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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Compare static loading and fast loading of a polymer (non linear elastic material) in terms of elastic potential generated
Imagine a non-linear elastic material such as a rubber band, nylon webbing or polyester webbing tensioned between two points.
Scenario 1:
A large mass is statically (no acceleration) loaded onto the ...
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1answer
44 views
Conservation of Energy and Quantum Fluctuations
Regarding conservation of mass-energy Wikipedia says: "this is an exact law, or more precisely, has never been shown to be violated."
However, regarding quantum fluctuations, Wikipedia says here: ...
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2answers
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What can be known about the formulas for energy only from the fact that it is conserved?
The question is to figure out how the energy can be derived knowing just one thing:
There is a quantity called Energy that is conserved over time.
The goal is to get an equation that somehow ...
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0answers
41 views
Jumping into the water from certain [closed]
a person who, starting from rest at the top of a cliff, swings down at the end of a rope, releases it, and falls into the water below. There are two paths by which the person can enter the water. ...
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3answers
160 views
Intuitive meaning of factor 2 in formula of vertical throw max height $h=v^2/2g$
This is a question about a simple thing. The simplified expression for maximum height in vertical throw is $h=\frac{v^2}{2g}$ , could anyone explain intuitively (analogies are welcome) why there is a ...
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0answers
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solid state physics [closed]
Show that the expression for the average energy of a system can be given by the relation $E=\dfrac{KT2d\log z}{dt}$ where Z-partition function for classical one dimensional system and is given by z= ...
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2answers
67 views
force applied not on the center of mass
when applying a force not on the center of mass of the body, the body will get both linear and angular mumentom (right?).
does the linear velocety from this force (that is applied not on the center of ...
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2answers
68 views
Can a hybrid vehicle ever be more efficient than a hydrocarbon-only vehicle built with the same parts?
Based on the laws of thermodynamics, shouldn't it be theoretically impossible for a non plug-in hybrid vehicle to ever be more fuel-efficient than a vehicle that connects the same engine directly to ...
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1answer
56 views
Short-duration forces
In circular motion, it is said that the centripetal force acts only for a very very short period of time, hence is able to only change the direction but not magnitude of the velocity.
Similarly in a ...
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1answer
41 views
Conservation of energy issue with pair creation/annihilation
Question: what is the energy balance in this situation?
(Fig A)
Two gamma rays collided and produced an electron/positron pair $(e-/e+)$.
(Fig B)
1. because all particles are accelerating ...
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0answers
51 views
Salisbury Screens and Energy Conservation
From Wikipedia on how a Salisbury Screen works:
1. When the radar wave strikes the front surface of the dielectric, it is split into two waves.
2. One wave is reflected from the glossy ...
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1answer
48 views
Why doesn't this equation for orbital motion change with position in the orbit?
The question and answer are on pg.8-10 of this PDF:
At first, I went through it, thinking nothing of it. But then, I wondered: "What if we picked a final state in which the space junk was NOT at ...
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2answers
64 views
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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2answers
48 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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3answers
178 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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2answers
102 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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1answer
53 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
173 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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1answer
84 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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2answers
79 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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6answers
387 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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4answers
221 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
88 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 ...
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2answers
60 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 $$\ddot{\vec{x}}_2= - G ...
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0answers
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Is resilience a measure of power expended upon unloading?
I read the definition of 'elasticity' and 'resilience on Wikipedia:
Elasticity is a property of materials which return to their original
shape after they are deformed.
Resilience is the ...
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1answer
89 views
Bremsstrahlung vs energy conservation
From Wikipedia:
Bremsstrahlung is electromagnetic radiation produced by the
deceleration of a charged particle when deflected by another charged
particle, typically an electron by an atomic ...
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9answers
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What makes running so much less energy-efficient than bicycling?
Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why?
According to the law of conservation of energy, bicycling should be more intensive because you have to ...
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1answer
40 views
About electrostatic potential energy
I consider an electron (charge $-e$) in $x=0$ and a constant electric field $E(x) \equiv E $. If the electron has initial velocity $v_0$ with the same direction of $E$, then its potential energy is
$$ ...
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0answers
45 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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1answer
138 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
69 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 ...
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2answers
130 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
498 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
47 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
208 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
141 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
204 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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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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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
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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
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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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3answers
346 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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6answers
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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
135 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
164 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
57 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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1answer
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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
80 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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3answers
74 views
computer vs heater coil comparison
If I have a computer and a heater coil that consume exactly the same amount of energy, which would be more efficient at heating my room? This is assuming that they both have the same fans and heat ...
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3answers
126 views
Redshifted photons
When a photon is emitted from a far away source and then measured by an observer, there is a loss of energy or redshift which takes place. Why does this happen?
I have read this similar post, however ...
