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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Why does the kinetic energy work theorem apply to inclined surfaces?

Work is supposed to be force x distance, how did we end up with work equaling the difference of hights x gravitational acceleration ? Sorry for my poor understanding :D
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Confusion about Conservation of Mechanical Energy

I'm having a bit of difficulty understanding how to apply the idea of conservation of mechanical energy to a problem. One of the questions on my homework reads: A physics student shoves a 0.50-kg ...
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2answers
27 views

Motion after a collision of objects not involved in it [closed]

I'm confused about collisions where there are parts of the objects not involved in it. How do these parts move after the collision? I'll make an example. Consider a cart with an incline attached ...
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1answer
46 views

Change in energy during orbital decay

Suppose we have a planet orbiting a star and the planet radiates away some amount of energy $\Delta E$. I want to find by how much the orbit decays (i.e the change in orbital radius). One way to do ...
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1answer
41 views

Kinetic Energy: Difference in energy for observers [duplicate]

Consider a stationary 10kg bike; if I apply a force of 10J then $KE=\frac{1}{2}mv^2$ so $v=\sqrt\frac{2KE}{m}=1m/s$. If we consider the same example again, but from the point of view of a stationary ...
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1st Law of thermodynamics (conservation of energy) and quantum tunneling

Doesn't quantum tunneling contradict the 1st Law of thermodynamics? As far as I remember my school physics teacher told us that quantum tunneling is the reason why we can observe the alpha decay, as ...
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81 views

How can I find the height for a looping pendulum? [closed]

I have a ball attached to a string, hanging on a rod placed $L$ meters above the floor. I release the ball and string at a 90 degree angle and let it loop around another horizontal bar after it ...
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64 views

Why has a molecule less energy than the uncombined atoms?

My book says, A molecule as compared to the atoms from which it is formed is more stable because it possesses energy lower than the energy of the uncombined atoms. This difference in energy is due ...
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1answer
107 views

Is there conservation of momentum if there's conservation of energy? [closed]

The equation for conservation of momentum: $$m_1\vec{v}_1 + m_2\vec{v}_2 = m_1\vec{u}_1 + m_2\vec{u}_2$$ The equation for conservation of energy: $$\frac 12m_1v_1^2 + \frac 12m_2v^2_2 = \frac ...
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1answer
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Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson? [closed]

Can we in the far future have an energy source made from vacuum energy, antimatter, dark energy, Higgs boson and all kind of these like stuffs?
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60 views

Where did all the photon energy go?

From the wikipedia page on dark energy, in reference to the total mass-energy of the universe: The mass–energy of dark matter and ordinary (baryonic) matter contribute 26.8% and 4.9%, ...
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62 views

How do I show that the total energy of a particle moving in an electrostatic field is constant?

Basically the Lorentz force says that: $$F=qE+qv × B$$ I am asked to show that if a particle moves in an electrostatic field $$E= - ∇V(x,y,z)$$ and ANY magnetic field, then the energy: ...
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59 views

Why does antimatter and matter fuse to release energy even though they will cancel each other perfectly?

I have seen many physics people talk about antimatter as a evil twin of matter and when they come in contact with each other leave massive amount of energy instead of only void. If antimatter and ...
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43 views

Energy and Momentum: Impact

Boat A has just lost another race to Boat B. In a fit of rage, the driver of Boat A (mass = 500 kg) turns his boat towards Boat B (mass = 450 kg) which is at rest as the driver gloats over yet ...
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0answers
27 views

Does a conservative form of the Chew-Goldberger-Low MHD equations exist?

I wonder if is it possible to bring Chew-Goldberger-Low MHD equations to a conservative form? Conservative form of the system of equations of ideal MHD with isotropic pressure is well known and can be ...
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2answers
47 views

Condition for looping the loop

Consider a ball tied to a string and it is imparted a velocity we have studied that condition for looping the loop is that tension at the uppermost point must be zero, but why is this condition ...
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0answers
31 views

Velocity of an object rolling down a spiral

Imagine a fiction less spring-like spiral, an object will roll down the wire path the spring creates. From conservation of energy, the initial PE will be equivalent to gravitational potential energy, ...
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1answer
38 views

Conservation of energy in the “ideal physics world”

Let us assume we are in the textbook ideal physics world, without any friction. Now, if I place a ball at the top of a parabola U and let it go under gravity, the ball should roll up and down forever ...
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1answer
35 views

Projectiles ability to do work to a box when connected by string? [closed]

I was wondering, the work-energy theorem states that KE can do work, as it is Mechanical energy. if the KE energy and thus Mechanical energy of a ball, if external...can do work on an object, ...
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1answer
57 views

Why does work done ultimately culminate as wasted heat?

Come to think of it, the work done on a body is converted into some form of energy.But why is it that it ultimately tends to produce heat? In physics we all talk about energy dissipation in the form ...
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74 views

Non-locality of gravitational energy

Gravitational energy is non-local which is essentially because of the equivalence principle. The equivalence principle says that you can always transform your frame so that you feel like in a ...
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4answers
172 views

Does the physical cross section of a projectile have an effect on the motion of a target? [closed]

Why can a baseball (moving with kinetic energy) move a baseball glove (or how a hammer displaces a nail),. yet a bullet can't move a person? I know the laws of conservation of energy, deformation, ...
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3answers
127 views

Where does the energy go when engine braking?

If you're in gear in a car and not accelerating, the car slows down faster than it would from just air resistance and tire deformation. In normal braking, the energy is turned into heat from the brake ...
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1answer
77 views

Harmonic oscillator: if $E=\frac{1}{2} q \dot{\theta}^2+\frac{1}{2} s \theta^2$ then $\omega=\sqrt{\frac{s}{q}}$?

Consider an harmonic oscillator. Suppose that I manage to write the mechanical energy as a function of a quantity, like the angle $\theta$ in this way $$E=\frac{1}{2} q \dot{\theta}^2+\frac{1}{2} s ...
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1answer
77 views

What is the reason behind why energy must always be conserved, apart from observation? [duplicate]

I know that we see in experiments (physical and thought) that energy is always transformed into something else, but what propels our universe to behave this way? What is happening at small levels that ...
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1answer
46 views

Equal kinetic energy in two bodies of different masses

Two bodies that are equal in their size rest in a table. There isn't friction between the bodies and the table. The mass of one of the bodies $(A)$ are $4$ times bigger than the mass of the second ...
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1answer
81 views

What is the probability of two bullets to get clashed? [closed]

I was surfing on Instagram, and I found this amazing proto whose description is "the probability of such an event to happen is incredibly small, so this is a really curious finding". Well.. I'm ...
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5answers
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How can escape velocity and gravitational potential be higher in weaker gravitational fields?

on Earth we have an escape velocity of about 11180 m/s and a gravitational field of g=9.81 m/s^2. It would seem obvious that the escape velocity was higher for stronger gravitational fields. Often it ...
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1answer
141 views

“Principle of least action” and “Principle of conservation of energy”: Which one is fundamental and which one is derived? [closed]

Suppose I throw a ball upwards. First it will rise under gravity and then fall under gravity. During the rising part the kinetic energy gradually decreases and the potential energy increases until ...
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1answer
167 views

Perpetual motion machine with magnets [duplicate]

I have been recently made aware of the following motor, which uses two magnets and a wheel to generate motion, and the creator of this machine claims that this motion is perpetual. Here is a ...
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1answer
43 views

Change of energy into friction?

If I am pushing a car and it does not move, I am doing no mechanical work. However, I am changing the energy that was stored in my cells into heat energy (maybe sound energy if my hand slips off the ...
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71 views

How does the energy remain conserved in a transformer?

The induced voltage in the secondary coil of a transformer is given as $\frac { { N }_{ S } }{ { N }_{ P } } *{ V }_{ P }$ (where ${ N }_{ P}$ and ${ N }_{ S }$ are the number of turns in the primary ...
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1answer
80 views

Does the kinetic energy change depending on the direction of the velocity? [closed]

The kinetic energy is calculated with the following formula: $$E_k = \frac 12 mv^2$$ Does $v$ represent the value of the velocity? I mean, if you have a specific direction of the velocity is the ...
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1answer
37 views

Kinematics contradicting conservation of energy?

I was having some conceptual difficulty reconciling my intuitive understanding of kinematics with conservation of energy, so I made up a short problem that tested my intuitions: Suppose I define an ...
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Radiation due to current

Generally we equate change in potential energy to change in kinetic energy but in case of a charged particle like electron this is inconsistent. Consider a case: An electron(of charge e)from rest is ...
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3answers
106 views

Where do the energy and momentum come from when light is perfectly reflected on a surface?

If you send a light beam to a surface which reflects the light perfectly I would say that on the surface is put a force of the light so the surface moves backwards. But where does that energy comes ...
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1answer
73 views

How is energy conserved in this electromagnetic scattering?

I have been pondering over an energy conservation issue for a few weeks and would like to share it with the hope that its interesting and someone will be able to answer. Suppose I have two positive ...
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1answer
84 views

How does an Atoms electric field overcome an electrons inertia?

An electron has mass, and therefore has inertia. How does an atoms electric field perpetually overcome an electrons inertia, necessary to hold it in its shell? Does this require continual work to be ...
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48 views

A Solid sphere rolling down an incline - find its radius

A solid sphere ( I = 2⁄5mr2 ) with unknown radius and 3 kg mass is rolling at 8 m/s. It travels 4 meters distance on a 30 degree ramp. What is the radius? This was given as a "trick" question ...
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Does the mass lost by merging black holes depend on how they merged?

We've all heard the news about the detection by gravitational waves of two black holes, one 29 solar masses and the other 36 solar masses, spiraling into each other to create a single black hole of 62 ...
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1answer
32 views

Energy conservation in Hubble Expansion [duplicate]

So space expands due to Hubble Flow. Some light is observed, which is seen to be red-shifted due to the space expansion. It is less energetic. Where did the energy go?
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2answers
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What was the initial source of energy?

According to law of conservation of energy; "Energy can neither be created nor be destroyed but may change it's form" Where did the energy come from which led to the creation of universe by big bang?
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171 views

Relating Poyntings theorem to Lenz and Faraday's law?

In system's similar to a motor, where the armature begins to accelerate simultaneously there is induced $-\epsilon$ to reduce the applied current(hence the applied power $P(t)$ is also reduced), or ...
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1answer
42 views

Energy method to solving equations of motion? Why does this method work and what is it called?

Given the stated system in the photo we are suppose to prove simple harmonic motion when given an initial displacement $x$ I first considered the total energy of the system which we will call $H$ ...
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Would superconducting mirrors in a superposition of spatial states make a difference related to the dynamical Casimir effect?

Related to the dynamical Casimir effect: https://www.technologyreview.com/s/424111/first-observation-of-the-dynamical-casimir-effect/ Related to superposition of macroscopic objects: ...
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182 views

Where Potential increase gets it's energy from?

Imagine there are two particles: A,B; of which, we observe A to be stationary, while B is moving towards A at velocity v. A,B has same mass; and they are charged both positive. B has a potential wrt ...
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Conservation of energy in vertical circular motion

Books usually use conservation of energy to solve problems in vertical circular motion. But, principle of conservation of energy for one particle is, If all the $n$ forces $\vec F_i ...
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“Carbon capture into fuel” - where does the energy come from?

I recently found an article titled "Scientists find a way to convert Carbon Dioxide from air into fuel" (here), and my immediate reaction was "surely not - the energy balance would kill you right ...
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Conservation of momentum and conservation of energy

I had the impression that conservation of momentum is valid only when we consider no loss of kinetic energy into heat etc. My tutor says that even if we have loss of kinetic energy as heat in a ...
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1answer
51 views

Relationship $v_A^2-v_B^2=2a_t\Delta s $ for motion with constant tangential acceleration [closed]

I can not understand a relationship in the general motion in the plane. I am considering a point, which follows a curved trajectory. Indicate A and B two different position of the trajectory and $v_A$ ...