The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.
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Kinetic energy of two charged balls at infinite distance between them
If I have two balls with masses and charges $m_1, q_1^{+}$, $m_2, q_2^{+}$, initially held at distance $d$, and then released, how can I know the kinetic energies of each of the balls at infinite ...
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4answers
559 views
Does matter with negative mass exist?
Or does it exist mathematically?
Is it really inconsistent with a common-sense, mathematics or known physical laws?
As far as I understand, if it exists, it must be far away from the "positive" ...
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3answers
165 views
Energy conserved… or not? Confused!
I am confused. Could someone kindly explain what's going on in this question?
A particle of mass $m$ and charge $e$ moves in the $x,y-$ plane. There is a constant magnetic field $B$ that points in ...
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1answer
25 views
Two dimensional elastic collisions with varying angle of incident
If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
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2answers
84 views
What would happen if energy was conserved but phase space volume wasn't? (and vice-versa)
I'm trying to understand the relationship between the two conservation laws. As I understand, Liouville's result is a weaker condition: it relies merely on the particular form assumed by Hamilton's ...
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0answers
28 views
Energy Conservation [migrated]
I'm working on a time integration scheme for my research. As a result I have come across an interesting phenomena. Somehow, the total energy of the scheme oscillates. At any given time the total ...
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4answers
546 views
Redshifting of Light and the expansion of the universe
So I have learned in class that light can get red-shifted as it travels through space. As I understand it, space itself expands and stretches out the wavelength of the light. This results in the light ...
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3answers
214 views
Energy Conservation Law validity
I will probably be laughed out of town for saying this, but why can't the Energy Conservation Law be broken?
Everybody thought electricity to motion was impossible until Faraday made his motor, ...
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2answers
310 views
Have red shifted photons lost energy and where did it go?
I think the title says it. Did expansion of the universe steal the energy somehow?
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3answers
444 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
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5answers
481 views
Does the amount of gravitational potential energy in the universe increase as it expands?
It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
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6answers
334 views
Electrostatic Potential Energy Derivation
How is the boxed step , physically as well as mathematically justified and correct ?
Source:Wiki http://en.wikipedia.org/wiki/Electric_potential_energy
As work done = $- \Delta U $. for Conservative ...
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1answer
35 views
Should the expression for energy conservation in alpha decay include the mass of electrons?
Let $M_P$, $M_d$, $m_\alpha$, $m_e$ and $Q$ the mass of the parent nucleus, daughter nucleus, alpha particle, electron and the disintegration energy, respectively. I understand that applying ...
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1answer
125 views
Energy conservation in nuclear reactions and radiactive decay
Reading "Fundamentals of Nuclear Physics" by Atam P. Arya, I understand that in a nuclear reaction, let say $x+X \to y+Y$ meaning that "when a particle $x$ strikes a target nucleus $X$, the outcome of ...
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2answers
586 views
Can a force in an explicitly time dependent classical system be conservative?
If I consider equations of motion derived from the pinciple of least action for an explicilty time dependend Lagrangian
$$\delta S[L[q(\text{t}),q'(\text{t}),{\bf t}]]=0,$$
under what ...
2
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1answer
64 views
Infinite reflection of light and the conservation of energy / momentum
First off, I confess I'm no physicist, but I have been asking people with a more extensive knowledge this one question, without a definitive answer so far.
Basically, I'm playing around with the idea ...
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1answer
49 views
Stopping distance of two objects with equal Kinetic Energy
I'm working on a problem regarding two objects with the same kinetic energy.
Two objects with masses of $m_1$ and $m_2$ have the same kinetic energy are both moving to the right. The same constant ...
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1answer
207 views
How should I apply conservation of energy to this problem?
A block rests on a table. Then the block is pushed by a spring and slides across the table until it falls to the ground. The mass of the block is $1.30\text{ kg}$, the spring constant $550\text{ ...
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4answers
194 views
Energy Gain with capacitor?
I have a question about energy gain in capacitors. Assume the following system:
As the electron gets accelerated inside the capacitor, it will have more kinetic energy coming out than going in. But ...
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1answer
86 views
How fast will I get if I burn a comet? [closed]
Assuming I have a spacecraft which is $30,000\,\mathrm{kg}$ (roughly the size of the Apollo spacecraft).
If I take a comet and (theoretically) electrolize it perfectly to hydrogen and oxygen. I know ...
3
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2answers
58 views
Two-body problem questions
I am self studying the two body problem and I'm stuck on the following:
I have given $$\ddot{\vec{x}}_1= - G m_2 \frac{\vec{x}_1-\vec{x}_2}{|\vec{x}_1-\vec{x}_2|^3}$$ and $$\ddot{\vec{x}}_2= - G ...
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0answers
15 views
Is resilience a measure of power expended upon unloading?
I read the definition of 'elasticity' and 'resilience on Wikipedia:
Elasticity is a property of materials which return to their original
shape after they are deformed.
Resilience is the ...
5
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1answer
87 views
Bremsstrahlung vs energy conservation
From Wikipedia:
Bremsstrahlung is electromagnetic radiation produced by the
deceleration of a charged particle when deflected by another charged
particle, typically an electron by an atomic ...
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9answers
3k views
What makes running so much less energy-efficient than bicycling?
Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?
According to the law of conservation of energy, bicycling should be more intensive because you have to ...
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1answer
35 views
About electrostatic potential energy
I consider an electron (charge $-e$) in $x=0$ and a constant electric field $E(x) \equiv E $. If the electron has initial velocity $v_0$ with the same direction of $E$, then its potential energy is
$$ ...
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0answers
44 views
Dynamics of an object hitting a spring [closed]
In a one dimensional universe, there is an object of mass m, heading towards a spring (which is attached to a wall of infinite mass) of spring constant k, at a constant speed of u (i.e. acceleration ...
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3answers
338 views
How Uncertainty Principle, Vacumm fluctuations and Energy Conservation coexist in QFT?
Recently I had a debate about the uncertainty principle in QFT that made me even more confused..
Because we use Furrier transforms in QFT we should have an analogue to the usual Heisenberg ...
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1answer
89 views
How do you find (initial) velocity using conservation of energy?
Without mass; only time, distance, and height is given. For example:
For this lab, the reference level was 100cm above ground therefore the height of the object was 10cm. I determined time and ...
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1answer
64 views
What Speed Would an object need to leave the earth at to reach L1? [closed]
Let's say the Earth is an airless sphere. What speed would an object weighing 1 kg need to leave the surface at in order to get to and be motionless at L1, where the Moon's gravity becomes stronger ...
2
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2answers
111 views
Elastic collision and spring
Bodies $A$ and $B$ are moving in the same direction in a straight line with a constant velocities on a frictionless surface. The mass and the velocity of $A$ are $2 \text{kg}$ and $10 \text{m/s}$. ...
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5answers
465 views
Why doesn't fusion contradict the 1st law of thermodynamics?
I was reading up on the 1st law of thermodynamics for my Chemistry exam and I was wondering why doesn't fusion contradict the 1st law of thermodynamics?
The 1st law states that
The energy of an ...
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1answer
45 views
Gravitational potential energy
Consider two places next to each other: Place 1, where there is a gravitational field whereas Place 2 - there's no field.
Now if we lifted a box in place 1, it gains potential energy. Then, we move ...
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1answer
142 views
How to understand the work-energy theorem?
How to understand the work-energy theorem?
I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
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1answer
134 views
The relation between Hamiltonian and Energy
I know Hamiltonian can be energy and be a constant of motion if and only if:
Lagrangian be time-independent,
potential be independent of velocity,
coordinate be time independent.
Otherwise
...
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3answers
190 views
Where do the conservation laws come from?
I know the conservation of energy comes from Noether's theorem via the time-translational symmetry, and if I remember correctly, the conservation of momentum comes from space-translational symmetry.
...
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9answers
740 views
Can a universe emerge from nothing?
If the Universe is flat and the total energy of the universe can be zero (we don't know if it is, but many theorists support the idea, i.e. at BB initial conditions:
t = 0, V = 0, E = 0) then is it ...
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0answers
37 views
Need help with relativistic dynamics
I understand the concept, but I'm having a hard time applying the consequences of conservation (energy/momentum). For example:
A proton with kinetic energy 437 MeV hits a proton at rest elastically ...
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4answers
550 views
Is there a deep reason why springs combine like capacitors?
I was solving a practice Physics GRE and there was a question about springs connected in series and parallel. I was too lazy to derive the way the spring constants add in each case. But I knew how ...
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0answers
17 views
How do I find the force from drop shock using material properties?
I am dropping a cylindrical cast iron bar with a know elastic modulus and poisson's ratio, $E_{1}$ and $v_{1}$, onto a flat beam of elastic modulus, $E_{2}$ and $v_{2}$ so there is tangential drop ...
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0answers
46 views
Where Is the mechanical energy flow in Poynting's theorem?
Poynting's theorem is usually interpreted as the statement of the conservation of energy, where the change in mechanical and electromagnetic energy within the volume equals the flow of electromagnetic ...
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0answers
71 views
How much money does an unused but plugged-in cellphone-charger waste in a year, if its not getting warm?
Is it right as xkcd states:
You can use heat flow to come up with simple rule of thumb: If an unused charger isn’t warm to the touch, it’s using less than a penny of electricity a day.
Or, more ...
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6answers
1k views
Why cannot we store light in form of light?
We can store cold (ice),heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnete. We can convert light into energy and then, if we want, back to ...
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1answer
118 views
Work being done without distance
I have this issue:
If you push a 40.0 kg crate at a constant speed of 1.40 m/s across a horizontal floor
(µk=0.25 ), at what rate (a) is work being done on the crate by you and (b) is the
energy ...
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2answers
139 views
Finding maximum speed in a work-energy problem
I have the following problem:
The Royal Gorge bridge over the Arkansas River is $310\text{ m}$ above
the river. A $57\text{ kg}$ bungee jumper has an elastic cord with an
unstressed length of ...
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0answers
53 views
Proof of Energy Conservation in Hydrogen collisions [closed]
This is a question from Kleppner's Mechanics:
A simple and very violent chemical reaction is $H + H = H_2$ + 5 eV
(1 eV = 1.6 $\times 10^{-19} J$, a healthy amount of energy on the atomic
...
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4answers
354 views
Origins of the universe questions
If the universe is expanding, what is it expanding into?
Similarly when the big bang happened where and how did it occur? - Where did the energy come from? Energy can not be created or destroyed does ...
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2answers
103 views
Does the mass of a falling body decrease?
Let's say a body with m=2kg falls from 100 meters. Obviously it's speed would be far lower than the speed of light so the change in mass (if it exists) would be very tiny. However, I know that if the ...
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1answer
38 views
When moving something by entrainment in a fluid stream, where does the energy come from?
The particular Something I had in mind here would be air bubbles that are pulled downwards against their buoyancy by a stream of water falling down a shaft. The work required to push those bubbles ...
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1answer
70 views
How to get the new direction of 2 disks colliding?
I'm developing a 2D game including collisions between many disks. I would like to know how I can get the angle corresponding to the new direction of each disk.
For every disk I have this information ...
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2answers
212 views
What happens to orbits at small radii in general relativity?
I know that (most) elliptic orbits precess due to the math of general relativity, like this:
source: http://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity
I also know that something ...
