Stack Exchange Network

Stack Exchange network consists of 174 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange

Questions tagged [energy-conservation]

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.

2
votes
1answer
70 views

Are these collisions equivalent?

Similar to the question if two cars with a velocity of 50 mph each colliding is the same as one car colliding with wall at 100 mph, I was wondering if the same amount of energy is produced when ...
0
votes
1answer
26 views

Conversion of Kinetic energy to electric potential energy

Two deuterium nuclei, separated by a large distance with same initial speed are travelling towards each other; the radius of the nucleus is given. How can I find the minimum speed of the nucleus? I ...
0
votes
1answer
36 views

Conservation of energy in rotational motion

Suppose a boy is standing on a platform which is free to rotate about an axis passing through its center. The Kinetic energy of the boy and the platform is K. If the boy stretches his hands so that ...
0
votes
0answers
26 views

Get relation from definition of stress-energy tensor and the conservation of energy

Starting from the following definition of stress-energy tensor for a perfect fluid in special relativity : $${\displaystyle T^{\mu \nu }=\left(\rho+{\frac {p}{c^{2}}}\right)\,v^{\mu }v^{\nu }-p\,\eta ...
0
votes
1answer
40 views

Is the mechanical power output of a car constant?

According to my textbook. The mechanical power output of a car is constant. My reasoning tells me otherwise though. According to a lot of pages on this forum related to the driving force of a car. ...
1
vote
0answers
16 views

Energy in magnets [duplicate]

If I have a magnet and an iron object, then the object would move towards the magnet, and the energy required to do so is stored in the magnet itself, but there can't simply be an infinite amount of ...
3
votes
3answers
231 views

Falling - is the gain in KE linear? [on hold]

When a ball falls from a high place, it experiences a gravitational force. Forces make objects accelerate (in this case, it is constantly increasing the velocity). Because $KE = \frac{1}{2}mv^2$ this ...
0
votes
1answer
27 views

Why does the initial bounce distance (from origin of release) for a ball increase with mass of the ball?

I have completed an experiment on the effects of the wetness of a ball on its trajectory when projected. What the data I collected says that the more wet a ball is (determined by the mass of the ball ...
-1
votes
3answers
66 views

Energy of an object

When an object goes up we say that it gained potential energy but it is doing positive work on the earth so it should lose energy.Please correct me.
1
vote
0answers
31 views

Internal energy of a system and its conservation

In textbooks of thermal physics, we usually find internal energy of a system defined as the energy measured in the center of mass frame. Now, in mechanics, we study the conservation of mechanical ...
1
vote
1answer
67 views

Can two semi-infinite plane waves undergo perfectly constructive interference?

This is building off of a question I asked here. When discussing the linked problem with some friends, the consensus seemed to be that the reason two identical semi-infinite plane waves cannot ...
1
vote
1answer
67 views

Lagrange Equation - Basics

The basic equation of Lagrange is given by, $$\frac{\mathrm d}{\mathrm dt} \frac{\partial L}{\partial \dot{q_j}} - \frac{\partial L}{\partial q_j} = Q_j \tag{1}$$ where $T$ is the kinetic energy, $V$ ...
1
vote
0answers
31 views

Is energy conserved in QFT in curved spacetime?

Energy is conserved in QFT in flat spacetime. Does energy conservation still hold in QFT in curved spacetime? I already searched this site and google about this but didn’t find a clear answer
5
votes
3answers
54 views

Energy conservation in the combination of two coherent beams

I was asked a question today by a student that I couldn't find a satisfactory answer to. Imagine we have two identical laser beams with average electric field strength $E$. Upon combining these beams ...
1
vote
1answer
31 views

Threshold energy for photoelectric effect

I'm trying to derive the threshold energy for the photon in photoelectric effect, but I'm not sure how to treat the electron. What can I assume about the final kinetic energy of the electron? I want ...
0
votes
1answer
70 views

Energy conservation in QM [duplicate]

Let's say I have a time independent Hamiltonian so my system conserves energy. It's initially in an energy eigenstate with $E=1$ in whatever energy units you like. I measure a different observable ...
0
votes
2answers
58 views

Why does an object in simple harmonic motion have kinetic energy at its equilibrium point? [closed]

While an object is undergoing simple harmonic motion, its kinetic energy tends to vary with its position. This kinetic energy is highest when it's at the point where the forces on it are at ...
1
vote
2answers
67 views

Can't understand energy conservation in simple thermodynamics example

SITUATION Consider a very big cylinder with a mobile piston in the middle separating two sections (A and B) filled with gas. Both the cylinder walls and the piston are adiabatic, so there is no ...
5
votes
2answers
80 views

How is it possible that the earth has the concept of energy equilibrium: the same amount of energy comes in and out?

I was reading this article on the Earth's Energy Budget, as well as a few other articles that basically contained the same information: That there must be an equilibrium of energy coming into the ...
0
votes
1answer
25 views

When finding the force of a falling raindrop, how to account for not knowing the collision time?

So my question is to find the force of impact on the ground of a raindrop of mass $m$ that falls at velocity $v$ right as it hits the ground. So what I did was find the momentum of the object, which ...
1
vote
1answer
30 views

A man on much faster spinning Earth decides to jump

Imagine that the Earth is spinning much faster than real situation and a man on the Earth's surface feels weightless. Suddenly the man decides to jump vertically with respect to the Earth. What will ...
0
votes
0answers
39 views

Impact parameter for particle capture

"Let a particle of mass m moving in a potential $$V(r)= \frac{-k}{r^3}$$ come in from infinity with speed $v_0$ and impact parameter $b$. What is the largest value of the impact parameter ($b_{max}$) ...
-4
votes
2answers
77 views

If there were no inertia would perpetual motion be possible? [closed]

Is it not inertia that makes a flywheel resist being put into motion? And if that is true, is inertia a consequence of being unable to create energy, i.e., why perpetual motion machines do not in fact ...
-1
votes
3answers
71 views

Can someone explain this equation more clearly: $\Delta E_{Total}=\Delta K + \Delta U = W_{External}$

I asked my Physics teacher and he explained it to me, but I am still confused on how to use it. \begin{equation} \Delta E_{Total}=\Delta K + \Delta U = W_{External} \end{equation} I think it's the "...
1
vote
2answers
48 views

Collision between two non-deformable objects

Let say we have a non-deformable object and we release it into free fall and it hits the non-deformable floor. What would happen? Here is the way I think. Since the floor and the object are both ...
12
votes
3answers
891 views

Humans have an average energy budget of $100$ Watts, but the power radiated from the body is $1000$ Watts?

On average a human consumes around $2000$ kilocalories per day. This converts to roughly $2000000$ calories / $86400$ seconds or around $100$ joules / second giving roughly $100$ watts. But if you ...
3
votes
1answer
49 views

How does a rolled piece of paper lose its Elastic Potential Energy

If you roll up a piece of paper and leave it for years it will stay rolled up. If you leave it rolled up for a minute it is easy to unroll. Of course this must be because the energy stored in that ...
0
votes
0answers
15 views

Can a car accelerate on a frictionless surface? [duplicate]

i have string of related questions. i know that the internal forces cannot result in acceleration of a body and so in daily life, car engine applies force to rotate wheels that produce friction force (...
0
votes
1answer
61 views

I can't understand why Y can not be higher than X in this following explanation of Richard Feynman in his lecture

We suppose, however, that there is such a thing—a reversible machine—which lowers one unit of weight (a pound or any other unit) by one unit of distance, and at the same time lifts a three-unit weight....
-1
votes
0answers
22 views

What is energy? (fundamentally) [duplicate]

This is a question I wonder about mostly that what is energy at fundamental level.Do we have a proposed particle that satisfies all properties.Can anyone explain what research is going on in this ...
-1
votes
1answer
84 views

Violation of energy conservation during collisions of a particle with different sections of a rod [closed]

Imagine a homogenous rod with total mass $M$ and length $l$ floating in free space without any force or constraint acting on it. Then, think about two possible scenarios. In the first, a particle with ...
-2
votes
1answer
31 views

What is the largest radius that the circular track can have such that the car remains in contact with the track? [closed]

A car is travelling at $u$ as it enters a vertical loop of radius $r$. What is the largest value of $r$ that the track can have such that the car goes up and around the track whilst remaining in ...
3
votes
1answer
42 views

Where does the energy come from when a magnet moves a compass? [duplicate]

My very limited understanding of magnetism is that it is essentially stored energy. However what I'm confused on is about the following situation: If you have a compass and you move a magnet close to ...
0
votes
1answer
74 views

How is red shifting explained with quantum mechanics? [duplicate]

Light travels as waves through space, and since space is expanding, the light gets stretched out. This is the explanation for red shifting I learned before. Now I know some more and that explanation ...
1
vote
1answer
43 views

CMB - Excess Energy? [duplicate]

Today in physics we were looking at how the energy of a photon is the product of Planck's constant and the frequency of the photon, therefore the lower the frequency, the lower the energy of the ...
-2
votes
3answers
29 views

Rolling friction and motion [duplicate]

The frictional force in a motion of a rolling body is along the direction of motion. Then what stops the motion of a rolling body?
-2
votes
2answers
57 views

How to identify if the law of conservation of energy is violated in a nuclear reaction? [closed]

I have encountered many textbook questions that ask for what laws of conservation is violated in a given nuclear reaction, and I can't seem to figure out how to identify if a nuclear reaction violates ...
1
vote
4answers
79 views

Why does force x distance not equal kinetic energy for objects rolling down an inclined plane?

Imagine two objects at rest at the top of an inclined plane which are capable of rolling and which are identical in every way (mass, radius, etc) with the exception of how their mass is distributed. ...
0
votes
0answers
55 views

Do we need infinitely many objects to make one have kinetic energy?

We have one ball at rest. How can we give it energy to make it move? One way is by striking it with another moving ball, for which we need another ball with energy. Then how can we make the other ball ...
1
vote
2answers
38 views

Force, work and the apparent disappearance of Mechanical Energy [duplicate]

A man exerts force on a wall of bricks. The man must have consumed the energy he possessed (mechanical energy?) to exert the force. The man sweats and tires himself out but the wall does not move. ...
-2
votes
1answer
31 views

Forces From Opposite Sides On A Block

Suppose we have an indestructible box in space , and we apply equal forces on two opposite side of box , the box wont move but then where will the energy go?(Conservation of energy)
0
votes
1answer
32 views

Photon frequency when transitioning from one medium to another

$E=h\nu$ where $\nu$ is frequency and $h$ is Planck's constant. The frequency, wavelength and speed are related as $c=\lambda\nu$. When transitioning from one medium to another the speed decreases by ...
0
votes
2answers
34 views

How does a bob oscillating tied at the end of rope obey the law of energy conservation?

A bob which has earned a certain potential energy being lifted and displaced from its equilibrium position. Then how does it oscillate back and fro to give so much energy. Or how can a little change ...
0
votes
0answers
22 views

Why can we use conservation of energy even when friction is involved? [duplicate]

A solid cylinder of mass $m$ and radius $r$ starts rolling down an inclined plane of inclination $\theta$. Friction is enough to prevent slipping. Find the speed of center of mass when its center of ...
1
vote
1answer
132 views

Where do “virtual” particles come from? [duplicate]

Quantum physics tells us that even a perfect vacuum be populated by "virtual" pairs of subatomic particles. I have read that these particles are allowed to violate the Conservation of Matter and ...
1
vote
1answer
38 views

How to simulate possible trajectories of particles after $\beta$ decay?

I'm a programmer trying to simulate the movement of the particles involved in $\beta^-$ decay, or at least an approximation of it, for fun, in a 2D universe. I would like to keep the simulation ...
0
votes
1answer
71 views

How to help the subway to save electric energy? [closed]

You get in the middle of a subway car. The subway accelerates, coasts, decelerates, optionally recouping energy. Is there anything you can do to make the subway use less energy? Walk or roll forward ...
0
votes
1answer
46 views

How do I show that $E\equiv \frac12 m \dot{x}^2 + U(x)$ is constant for a classical particle moving under the action of $F(x) = -dU/dx$? [closed]

A mass $m$ moves in one dimension $x$ under the action of a (net) force $F(x)$. The force can be derived from a function $U(x)$ according to the formula $$ F(x) = -dU/dx. $$ Show that the quantity ...
1
vote
1answer
52 views

Is the cosmological Continuity Equation an expression for energy conservation in General Relativity?

Is the cosmological continuity equation $$ \frac{d\rho}{dt} + 3H(\rho + p/c^2) = 0 $$ an expression for energy conservation in General Relativity?
0
votes
1answer
54 views

Does energy conservation take into account the dimension of time?

We know that there are only 3 spacial dimensions because if we light a light bulb, we are able to measure the energy at any point in the 3 spacial dimensions and we can see that no energy is leaking ...