The statement that a property of a system does not change if the system is isolated.

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60 views

Does inertia have anything to do with conservation of energy? [closed]

I have googled this and not found anything. It seems to me that if objects at rest did not resist being accelerated (which is what I once thought was the case in outside of gravity like in outer ...
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1answer
93 views

Rocket equation derivation mistake by my professor

In one of my lectures our physics professor gave a derivation of the ideal rocket equation as follows: Let $v_G > 0$ be the velocity at which the gas is emitted from the rocket. Let $m$ and $v$ be ...
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44 views

Apart from friction what can be considered a non-conservative force?

I know that friction is a non-conservative force. My question is if we have any non-conservative forces or other fields in electrodynamics? second part question for a force that conservative we have ...
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2answers
41 views

What is the reason for Earth's rotation and revolution?

Is it because of the gravitational effect of the other planets and the sun then why do the other planets rotate and revolve too? And well does the Sun rotate and revolve too?
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0answers
26 views

Alpha Radiation - Charges [duplicate]

Has the radiating material that emits only alpha radiation a negative charge? I am just wondering, because alpha radiation is a positive charged helium ion - so where are the electrons going?
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1answer
106 views

Is there conservation of momentum if there's conservation of energy? [closed]

The equation for conservation of momentum: $$m_1\vec{v}_1 + m_2\vec{v}_2 = m_1\vec{u}_1 + m_2\vec{u}_2$$ The equation for conservation of energy: $$\frac 12m_1v_1^2 + \frac 12m_2v^2_2 = \frac ...
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1answer
85 views

Deeper principles in classical mechanics

While teaching introductory physics, my professor explained that the conservation of linear momentum, conservation of energy and conservation of angular momentum are based on deeper principles in ...
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2answers
122 views

Why is this specific definition of acceleration used in deriving a general equation for kinetic energy?

$$\sum_{i=0}^n m_i v_i$$ as well as $$\frac{1}{2}\sum_{i=1}^n m_i v^2_i$$ Have been proved to be conserved by Newton & Leibniz, respectively. (http://en.wikipedia.org/wiki/Vis_viva) I don't ...
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37 views

Does eq. of continuity hold in vacuum too?

When a streamlined flow of water flows down through an ordinary tap, it's cross-sectional area decreases according to eq. of continuity due to atmospheric pressure. If the same apparatus were to be ...
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2answers
73 views

What formula do I use to calculate the force of impact of a falling object?

I am trying to calculate the force of impact of a falling object. I did my egg drop project. I dropped the egg from 10m with a mass of 126kg and with the velocity of 14.1 m/s. Which formula should I ...
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2answers
85 views

Newton's third law & punching walls [closed]

N-law 3: if you punched a wall as hard and as you could, would it possible to retract your hand before the reaction from the wall could affect your hand less than if you wouldn't retract at all? ...
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1answer
57 views

What would happen if light was converted into matter as it is propogating?

If light is a form of energy and can be converted into matter then does light slow down since no mass full object can reach the speed of light
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1answer
69 views

How is momentum conserved in magnetic field? [duplicate]

In a uniform perpendicular magnetic field , force on charged particle cause change in direction of momentum. How do we explain it using conservation of momentum? I don't know what is Hamilton or ...
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2answers
150 views

Does the Inertia of a Cue Ball Affect its Reflection Angle off a Resting Billiard Ball?

Consider the following Diagram in which a Cue Ball (A) of mass M is shot twice at another pool ball with identical mass M. When the force with which the cue ball (A) is hit (v1) is increased (v2) ...
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1answer
40 views

Question about 3d collision of 2 particles [duplicate]

I'm wondering about how to calculate the resultant velocity vectors of an elastic collision of two particles of same mass, in 3d. I've seen a post on the subject in which you answered, but I didn't ...
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2answers
61 views

Total Momentum in the Center of Mass Reference Frame

For solving problems involving elastic collisions it is useful to use the center of mass reference frame as to avoid quadratic equations. However, I am confused about why the total momentum in the ...
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5answers
297 views

How does constant thrust avoid quadratic kinetic energy accumulation?

I haven't found the right search terms for this question, so if it has been answered, references would be welcome. Suppose we start from experimental station in deep space (interstellar space if need ...
2
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1answer
77 views

What is the reason behind why energy must always be conserved, apart from observation? [duplicate]

I know that we see in experiments (physical and thought) that energy is always transformed into something else, but what propels our universe to behave this way? What is happening at small levels that ...
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1answer
25 views

Lorentz Force and Apparent Conservation of Momentum Violation Useful for Unidirectional Force?

My understanding is that the apparent violation of Newton's Third Law by the Lorentz Force necessitates a description of the system that describes the "missing" momentum as being absorbed/carried by ...
2
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1answer
42 views

Why is the matter orbiting a black hole in the shape of a disk? [duplicate]

More generally, if a blackhole is a singularity...a point in space...why is matter only circling in a flat disk-like region. It would seem to me matter would circle it in a sphere surrounding the ...
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2answers
161 views

Griffiths' argument that Newton's third law is invalid in special relativity [duplicate]

I believe that Newton's third law is valid in special relativity. But I read an argument in Griffiths' textbook Introduction to Electrodynamics (3rd edition), where he argued on page 517, that ...
4
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2answers
130 views

Does the conservation of the Wronskian follow from Noether's principle?

Noether's principle is the paradigm that symmetries of Hamiltonian and Lagrangian systems correspond to conservation laws of various kinds. Consider a one-dimensional harmonic oscillator $$\tag{*} ...
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1answer
42 views

Deriving a diffusion equation from a given mass flux density [closed]

I am given that the mass flux density $q$ of a substance in a medium obeys the following physical law. $$q=-D\frac{n^3}{n-n_0}\frac{\partial n}{\partial x}+nv,$$ where $n(x,t)$ is the ...
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0answers
39 views

Symmetries of a Lagrangian density

Given some Lagrangian density as this how in general can one finds it's symmetries that give conserved currents? For example in this case U(1) is ok, but are there others? Do you know some book ...
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1answer
41 views

Entropy in ideal fluids [closed]

Can you derive a conservation law for entropy per unit mass? I know that there is a proof for this conservation. My assumptions are: $(\nabla p)/n = D \vec{v} + \vec{v} \cdot \nabla\vec{v}$ (Euler ...
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0answers
35 views

Separation of two particles

An object ofass $5$ kg is projected with a velocity of $20m/s$ at an angle of $60degress$ to the horizontal. At the highest point of its path projectile explodes and breaks up into $2$ fragments of ...
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1answer
37 views

Total energy of the fluid

Consider a one dimensional system with fluid in it. Mass and momentum balance equation of the system are (in the absence of external forces and assuming Newtonian behaviour valid for viscocity), ...
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0answers
45 views

How is momentum conserved in this electromagnetic scattering?

It is well known that verifying momentum and energy conservation in the presence of electromagnetic fields requires care as the fields themselves carry energy and momentum (see Griffiths chapter 8 for ...
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2answers
42 views

Ball collides with another. Find velocities [closed]

A ball moving at a velocity of 5.0 m/s east when it collided with an identical ball at rest. Both balls are the same mass. The first ball moved at 60 degrees north of east. The second went 30 ...
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1answer
81 views

What does Liouville's Theorem actually mean?

Basically, the mathematical statement of Liouville's theorem is: $$\frac{\partial \rho }{\partial t}= -\sum_{i}\left(\frac{\partial \rho}{\partial q_i}\,\dot{q_i}+\frac{\partial\rho}{\partial ...
2
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2answers
80 views

Can a spinning object stop spinning?

Suppose there's an object spinning in empty space. That object has mass and angular momentum. Is it possible for that object to stop spinning without any external intervention, say due to a chemical ...
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3answers
104 views

Where do the energy and momentum come from when light is perfectly reflected on a surface?

If you send a light beam to a surface which reflects the light perfectly I would say that on the surface is put a force of the light so the surface moves backwards. But where does that energy comes ...
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2answers
64 views

How is water falling from a tap an example of the conservation of mass?

In a book, Conceptual Physics by Benjamin Corral, it was given that when water falls down from a tap, it is a case of conservation of mass. They said that this is because water is thick when just ...
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2answers
92 views

What was the initial source of energy?

According to law of conservation of energy; "Energy can neither be created nor be destroyed but may change it's form" Where did the energy come from which led to the creation of universe by big bang?
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116 views

Conservation of momentum and conservation of energy

I had the impression that conservation of momentum is valid only when we consider no loss of kinetic energy into heat etc. My tutor says that even if we have loss of kinetic energy as heat in a ...
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0answers
41 views

Is the conservation of probability in the Schroedinger's equation unique?

The Schroedinger's equation can be viewed as a diffusion equation with imaginary constants $a$ and $b$ satisfying, $$(1) \quad \Psi_t=a \cdot \Delta \Psi-b \cdot V(x,t) \cdot \Psi$$ However if $a$ ...
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1answer
156 views

Why is the decay of a neutral pion $\to$ electron-positron loop suppressed?

To my understanding the decay of a neutral pion into an electron-positron pair can only happen by the electromagnetic force and the mediation of two virtual photons in a triangle-diagram, so it is ...
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2answers
73 views

When a ball bounces on a wall at an angle, why does the angle of incidence equal angle of reflection?

When a ball is thrown to a wall without spin and bounces, supposing it's an elastic collision, why does the angle of incidence equal the angle "reflection" (relatively to the normal), why isn't it any ...
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0answers
49 views

Continuity equation - water mass conservation

I have a problem to understand a passage in the article "Modeling the dynamics of pressure propagation and diameter variation in tree sapwood" that you can find here: ...
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0answers
22 views

How can intuitively guess what conserved quantities has the system that I am studying?

I'm taking a course in Classical Electrodynamics and in one problem my teacher introduced us to a triplet of fields ($\phi^a$) invariant under internal rotations, i.e. transformations like: ...
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0answers
43 views

Higher order versions of momentum? Can conservation principles be established and used? [closed]

Question Can higher order derivatives of momentum be useful in creating theories of dynamics if they have conservation principles? Even if they aren't needed, could it be done in theory? For ...
2
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1answer
62 views

Nail clippers and conservation of momentum

As I clip my fingernails using nail clippers, pieces of nails sometimes fly off violently, reaching the other end of the room. What's happening in terms of conservation of momentum, how are the pieces ...
1
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1answer
91 views

conservation of momentum? [closed]

At hyperphysics I got this image, with the same description in text as is in this image It says that when a massive particle (say $A$) moving with a velocity collides with an object having a ...
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1answer
59 views

Conservation of momentum always? [closed]

Consider two atoms A and B, with A at rest and B in motion towards A. As soon as B is in the atomic field of A, will it stop with its energy going into B or will it just move up to a postion where ...
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1answer
89 views

Newton's third law at the quantum level? [closed]

let's look at force at the atomic level to understand the newtons third law of motion. I'll use Helium atoms as an example. Now imagine we start with one atom HE2 stationary, and throw another atom ...
3
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1answer
65 views

Second Kepler's law implies that constant velocity

I was trying deduce that if we suppose that the planet's orbits are circular and de Kepler's secong law is true: A line joining a planet and the Sun sweeps out equal areas during equal intervals of ...
5
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1answer
98 views

Why doesn't the 9th ball move in the break in the nine-ball pool game?

In the game of nine-ball pool, we break the rack like shown below: In the break, we hit the 1st ball with the cue ball. Many people familiar with pool games say that if the rack is constructed ...
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2answers
44 views

Why can't a force between two particles exert a torque?

Let's say I have two particles which exert a mysterious force between them. Say they lie on the x-axis. Newton's 3rd law tells us that the force on the two particles due to the other must be opposite ...
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1answer
97 views

Mathematical proof of an electron cant absorb a photon [duplicate]

How can we mathematically prove that a free electron cant absorb a photon totally?
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2answers
163 views

Multiple conservation of momentum in one [closed]

I am in a Grade 12 Physics course and my teacher gave this question for bonus marks, to be solved with only highschool concepts. I suspect that this problem has to do with conservation of momentum, ...