# Tag Info

### What gives particles mass? Also why are particles like photons considered massless when they have energy and momentum?

The idea that a photon is massless is a pedagogic convention, that has not always held sway. When I was taught special relativity in the '70s, there were two kinds of mass that were distinguished. ...
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### Does this prove that the force and momentum formulas are wrong?

Assuming that OP means Newton's second law by force formula, Note that it says $$\mathbf{F}_\text{net}=m\mathbf{a}_\text{net}$$ In you situation, you are putting two forces from opposite sizes so that ...
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### Energy to momentum

Yes, light has momentum, so you could use your collected electrical energy to create a beam of light to propel the satellite. However, you need to bear two points in mind- firstly, the recoil from the ...
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### Is it possible to move without throwing or pushing another object or energy?

Is there any way that a closed system can move without throwing or pushing anything outside of it? Yes, it can move at a constant velocity without throwing or pushing anything outside of it. Edit: in ...
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### Is it possible to move without throwing or pushing another object or energy?

Please restrict the answer to classical physics. The center of mass of your object cannot change its momentum without transferring momentum to or from some other object. You could make the object ...
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1 vote
Accepted

### Do Einstein field equations only relate local spacetime curvature to local energy-momentum of matter?

Yes, they cannot be extended to relate global curvature to global energy-momentum, not in general at least. You can see this by noting that the Einstein Field Equations can be derived by demanding ...
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1 vote

### Energy to momentum

You're right, the only way to change linear motion in space is by throwing stuff out, and why is that a problem? Instead consider, most obviously, a nuclear submarine which any number of people think ...
1 vote

### Energy to momentum

The most direct method is to use the Earth's magnetic field. It is generally used only for attitude control (i.e. generating angular momentum), but nothing (except the low efficiency, of course) stops ...
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1 vote

### Energy to momentum

You should read up on solar sails, electric sails, and magnetic sails if you are interested in propulsion near stars. A solar sail uses the momentum of solar photons to drive a ship, while electric ...
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1 vote

### On what ground do you multiply $m$ with $v$ in the momentum equation $p=mv$?

Strictly speaking, a definition doesn’t need justification. However, some definitions are useful and important, and it is relatively easy to show why they are useful. In the case of momentum, ...
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1 vote

### Moving cart, falling rain, and constant velocity

$F_{net} = \frac {d}{dt} (mv) = v\frac {dm}{dt} + m\frac {dv}{dt} = v\frac{dm}{dt}$ for constant velocity $v$, mass $m$, rate of change of mass $dm/dt$ (the value you listed as $x$ in the question). ...
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1 vote

### Intuitively Understanding Work and Energy

"Most of the fundamental ideas of science are essentially simple, and may, as a rule, be expressed in a language comprehensible to everyone." (Einstein and Infeld, The Evolution of Physics) ...
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