# Questions tagged [approximations]

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5answers
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### Wave Equation derivation

I'm curious about part of the derivation of the wave equation as is done in all references that I've seen so far (I'm gonna reproduce only the part that's puzzling me). We apply Newton's second law ...
0answers
50 views

### Ignoring 2nd order terms in QM definitions of translation and momentum operators

In Sakurai's 'Modern Quantum Mechanics', he defines the infinitesimal translation operator as: $\mathcal{J}(d\mathbf{x}')=1-i\mathbf{K}\cdot d\mathbf{x}'$, and then he goes on to prove this ...
2answers
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### Why do we don't get exact answers to our experiment but rather an approx value to it?

Why do we face a problem of not getting to the exact answer but to an approx to it, mostly everything we come through in practical physics is a approx but not the exact one.
0answers
23 views

### How to find the terms of Post-Newtonian approximation

I am studying the Damour paper on the Post-Minkowskian approximation to the 2 bodies problem in General Relativity (paper) but can't really understand the explanation of the previous state of the art, ...
0answers
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### What is the static exchange approximation?

In this paper, on the 4th page (and throughout), they talk about studying electron-helium scattering in the "static-exchange approximation". I have scoured the literature and have not been able to ...
2answers
696 views

### Approximating an expression for a potential

In a problem which I was doing, I came across an expression for the potential $V$ of a system as follows $$V = k\left(\frac{1}{l - x} + \frac{1}{l + x}\right)\tag{1}\label{1}$$ where $k$ is a constant,...
1answer
57 views

### AC currents for self-inductance of a wire

I've been currently reading some stuff on self-inductance of a wire. There are different regimes where different approximations are used. For the AC currents the notes state that an approximation can ...
2answers
115 views

### Are the infinitesimal lengths in integrals bounded by the Planck length? [closed]

When we integrate something say work, $\int F\cdot ds$ then we will get work but what exactly is $ds$? how much is ds? Is it the Planck length? Are we just pretending there exists some infinitesimals ...
1answer
60 views

### Why is the Earth not an inertial frame of reference?

From many sources I have found the explanation that the Earth is not an inertial frame of reference because it rotates around its axis. However, nobody mentions the rotation about the Sun. What I ...
2answers
36 views

### Why is the velocity in the horizontal direction constant for 2D projectile motion?

For 2D projectile motion, why is the velocity in the horizontal direction constant? I learned that we neglect air resistance, but why? And isn't there horizontal acceleration at the beginning when ...
0answers
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2answers
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### Thick vs thin lens

What is the criteria to know whether a lens is thin or thick ? Suppose we have a lens of r1= 10 mm , r2= -10 mm and thickness is 5 mm. So, with this information what can we say about the lens whether ...
0answers
91 views

### Perturbation to the flat space metric

from the geodesic equation for non-relativistic case where $$v_i\ll c$$ $$\frac{dx^i}{dt}\ll1,{\rm for }\ c =1$$ $$\frac{dx^i}{d\tau}\ll\frac{dt}{d\tau}$$using this the geodesic equation for proper ...
1answer
141 views

### Coulomb's Law Question

The presentation of Coulomb's Law in various books occasionally has a note that the test charge, q2, must be small enough that it doesn"t alter the field of the first charge, q1. The same limitation ...
0answers
37 views

### QM limit of QFT in Schwartz [duplicate]

In Matthew Schwartz's QFT text, he derives the Schrodinger Equation in the low-energy limit. I got lost on one of the steps. First he mentions that $$\Psi (x) = <x| \Psi>,\tag{2.83}$$ ...