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

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1answer
64 views

Using Conservation of momentum and Energy to solve a problem [closed]

A 9kg bullet is fired horizontally into a 10 kg block of wood suspended by a rope from the ceiling. The block swings in an arc, rising 6mm above its lowest position. Find the velocity of the bullet. ...
7
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6answers
846 views

Is (rest) mass conserved in special relativity?

I don't understand why it is said that the (rest) mass of a system is not conserved in relativity. I mean, the momentum of a system is conserved (i.e.: it remains constant in a frame of reference ...
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4answers
85 views

Sun and planets orbit each other

Do not the planets and the Sun revolve in orbits around each other and the shape of the orbit depends on where the center of gravity of the system is? The greater the mass of the Sun, the closer the ...
3
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2answers
94 views

Why is $p_\phi$ conserved in a Schwarzschild orbit?

This arises from the question What is the relationship between $a$ and $m$, which I'm afraid I answered just by looking it up in Schutz's book. However Schutz (as he frequently does) glosses over ...
2
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1answer
77 views

Pair-annihilation why does it occour? [duplicate]

Why does pair annihilation occur with particles and only their matching anti-particle? E.g., electrons and positrons, but not protons and positrons? What is the difference?
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2answers
107 views

Pass to globally conserved currents from locally conserved currents in curved spacetime

Let us begin with a Lagrangian of the form $$\mathscr L= \frac 12 \sqrt{-g}g^{\mu\nu}\partial_\mu\phi(x)\partial_\nu\phi(x)+\mathscr L_g,$$ where $$\mathscr L_g=\frac 1{16\pi k}\sqrt{-g}R.$$ ...
1
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1answer
68 views

Collisions between an object and a wall

Is momentum conserved when an object bounces back against a wall? The wall doesn’t move, but the object moves in the opposite direction. Assume this is an ideal, elastic collision. If, initially, the ...
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1answer
51 views

Two Bodies Orbiting Around Each Other and Kepler’s Laws?

If two bodies are orbiting around a central center of gravity, how does Kepler’s first law (the one regarding the ellipse) apply?
5
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1answer
383 views

Why can't Compton scattering happen in leading order of perturbation theory?

Why is the matrix element of Compton scattering in leading order of perturbation theory equal to zero? Why can this process only be described in second order of perturbation theory, i.e. with exchange ...
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2answers
96 views

Comparing the orbit radius of two spherical objects [duplicate]

Assume the mass of star 2 is 4 times the mass of star 1. Compare the radius of the orbit of star 1 to that of star 2. Possible answers: R1:R2=1:4 R1:R2=1:2 R1:R2=2:1 R1:R2=4:1 R1:R2=16:1 ...
2
votes
1answer
48 views

Do all “normal” black holes rotate?

Can we assume that most (if not all) black holes are rotating, due to conservation of momentum? I am excluding the micro world from this question, just thinking of the range of stars on the main ...
1
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0answers
69 views

Noether's first and second theorems

My understanding of Noether's first theorem is as follows. Consider a set of infinitesimal transformations that leave the action invariant, that are indexed by $n$ linearly independent parameters, ...
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2answers
56 views

Eulerian mass conservation on a stream line to Lagrangian mass conservation

if the density of a fluid particle is conserved on a streamline, $$\frac{d\rho}{dt}=0.$$ Why does this mean $$\frac{\partial \rho}{\partial t}+(\mathbf{v}\cdot\nabla)\rho=0$$ is true everywhere? Why ...
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2answers
108 views

Can gyroscope work in zero gravity?

Most ships have two or more gyroscopes to balance on water, man made satellites uses gyroscope for orientation as they fall around earth. All these applications seems to be associated with gravity, ...
0
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2answers
67 views

Conservation of angular momentum in a planetary system

Why is angular momentum conserved when a planet revolves about sun in an elliptical orbit? Why is linear momentum not conserved in this case? Please use the minimum amount of equations and try to ...
1
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1answer
46 views

Continuity Equation for Momentum

Momentum is a conserved quantity, which makes me wonder if we can write an equation for the local conservation of momentum in the form of a continuity equation. If we're considering a system of ...
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0answers
42 views

Expected value, Fluid Mechanics and Momentum balance

I've been doing some problems on Fluid Mechanics involving the balance of Matter and Momentum and I have a question I've been wondering about. Suppose the following: We have a (viscous and ...
2
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3answers
203 views

Confusion regarding rotational motion!

Let us assume I have a rod of some mass m, moment of inertia I, length l and center C. If I apply a force F on C for a duration of time t, it will accelerate forward. If I apply it elsewhere, the ...
3
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1answer
89 views

Physics simulator based on conservation laws?

Reading the article: http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion#Relationship_to_the_conservation_laws there's a section stating that: ...
3
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3answers
87 views

How can I find the angular and linear velocity of a 2D body that breaks into two bodies?

Afternoon. This is my first question, so do let me know if I'm doing anything wrong. Looking for help on building a 2D physics game engine with bodies that split in half: I have a two dimensional ...
22
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5answers
8k views

Is it possible to shoot bullets in space or would the recoil of the gun be too strong?

I've read a few articles that say that astronauts have already brought guns in space and that shooting bullets in space is possible. But won't the recoil of the gun be too strong? Law of ...
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3answers
237 views

What is actually a conservation law?

Though in his lectures, Feynman didn't define conservation law, he did use it while explaining divergence theorem: [...] heat is conserved. That is, no heat is generated inside the material and ...
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0answers
71 views

Antimatter universe and Noether's theorem

I am studying Feynman's "symmetry in physical laws", where he talks about conservation laws for corresponding symmetries. (I know this is Noether's theorem, I am studying this from David Tong's ...
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0answers
26 views

Interpretation of Mass Continuity Equation in MHD [duplicate]

I'm writing up my final-year dissertation and I'm required to give, as part of the introduction, an analysis of all the equations (and their terms) of which I use. Embarrassingly, whilst of course ...
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2answers
33 views

Conservation of Sea water Vs Conservation of Matter

I have been listening to people, since my childhood, saying that amount of water in Sea always remains constant even if we draw out much from it, it never changes. They used to emphasize by saying ...
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1answer
39 views

Time evolution of generalized angular momentum operator

We define this operator : $$M^{\mu\nu} = \int d^3x~(x^{\mu}T^{0\nu} - x^{\nu}T^{0\mu})$$ where $T_{\mu\nu}$ is the energy momentum tensor (see e.g. Energy momentum tensor from Noether's theorem) ...
2
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1answer
55 views

Finding final velocity in inelastic collision [closed]

Information: In a shipping company distribution center, an open cart of mass 49.0-kg is rolling to the left at a speed of 5.40-m/s (see the figure). You can ...
0
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1answer
86 views

Why is $\int_{s} \mathbf{h}\cdot \mathbf{n} da = - \dfrac{dQ}{dt}$ & not $\int_{s} \mathbf{h} \cdot\mathbf{n} da = - \dfrac{dQ}{dt} .{dt}$?

I was reading the Lectures of Feynman about surface integral where a situation in which heat is conserved has been dealt. Let there be $Q$ heat energy present inside a body. Now, if there is net heat ...
-1
votes
1answer
190 views

Continuity equation in fluid mechanics

The continuity equation in fluid mechanics states that $$ \frac{\partial\rho}{\partial t} + \nabla\cdot(ρ\mathbf u)=0 $$ Can you explain to me what is the physical meaning of each term of the ...
1
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1answer
62 views

What is the definition of parity conservation?

I searched quite hard, and am still confused what is the exact definition of parity conservation? For example, we have quantum system with initial state $\Phi_i$, and after decaying it comes to final ...
0
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1answer
121 views

Proving the conservation of 4-momentum for a particle collision $A+B\to C+D$

Let me say that particle A hits particle B and two particles come out - C and D; In system S I can write: $$p_A^μ+p_B^μ=p_C^μ+p_D^μ;\tag{1}$$ here $p_N^μ$ is the 4-momentum. Using the Lorentz ...
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2answers
110 views

Two balls travelling at different speeds collide in two referentials

In a referential R1, one ball B1 travels at 100 m/s and hits and another identitcal ball B2 that travels at 50 m/s in the same direction. Assuming the material in which the balls are made is such that ...
0
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1answer
70 views

Calculation of velocity via kinetic energy and momentum yielding different answer

I am attacking the given problem (as a preface I'm not asking to be spoon fed any answers, just looking for clarity from people much smarter than myself) A 15.0kg block is attached to a very light ...
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0answers
50 views

During perfectly elastic collision of two objects $A$ and $B$, how much is the initial speed of object $A$ affecting the force applied to $A$ itself?

I got stuck in what seemed to be an easy problem. If two bodies $A$ and $B$ collide perfectly elastically and head-on what is the equation that gives us the forces applied to $A$ and $B$, given a ...
1
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2answers
95 views

Why is momentum conserved in inelastic collisions? How is it related to momentum-impulse theory?

First of all I want to mention, that I've found many questions around this site and in other websites dealing with my question; however, I don't think they answer my question fully. So I am here to ...
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3answers
385 views

The momentum of a swinging sword

Suppose you are faced with a zombie, and the only way to kill it and save yourself is to chop its head off with your sword. However, you are very weak from illness, and can only afford to strike once. ...
3
votes
5answers
298 views

How is Angular Momentum Conserved when Mass is Released?

I am not a physicist (math/comp-sci) but I understand that Angular Momentum is supposed to be conserved. I find this confusing because there seems to be many simple, common cases where a restrained, ...
1
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1answer
80 views

Angular momentum in planetary disk formation

This question is actually more linked to astronomy and astrophysics than to pure physics. I tried posting it on the astronomy page, however it got no answers, so I though this page might help. ...
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0answers
22 views

Averaged energies in particle collisions

Let's have (in CM frame) process $x + y \to x + y + z$, where $x, y, z$ correspond to (in general) different particles with non-zero masses. The total energy of process is $E$. How to calculate ...
3
votes
1answer
84 views

If there are 4 dimensions, shouldn't objects appear and disappear in 3D space?

If there are 4 (or more) physical dimensions, and physical objects can move through the 4th dimension in paths perpendicular to our 3 dimensions, the physical objects must pass through our ...
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2answers
45 views

How did the planets (in the Solar System) start to revolve around the sun if they were attracted towards the Sun via the gravitational force? [duplicate]

The planets in the Solar System revolve around the Sun in almost circular paths called orbits. The Sun pulls the planets with the gravitational force,but the planets do not get drawn to the Sun but ...
0
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2answers
38 views

How to define conserved charges in Euclidean field theory?

In a field theory with signature (1,d), conserved charges are obtained by integrating the time component of a conserved current over a spatial region. What are the corresponding equations and ...
0
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1answer
87 views

Equations for a collision between two particles

Say I have two particles on a 2D plane, they have a x and y coordinate, a x and y velocity, a mass, a coefficient of restitution and a coefficient of friction. What formulae would I need to determine ...
0
votes
2answers
72 views

How do we prove that the initial velocity is equal to final velocity relative to centre of mass?

In elastic collision it is stated that the initial velocity relative to centre of mass is equivalent to final velocity of centre of mass of the same object. How do we prove that?
0
votes
1answer
90 views

How does the Earth rotate, given that the torque acting on it while revolving is zero?

I've come to understand that the torque acting on the Earth while revolving the Earth is zero. Torque is the force responsible for rotation of a body. So how does the Earth rotate?
3
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2answers
886 views

Can conservation of momentum be violated?

The law of the conservation of momentum has been established for hundred of years. Even in Quantum field theory every particle collision must be momentum-conserving if there is homogenity in space. ...
0
votes
1answer
158 views

How does frequency change with centripetal force?

Using the equation $$F_c = {{mv^2}\over{r}}$$ I can see that mass and velocity are directly proportional to centripetal force. I can also see that the radius length is inversely proportional to ...
32
votes
4answers
2k views

Intuition as to why the orientation (of a 3D object) is not a conserved quantity?

Say you start off floating in space, in a fixed position and orientation, with zero linear and angular velocity, with no external forces. So you are a closed mechanical system. By twisting your body ...
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1answer
311 views
0
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1answer
124 views

Basic question about angular momentum

I've learned that the angular momentum of an object rotating about a fixed axis is $I \omega $. Also, in absence of external torques, $I_1 \omega_1 = I_2 \omega_2 $ (meaning, two different events). I ...