a black hole, after a ...
Accepted

### 'Elementary' particles (electrons) vs non-elementary particles (protons) regarding relativistic mass

Everything inside the proton transforms in the same way as an electron would, including intra-quark binding energies and such, so no, the observed energy ("relativistic mass") of composite ...

### How can the graviton be both massless and self-interacting?

Gravitons are charged under gravity. They don't have mass, but they do have energy and momentum. And in general relativity, energy and momentum gravitate just as mass does. So gravitons gravitate (...
Gravitons couple to energy-momentum tensor $T_{\mu \nu}$, not mass (which captures only the $00$ component of $T_{\mu \nu}$). That's why photons (which are also massless, like gravitons) interact with ...