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.

learn more… | top users | synonyms

0
votes
1answer
20 views

Greater momentum than initial?

The question is : Heavier object A, initially at rest, is struck by lighter object B. Is it possible for object A to have a larger final momentum than the initial momentum of object B? The ...
-1
votes
2answers
40 views

Finding the initial velocity of projectile [on hold]

What is the formula for calculating initial velocity of projectile using the conservation of energy given the mass, height, angle ?
-2
votes
1answer
55 views

Why does this not make infinite energy?

Here's an idea I had for infinite energy: Get into a spaceship, and fly to a star or other large body and get into orbit. Then just harvest your kinetic energy somehow and keep going indefinitely. My ...
2
votes
3answers
86 views

Violation of conservation of energy?

Imagine a perfectly spherical mirror with 100% reflection. Imagine a point source of light in the center. The point source keeps radiating light. Will the light undergo destructive interference ...
1
vote
2answers
79 views

Why doesn't Beta Decay violate the laws of physics?

In Beta decay, a neutron decays into a proton, and "throws out" an electron at high speed. However, this, to me, suggests that the law of conservation of mass is not being kept here. Neutrons have a ...
1
vote
1answer
35 views

Why did the electric potential energy fall in this situation?

Suppose you have two capacitors configured as shown in the image below. One of them has charge $q$ and the other one is uncharged. Then, you close the switch, allowing charges to flow. After ...
6
votes
3answers
649 views

Where does the majority of the mass of the usual matter come from? [duplicate]

I apologize in advance to experts for the naivety of the question. It should be a duplicate but I didn't find any satifying question or answer about that. The proton is composed by two up quarks ...
6
votes
2answers
166 views

Do virtual particles actually physically exist?

I have heard virtual particles pop in and out of existence all the time, most notable being the pairs that pop out beside black holes and while one gets pulled away. But wouldn't this actually violate ...
0
votes
3answers
32 views

Velocity and transfer of energy for a body in free fall

I just need to check if my understanding of the transfer of gravitational potential energy to kinetic energy is correct. Is anything wrong below? Say if a body were suspended at a height h above the ...
0
votes
2answers
46 views

a question about work needed to move a charge towards another one [duplicate]

Consider two charges $q_1>0$ & $q_2>0$ separated by a distance $d_1$, and we want to move charge $q_2$ to a distance $d_2$ from $d_1$ where $d_1-d_2>0$. I need to calculate the work ...
1
vote
1answer
43 views

A captious work problem: same paths but same forces?

A man jumps onto a chair. A man climbs onto a chair by putting a leg first and then the other. In both cases, the work has been the same. TRUE or FALSE...? Spoiler!: The path is the same, so the ...
0
votes
0answers
48 views

How to model the dissipation term

When a solid particle which is moving with $V_p$ collides with a droplet which is falling with $V_d$, based on what reported in literature, a liquid skirt will be formed as shown in figure below. One ...
4
votes
3answers
75 views

Ball on a slope with hollow

The experiment shown in the image suggests that ball B will reach the goal faster than ball A although the balls have identical properties and they start from the same height. The authors even ...
4
votes
2answers
234 views

Question on particle decay process, conservation of energy

Why is $$\Sigma^0 \rightarrow \Lambda +\pi^0$$ not a possible process? Charge and baryon number both are conserved. There's no issue with strangeness that I can tell. The masses in $\frac{Mev}{c^2}$ ...
1
vote
3answers
64 views

Why does it hurt more to fall from 10 stories, than 1?

$F=ma$. Falling from any distance, mass stays the same, and acceleration due to gravity stays the same. So, why does it hurt more, the longer you fall?
0
votes
1answer
31 views

Question regarding the sail attached to a boat. [closed]

I came across a physics numerical in which the boat is travelling 45 degrees south-east. The direction of wind is towards north and the force that it exerts is constant force of 100 N on the sail. The ...
2
votes
3answers
61 views

Does (mass $\rightarrow$ pure energy) conversion need both matter $and$ antimatter?

A question in my revision guide: "Explain why the mass of a tree cannot be converted directly into energy." The answer explains that the tree contains particles but not their corresponding ...
1
vote
1answer
121 views

Positions and corresponding terminology of “acoustics energy”?

From limited knowledge, decades ago, dating back to the nineteenth century: Ohm's law of specific acoustic energies was the first biological application of Fourier's theorem. Actually, it was ...
1
vote
1answer
45 views

How can gravitational energy cancel out the rest energy of mass in the universe [duplicate]

I've heard that some physicists think that the net energy of the universe is zero. For this to happen, I would assume that the negative gravitational energy of a body ought to cancel out its rest ...
2
votes
0answers
25 views

Spinning, decelerating coin on a table

A coin spun on a table will eventually end up flat on the table, ignoring the cases where it remains balanced and comes to rest on its edge (or falls off etc). Before the coin comes to rest on its ...
1
vote
2answers
32 views

Inelastic Collision and Energy

How is is possible for momentum to be conserved and for KE to not be conserved? Momentum is related to velocity and velocity is related to KE, therefore if KE was lost, how can the system have the ...
2
votes
2answers
59 views

Alternatives to the neutrino in the beta spectrum?

It is well known that the neutrino appeared to explain the power distribution in the beta decay spectrum. (see PSE) What other explanations candidates were available? It is stated that the ...
2
votes
0answers
49 views

Is refraction of light a thermodynamic process?

Is refraction of light a thermodynamic process? Can it be explained by conservation of energy? If so, does temperature has an effect on refraction of light?
2
votes
4answers
219 views

If a bell is rung in a perfect vacuum and is suspended by opposing magnets will it ever stop ringing?

As far as I am aware a bell rung in a perfect vacuum would only lose energy and stop ringing due to friction between the bell and the hanging mechanism (due to the lack of air resistance). Therefore ...
2
votes
1answer
100 views

Weightless person in a cart on a rail [closed]

In a problem I'm trying to solve I'm having a conflict between my understanding and the proposed solution of the problem. In the problem, a cart of mass $m$ is put at rest in the start of a ...
2
votes
2answers
143 views

Dropping a weight onto a spring scale

Say I drop a 5kg weight from a height of 1 meters onto a spring scale like many people have in their bathrooms. On impact the scale will show a higher weight than 5kg. Question: Which quantities ...
2
votes
2answers
96 views

Where did energy come from?

So it is my understanding that before the big bang there was a very small point that held all of the matter in the universe. My question is what form did energy of the universe take on when it was in ...
21
votes
2answers
4k views

Why can't I do this to get infinite energy?

I know that I cannot do this because of conservation of energy, so I am looking for an answer as to why this will not work. So by my understanding of Einstein's whole famous $E=mc^2$ thing it is ...
1
vote
5answers
42 views

How does mechanical energy conservation work? [closed]

I am a little confused about how mechanical energy conservation operates when it comes to things like predicting velocity. I know that if conservative forces are the only forces acting on a body, then ...
0
votes
1answer
50 views

Finding the equation of motion

I am currently working on a problem where a particle of mass $m$ can move along the x-axis with no friction. The particle is moving in a gravitational field and the potential energy is given by: ...
-2
votes
1answer
55 views

Why is it necessary to supply constant electricity to make a laser work?

As we know that in lasers there are excited atoms. When energy is provided in the form of light, heat or electricity to these atoms, these excited atoms after sometime go to a lower state of energy ...
4
votes
4answers
83 views

Different kinds of energies in the form of $\frac{a_1a_2^2}{2}$

It seems to me that the energy of some kind frequently takes the form $$\frac{a_1a_2^2}{2}$$ Where $a_1$ and $a_2$ are some variables. So, for example, kinetic energy has $m$ and $v$; rotational ...
0
votes
3answers
38 views

When moving from one position to another at a constant velocity, how does the conservation of energy hold?

I know that this might be a duplicate question, but I have not found any satisfactory answers that clear up my lack of understanding. Here is my question. Say a sloth hangs on a tree in the middle of ...
1
vote
1answer
29 views

Velocity of mass colliding with another mass under the influence of gravity

If two spherical objects of mass $m$ and mass $4m$ are in outer space with no other forces other than the gravitational attraction between them and they're separated by distance $d$, what velocity ...
1
vote
1answer
68 views

Pendulum problem where it hits a peg swings around and lands on top of it

Using energy equation from when it is released to when it reaches its highest point after hitting the peg I get change in kinetic energy = 0 and change in potential due to gravity = 0 Does the string ...
0
votes
3answers
38 views

Work and energy

when a ball of mass m is brought with uniform velocity from infinity into the g field of the earth at a distance r from it, the potential energy of the ball earth system decreases from 0 to -GMm/r. ...
1
vote
2answers
74 views

Energy conversion (conceptual) question

Suppose I have a electronic car. It runs by electrical energy. So when wheels rotate, mechanical energy is produced. Then why can't we convert it back to electric energy and produce somewhat unlimited ...
0
votes
2answers
48 views

Approximating energy loss caused by drag force

We know the usual aerodynamic drag equation is given by: $$F_d = -bv^2$$ Where $b$ here is just some constants combination related to the property of the fluid and the material passing through. My ...
7
votes
5answers
288 views

In an elastic collision, can we choose between cons. of energy and cons. of momentum?

Suppose we have two solid spheres with masses $m$ and $M$, respectively, and that $m$ is significantly less than $M$. The lighter sphere is placed directly on top of the heavier one, and the two are ...
2
votes
0answers
26 views

Fluid approximation for inflaton?

In cosmology there is a widely used fluid approximation which applies when the mean free path is very small compared to the scale of observation and then all the properties of stress energy tensor can ...
0
votes
2answers
75 views

Energy conservation $\iff \frac{dE}{dt} = 0\ $?

If I'm asked to prove that a system is/ isn't conservative and compare it to whether or not the Hamiltonian is conserved, does that mean I need to compute the time derivative of energy $(T+U)$? Doing ...
0
votes
1answer
54 views

Difference between speed of light and of bullet after passing through a barrier [duplicate]

I just read that, when a ray of light traveling in vacuum at $c$ strikes a glass slab, its speed decreases and then when it re-emerges it gets back to its original speed i.e $c$. If I draw a ...
0
votes
2answers
65 views

Is potential energy an energy exists or potentiality of energy? [closed]

I want to ask whether Potential Energy is truly existing Energy - though i know the energy concept is abstract and we do not know what is energy truly as Feynman said - or it is the potentiality that ...
0
votes
1answer
46 views

Doesn't the Big Bang negate the law of conservation of matter? [duplicate]

I was watching Hawking (2004 film) and in the movie, Hawking has a sudden insight that all the matter and energy observed in the known universe may, in fact, have sprung out from nothing (singularity) ...
1
vote
1answer
40 views

Ideally black plates

Let's consider $m$ ideally black, thin, infinite plates. Let $T_i$ be the temperature of the $i$-th plate. For given $T_1 > T_m$, why is the radiant flux between any two adjacent plates the same ...
2
votes
1answer
44 views

How does energy get transfered from destructive interference point to constructive one?

If there are two pressure waves (like sound waves) that travel in opposite directions and have the same amplitude then destructive interference occurs: one wave will compress the air particle (here ...
2
votes
2answers
94 views

Blackbody radiation and emissive power

According to blackbody radiation theory, and thanks to Planck, we now know that there is a energy density, $u(\lambda,T)$ [$J/m^3$], associated with a certain wavelength at a particular temperature. ...
0
votes
0answers
41 views

How can electron move and yet have zero total energy?

Imagine an electron positioned at a zero electric potential plate with zero kinetic energy. The zero electric potential plate is combined in series with another plate with positive electric potential ...
0
votes
0answers
22 views

Why is the total energy of current universe conserved? [duplicate]

I began to think about this problem a long time ago and also asked some classmates as well, but none of them can give me a creative solution. I actually developed an original theory that our ...
0
votes
2answers
34 views

Find max height [closed]

I have stucked with finding max height of an $m$, given only $m, M$ and initial velocity of $m$ - $\vec{V}$. There is no friction in the system. As I understood I need to find $\vec{V_0}$ - velocity ...