# Tag Info

### Does A Pivot Exert A Force

Answers from the comments From @mmesser314 A pivot does exert a force. You can see this by hanging a pendulum from one. The pendulum does not fall down. If you hold the bob off to the side, gravity ...

### Why particle is certain to escape the conservative field when the mechanical energy is positive?

From the comments: By @naturallyInconsistent For convenience, we chose to put the conventional dividing line for this thing so that zero is the limiting case. i.e. it is a choice, a very natural ...
1 vote

### Energy and momentum conservation for photon absorption

As you point out, not all of the energy of the photon goes into exciting the electron, Similarly when stationary atom emits a photon, some of the electon level enenrgy goes into the recoil kinetic ...
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1 vote

### What are the forces that do work when a spring between two masses pulls inwards?

There is no kinetic energy change in the second system (1,2, spring). So there is no net work done, so no need to ask which external forces that do the work. The constituent particles of the system do ...
• 51.5k
Accepted

### Where does the energy in a fundamental interaction come from?

In your electron example, there are two problems: Although independently both electrons gain energy, you would never find two electrons close to one another to begin with, because such repulsion ...
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### With two balls connected to a string find minimum upward velocity that can be given to one of the balls such that the other leaves the ground

I have done upto this, but I don't know how to further solve this second order differential equation.
1 vote

### Validity of $\mbox{d}H/\mbox{d}t=\partial H/\partial t$ for dissipative systems

I'll split my answer in some paragraphs: 1. Newton's mechanics in strong and weak forms; 2. Lagrangian mechanics; 3. Hamiltonian mechanics. 1.a Newton's mechanics: strong form. Newton's second ...
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Accepted

### Strogatz's condition on definition of energy

If $E$ does not depend on $x(t)$, then $\frac{dE}{dt}=0$ doesn't give you any information about $x(t)$. Since the entire goal of classical mechanics is to solve for $x(t)$, this means such an $E$ ...
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### Validity of $\mbox{d}H/\mbox{d}t=\partial H/\partial t$ for dissipative systems

In principle you can use Hamiltonian mechanics also for dissipative systems, or fon systems which cannot be completely described by a Lagrangian. When a Lagrangian is provided, the Legendre ...
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Accepted

### Spring potential energy, conversion

The problem in your book assumes an ideal (lossless) and massless spring. That is typical of entry level physics problems. Without the ball you would need to consider, at a minimum, that the spring ...
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### Spring potential energy, conversion

In the case where there is no ball (and assuming there is no friction/other dissipative forces), the spring will expand beyond its natural length due to the momentum it gains while unstretching. At ...

### Does a rocket moving in a circle expel exhaust at a greater velocity?

Accordingly, must not the exhaust now obtain the entirety of the spent energy? Yes. From the rocket's frame of reference, would the exhaust gases be perceived as exiting at a greater velocity than ...
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1 vote

### Does a rocket moving in a circle expel exhaust at a greater velocity?

Energy is frame dependent. In a inertial frame that is momentarily comoving with the rocket, its kinetic energy is zero, and the kinetic energy of the gas is the same for both cases, if we suppose the ...
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### Does a rocket moving in a circle expel exhaust at a greater velocity?

This is an interesting question, because when it is moving in a circle, the magnitude of its tangential velocity is constant and its angular velocity is also constant. Therefore the total kinetic ...
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### Does a rocket moving in a circle expel exhaust at a greater velocity?

The energy of the fuel does become the kinetic energy of the exhaust. Let us ignore the fact that the rocket becomes lighter as it burns fuel, and that it will eventually run out of fuel. The rocket ...
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### Why in two block system, both block tends to have same velocity at maximum potential energy when no external force is applied?

Answer to problem 2: 2 blocks + spring The system of two blocks being isolated, its barycenter is in uniform tranlation, and defines the barycentric Galilean referential, at velovity \vec{V}=(m_1\...
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### With two balls connected to a string find minimum upward velocity that can be given to one of the balls such that the other leaves the ground

This statement that the both balls have equal horizontal velocity is not correct. When string is vertical let velocity of A be v(A) and velocity of B be v(B). Then, mv(B)²/(L/2)=T+mg mv(A)²/(L/2)=T-mg(...
1 vote

### Energy Transfer and Work in Action-Reaction Forces

Because doing work on something doesn't make you gain kinetic energy. Now I have said this multiple times on this site, and will continue: Bringing the human body into physics never, ever, helps. It ...
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### Is it possible to lower the energy of the vacuum?

I agree with what others have said here, QFT should be learned in university, but to actually answer your question: let's say your description of quantum vacuum energy is correct. No, you can't alter ...
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### Why doesn't a hydraulic lever violate conservation of energy?

But like using the kinematics formula we get equal displacement. Like we know that mass of the lever increases proportionately with Area so F1/m1 = F2/m2 Now, S = ut + 1/2at^2 S = 1/2at^2 S1 = 1/2F1/...
1 vote

### Do off-centre forces create additional energy?

There is a standard demonstration in which an off-centre force is provided by a string wrapped around a cylinder on a air-table. Another string is connected to the c-of-mass of an otherwise ...
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### Is it possible there can be a non-Fourier model of string vibration? Is there an exact solution?

Assuming linearity, the response of the system can be written as the superposition of its modes. How many modes does a continuous medium have? An infinite number, in general. How many modes have non-...
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### Is it possible there can be a non-Fourier model of string vibration? Is there an exact solution?

A vibrating string can support a very large number of vibrational modes simultaneously. This is because waves on strings superimpose linearly. Note here that an electric guitar string struck hard with ...
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Accepted

### Do off-centre forces create additional energy?

If the line of action of the force is not through the centre of mass then body can be thought of as being acted on by a force whose line of action is through the centre of mass which produces a linear ...
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Accepted

### Energy of a flowing fluid

Shouldn't the net force acting on the fluid element by the surrounding fluid, as it is pushed through a distance $dx$, be equal to the changes in the kinetic and potential energies of the fluid ...
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### Energy of a flowing fluid

I hope you can find the answer to all your questions in the balance of kinetic energy. Differential form. The differential form of this equation is derived from the scalar product of the momentum ...
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### Does a small domino toppling a large domino violate conservation of energy?

A small domino should be more massive than the larger domino . Otherwise the initial energy applied to it should enough to push other dominos and eventually it will stop due to loss of energy So ther'...
1 vote

### Does a small domino toppling a large domino violate conservation of energy?

There is no paradox at all. With a small activation energy it is possible to initiate a process that releases a lot of energy that was stored in a metastable state. Combustion is an example: with a ...
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Accepted

### "Friction at a contact point", rubber-band experiment

Yes, there will be some heat produced as a result of friction between the rubber band and the board, and also some heat produced as a result of internal friction within the rubber band itself, as it ...
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### Does life erode Earth's mass over time?

Good question! No, life does not reduce the mass of Earth. Life is cycling elements such as C, O, N and H. It is fuelled nearly completely by solar energy. This is converted to heat, which at the end ...
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### Does life erode Earth's mass over time?

Remember that nearly all of life's energy ultimately comes from the sun. Life uses energy that is received from the sun, "mixes" it with matter from the earth, and eventually that energy ...
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1 vote

### Is the work I do on the object always equal in magnitude but opposite in sign to the work the object does on me?

Does Newton's third law also hold in the work-energy theorem? In other words, are the 'work I do on the object' and the 'work the object does on me' equal in magnitude but opposite in sign? Sometimes ...
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