1
vote
1answer
105 views

How much fuel is needed for a constant-acceleration rocket?

My son and I have been discussing the concept of a constant-acceleration rocket, as described here and here. We're willing to assume advanced technology such as a fusion rocket, which, according to ...
0
votes
0answers
15 views

Help with Kinematics and trajectory [duplicate]

I am asking this again because it was closed and I didn't receive a response. So a baseball is hit with a velocity of 30 m/s at 42.6 degrees to the ground. A fielder who is supposed to catch the ...
0
votes
0answers
11 views

Projectile motion help needed [duplicate]

I have a question. So a baseball is hit with a velocity of 30 m/s at 42.6 degrees to the ground. A fielder who is supposed to catch the ball is smart and knows that he is exactly 8 meters from where ...
0
votes
1answer
319 views

Potential Differences (Voltage) from Point Charges [closed]

Identical +1.8 micro Colomb charges are fixed to adjacent corners of a square. What charge (magnitude and algebraic sign) should be fixed to one of the empty corners, so that the total electric ...
0
votes
2answers
106 views

Timelike curves in Special Relativity

I have a question that probably might sound silly to most of you. We know that a natural Lorentz-invariant parametrization of a timelike curve is provided by: $$\tau$$ the Lorentz-invariant proper ...
3
votes
1answer
50 views

What is the best angle to see water

Sometimes when I clean my house, I miss some of the water since it seems invisible, but when I stand in different angle from the water I can see it clearly, whereas some angles offer partial ...
1
vote
1answer
228 views

Problem in ray diagrams in convex mirrors

I just found out about the Desmos online graphing calculator, and it is really a very nice tool. Just while playing with it, I had the idea of creating simulations. I decided to make a simulation of ...
2
votes
1answer
90 views

Why eigenvector points to principal stress plane?

I can represent a tensor by a matrix. Suppose we are talking about a 2nd order tensor, and the matrix is therefore 3x3. If I find one eigenvector of that matrix; that vector represents normal vector ...
1
vote
0answers
97 views

What is physical significance of product of Inertia? [duplicate]

I have read in available sources that product of inertia is just a term that is defined because it is useful in calculating the minimum and maximum moments of inertia of a body and also in finding the ...
2
votes
3answers
184 views

Has the universe we live in started as a black hole that is imploding?

As discussed in this question, as far back as the 1960s it was suggested that the Schwarzschild metric can be smoothly joined to a de Sitter metric. For example the idea has been used by Lee Smolin in ...
2
votes
0answers
50 views

Left (right) invariant vector fields on superspace

I read Freed'd book on "Five lectures on supersymmetry". For any vector space $V$ with metric of signature $(1,n-1)$ he constructs super Lie algebra $$ L=V \oplus S^*, $$ where $S$ is space of ...
1
vote
3answers
63 views

Difference between high-level and low-levels of electromagnetic radiation

can someone please explain me what we mean by 'high-level' or 'low-level' in electromagnetic radiation? for example, it is believed that high-level microwave radiation is harmful to human but not the ...
1
vote
2answers
84 views

DC Generator with magnet as rotor

DC generators convert the AC current in them by split ring commutators right and the graph of the current will be like this but the question is how would be the graph if the magnet is the rotor and ...
2
votes
2answers
133 views

How should I quote errors when measurements are asymetrically clustered?

Suppose five people measure the length of a stick and report the following values 4.90cm 4.92cm 4.93cm 4.94cm 4.94cm In high school science we are told that in ...
1
vote
0answers
77 views

Why is graphene the only (stable) 2D sheet structure? [duplicate]

I know that Carbon molecules can form different structures depending on how they bond with each other: graphite, diamond, graphene and fullerene. As far as I understand, graphene is just a "sheet" of ...
-2
votes
3answers
108 views

Why airports are generally near sea? [closed]

The generic way to measure the height is from the sea-level. If the airport is near the sea i.e. not at a very height from the sea, then the fuel required to make the air-plane above the sea level ...
0
votes
4answers
129 views

Can conservation of momentum and conservation of energy explain every possible event in the Universe?

I heard my friend, a researcher, say that we can, in theory, explain every event happening in the universe using the Conservation of momentum and energy. He added that we may not be able to do that ...
1
vote
2answers
191 views

Scattering theory textbooks

I am looking for a possibly extensive list of great textbooks on elastic and inelastic scattering of particles within quantum field theory. So far I am familiar with: Peskin and Schroeder: An ...
0
votes
1answer
64 views

Commutativity of Position Operators

Does the position operator $q_{i}$ of one harmonic oscillator commute with the position operator $q_{j}$ of another different harmonic oscillator? In other words, is $q_{i} q_{j} = q_{j} q_{i}$ true? ...
2
votes
0answers
47 views

Strength of Magnet in Magnetic Bearings

While reading about magnetic bearings, one reads about passive/active magnetic bearings. By passive, one means no electric current is put into the bearing for attaining magnetic levitation/flux. In ...
0
votes
1answer
26 views

Can electrons move back into the donor level

In a n-type semiconductor electrons from the donor level move up into the conduction band but can they move back down? Also in a p-type semiconductor electrons move to the acceptor level from the ...
1
vote
1answer
140 views

Time evolution of a quantum system

A quantum system has Hamiltonian $H$ with normalised eigenstates $\psi_n$ and corresponding energies $E_n$ ($n = 1,2,3...$). A linear operator $Q$ is defined by its action on these states: $$ ...
3
votes
2answers
926 views

Will a microwave heat sand?

I want to cook Turkish coffee on heated sand at school. I have difficulty accessing some easier method of heating, so I was going to try to heat sand in a microwave. It was then pointed out to me that ...
0
votes
1answer
70 views

Direction of EMF

I was reading electromagnetic induction and came across EMF.Now in my text book as the theory has been developed ,they have many a times talked about direction of EMF. But they have not clearly ...
3
votes
4answers
167 views

How are theories in physics validated against existing knowledge in practice?

Often you hear people mention how most attempts at Unified Theories haven't "accounted for all the experimental observations". The question is, how do you know you have accounted for all of the ...
2
votes
6answers
129 views

Thermodynamics only deals with homogenous systems?

In Thermodynamics quantities like pressure, temperature and entropy are associated with overall states of a macroscopic system. In that case, we do not talk about "the quantity $Q$ at the point $p$ of ...
1
vote
0answers
33 views

Momentum of electron problem [duplicate]

Recently, my friend bemused me with a question related to the momentum of an electron. The confusing logic is stated below: Since an electron is a particle and according to classical physics, we know ...
1
vote
3answers
385 views

Propellers and the Speed of sound

I was reading this website that described a novel wind turbine technology and it has this quote: In the mid-20th century, open propeller-driven planes with piston engines reached performance ...
0
votes
1answer
60 views

Broken Thin Lens Algorithm [closed]

We all know the thin lens equation. For $o$ being a horizontal object distance and $f$ being the focal length, the horizontal image distance $i$ is described by: $$\frac{1}{f} = \frac{1}{o} + ...
1
vote
2answers
61 views

Standard Usage of the word “per”

Math guy here. What is the usual meaning of "x per y per z?" Is this (ignoring details) (x/y)/z or x/(y/z)? Sorry to be mundane.
0
votes
1answer
118 views

Two-block system acceleration when applying a force in the $x$ direction

Two blocks are connected by a light-weight string that passes over a friction-less pulley (a.k.a. Atwood's machine). Block # 1 (called $m_1$) hangs from the edge of a friction-less table. Block # 2 ...
0
votes
0answers
21 views

Two Dimensional Kinematics and Projectile motion [duplicate]

I have a question. So a baseball is hit with a velocity of 30 m/s at 42.6 degrees to the ground. A fielder who is supposed to catch the ball is smart and knows that he is exactly 8 meters from where ...
4
votes
2answers
212 views

Why is the phase space a symplectic manifold rather than a manifold with a metric?

Why does phase space require a symplectic geometry rather than a metric? Is there some scenario where a metric is unable to describe the notion of length in phase space, specifically in relation to ...
2
votes
1answer
148 views

Does air conduct heat better than saturated steam?

This engineering toolbox table shows thermal conductivity of steam at 0.016. I understand that water is better in conducting heat than air, but if I read this correctly, steam is worse in conducting ...
1
vote
3answers
93 views

What is special about the ratio $Q/V$ that we give it the name Capacitance?

Why is the ratio Charge/Potential important? Also, usually when we add charge, the potential changes. Then why do we care how much charge we can put on a conductor for a given potential.
0
votes
1answer
60 views

Upcoming 'things to watch' in physics [closed]

What are the 'things to watch' in physics happening in the next year or so? E.g. A big astronomical event that happens only every n thousand years, or an experiment in particle physics which will ...
1
vote
2answers
88 views

Draw the charge configuration according to the electric field [closed]

I'm trying to draw the charge configuration outside the regions that lead to the given electric field patterns. Can someone show me how?
1
vote
0answers
58 views
1
vote
1answer
66 views

How does velocity relate to energy difference in Compton scattering?

I'm having trouble understanding what my professor is getting at asking in this question. I just visited her office and her explanation minutely helped. I'm hoping to get a bit more clarity on what is ...
3
votes
1answer
181 views

Why is the Earth's moon so big?

It could just be me, but it seems like our Moon is WAY bigger than it should be for a planet of our size. If you look at satellite-to-planet mass ratios for the largest moons of Jupiter, Saturn, ...
1
vote
0answers
26 views

Why isn't the sky violet? [duplicate]

Why isn't the sky violet? Is there any physical reason why the sky isn't perceived to be violet or is it all in our cones?
1
vote
0answers
23 views

What is “full clutching flow” in fluid-dynamics with regards venturi design?

As the title says, what is "full clutching flow" in fluid-dynamics with regards venturi design? I came across this reference a few times and can not find any further info on it, I'm currently ...
1
vote
1answer
536 views

Spring Powered Car - Which Design Is Best? [closed]

I'm in the preliminary stages of building a spring powered car to race over a distance of 10 metres. It is a yearly event held in my local area and so far we've only seen one spring powered car. I ...
2
votes
1answer
77 views

Can a lone black hole in a closed Universe evaporate?

If there is a closed Universe which only has a black hole in it, can that black hole evaporate? As the black hole evaporates, it gives off energy, which will eventually come back and be re-absorbed ...
1
vote
3answers
4k views

When is momentum not conserved?

What are some common examples where momentum is not conserved? This question arose in my mind when I read that a ball dropped from a height penetrates into a bed of sand and that momentum is ...
0
votes
1answer
31 views

N-type semiconductors and the number of holes?

Let us say we have Germanium which has been doped with antimony. Antinomy has a a fifth valence electron not involved in bonding. Is this electron in the valence band? I ask because all texts on the ...
2
votes
1answer
65 views

Is there a Fermi estimations toolbox?

I am a theoretical physics student and am a little ashamed at my inability to estimate any measurable quantity. I would like to develop my skills at Fermi estimations. Although it is hopeless to ...
0
votes
1answer
40 views

Why part of debroglie wave is negative?

I mean de Broglie wave for a particle represents the probability of the particle existing at a point, then, how can it be negative as well as positive?
0
votes
1answer
55 views

Why do roads appear wet on hot days? I do not understand [duplicate]

Why do roads appear wet on hot days? I don't understand the bending of light rays in relation to the density fluctuations. How are they related? Why do we see mirror images of cars on road in hot ...
0
votes
1answer
178 views

Is antimatter power theoretically possible?

There is a quote and citation on the "Antimatter" page of the obvious encyclopedic site (https://en.wikipedia.org/wiki/Antimatter), saying that positrons could be produced (in the future) at a rate of ...

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