# Tag Info

### Work done by a spring on objects of different masses

Momentum is conserved. So with trolley $A$ and $B$: $$p_A + p_B = 0$$ hence: $$p_A = - p_B \equiv p$$ So the kinetic energies are: $$T_A = \frac{p^2}{2M_A}$$ $$T_B = \frac{p^2}{2M_B}$$ which are ...
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1 vote

### Work done by a spring on objects of different masses

A simple thought experiment would say NO, but it depends on some clarification. Work done on one of the masses is equal to the integral of spring force over displacement. In the limit of one of the ...
• 757
1 vote
Accepted

### Energy Conservation Spring Problem

OK I wrote the wrong answer previously, here is a corrected version. The main observation is that conservation of energy doesn't apply. At least not in the way you are writing it. Why? Because if ...
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### How hot can one heat a single atom?

I have to begin by being pedantic. Thermodynamics (the subfield of physics where temperatures, entropy, and pressures are explained) does not make statements about the state of a single atom. Like ...
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### How hot can one heat a single atom?

I'll start with question #1: For the radiation to have any effect on the hydrogen atom, it must have a frequency (and therefore a well-defined energy content) that the atom is capable of responding to....
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### How hot can one heat a single atom?

Heat is a thermodynamic concept, applicable to systems with large number of particles (taking Avogadro number $N_A\propto 10^{23}$ as a typical number of particle sin the system). In fact, ...
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Accepted

### What does the equation of the law of conservation of total mechanical energy with deformation energy look like?

The reason you "neglect" deformation energy is that it isn't necessary for the calculation, especially in the case of perfectly elastic contact. The only time you might want to include it ...
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### When do we use $ΔU = mcΔT$?

There is no "$c$", you must always specify under what conditions you measure the heat capacity (OK, sometimes people are lazy and only the context tells you what subscript is operational). ...
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### When do we use $ΔU = mcΔT$?

In classical thermodynamics, we usually start with defining the exact differential of the property we want to examine (because we are interested mostly in differences) rather than exact quantities. ...
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### Motion of the COM of 2-body system

For your block and wedge system the forces acting on and within it are as shown below with the block treated as a point mass and only the directions shown for the forces $N'''$ and $Mg$. In terms of ...
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### Motion of the COM of 2-body system

As there were no external forces on the system from when the block was given the velocity 𝑣... The net external force on the system is $\left[N - \left(M+m\right)g\right]{\hat y}$, where $N$ is the ...
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### Motion of the COM of 2-body system

There is a normal (vertical) force from the ground to the wedge of mass $M$ that allows for a displacement of the COM in the vertical direction. In other words, this is not an isolated two-body system....
Accepted

### Total Work Done and Energy calculation "error"? - Bike cycling up bridge

I belive the answer should be -410 J provided from the equation: $$[W=\Delta KE]$$ The total work is the work by all forces.
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

It looks like a lot of people are not understanding @BioPhysicist correct answer. So I will attempt to help... Buoyant Force is defined as -pgV (p=fluid density, g=gravitational acceleration, V=...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

Blowing the ballast takes energy, but that isn't the question. The energy to rise is from gravity. The submarine is now lighter than the seawater surrounding it ... which is another way of saying the ...

### Prediction and Work

It has nothing to do with "prediction" If something moves it has kinetic energy, you can change it to any other kind of energy. Thats how one uses moving water or air or a dynamo on a ...
• 6,559
1 vote
Accepted

### Prediction and Work

I've had this idea that if you can predict something, you can theoretically extract work from it. I don't think that is always true. For example, if you have a closed system (i.e. perfectly isolated ...
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### Home position of a wind turbine

If the blades were properly balanced and equal in mass, there should be no preferred resting position. By symmetry, the centre of mass of a three bladed propeller or turbine will be at the axis of ...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

As the submarine sinks, it pushes the water up. If a submarine of volume $V$ goes down a distance $d$ in a fluid of density $\rho$ in a gravitational field of $g$, then it has done $\rho gdV$ of work ...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

It doesn't take a quantity of energy to lift the submarine, if it achieves neutral buoyancy, because the rising mass of submarine is balanced by the falling of ocean water into its previous location. ...
• 10.1k
1 vote

### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

This doesn't reconcile with the fact that a neutrally buoyant submarine can jettison arbitrarily little ballast to surface. You claim that when going to twice the depth, you need to spend twice the ...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

The energy came from the thermal energy of the compressed air used to blow the tanks. As the water is blown out, the air molecules collide with the water surface; as the water surface is moving away ...
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### Waves, energy and matter

I'm not a physicist, and I'm not sure how the physicists will vote on this one; but IMO, instead of saying that a wave transports energy, I think it might be closer to the truth to say that a wave is ...
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1 vote

### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

@BowlOfRed's answer is the correct one. But to further clarify that, let's illustrate the idea with a perpetual motion machine rather than your submarine, as follows. To begin with, let's construct a ...
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### Waves, energy and matter

What is the physical reason why a wave (mechanical or electromagnetic) can transport only energy and not matter? An electromagnetic wave, by definition, describes wave behavior related to photons. ...

### Waves, energy and matter

We like to deal with abstractions that make easier to explain reality. For example, there is no mechanical waves in an idealized rigid body. Any movement at any of its points is instantaneously ...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

The high-pressure air in the compressed air system inside a submarine has a tremendous amount of energy stored in it. That stored energy lets the air perform work against the surrounding high-pressure ...
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### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

Now at depth it blows the tanks and becomes positively buoyant. This is not free. As the openings are now at depth, it requires a large amount of energy to remove the water. The energy to remove ...
• 41.9k
Accepted

### When a submarine blows its ballast and rises, where did the energy for the ascent come from?

Buoyancy comes from a fluid exerting a net upward force pushing on a submerged object. As a floating object rises, the buoyant force does positive work on the object, and hence negative work is being ...
• 58.1k
1 vote

### Do gravitational waves cause matter to radiate?

It depends on your exact setup but in theory you could arrange for gravitational waves to generate electromagnetic radiation, however in practice I don't think this scenario is very plausible to occur ...
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### Do gravitational waves cause matter to radiate?

Let us assume that our gravitational waves are described by a linearized Einstein equation, so that the metric is of the form $$g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu}$$ Where $\eta_{\mu\nu}$ is the ...
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### How does gravitational potential energy work in a very large distance?

Initially, they are millions of light-years apart, with the asteroid slowly moving away from the planet at a velocity exceeding the escape velocity at that distance (1 cm/century). Conventionally, ...
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### What is the potential energy and kinetic energy for capacitor and inductor?

In an antenna energy exchange occurs between its electric field and moving current. The dynamics of a half wave dipole are similar to sloshing water in a bathtub. At the instant the water stops ...
• 1
Accepted

### Could thermal or quantum fluctuations in the far future ionize matter?

Yes, this is a consequence of the Herzfeld "paradox", first noted in 1912. The partition function for a sole hydrogen atom is not well behaved, diverging when one takes the infinite number ...
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### What is the potential energy and kinetic energy for capacitor and inductor?

First, let me collect my earlier comments here: In specifying a Lagrangian, one typically specifies the configuration space and generalized coordinates on it. Terms in the lagrangian are often ...
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1 vote

### What is the potential energy and kinetic energy for capacitor and inductor?

Kinetic energy is $1/2mv^2$ of a moving mass. The kinetic energy of a current is tiny. First, electrons are $1800$ times lighter than protons and neutrons. And only a fraction of the electrons of ...
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