# Tag Info

80

The answer to the question is in the question itself. You wrote: Suppose 2 objects are placed not too far away from themselves. These objects start moving towards each other due to gravity. Then asked: Where does the energy come from? The energy comes from whoever placed the two objects apart. They had to do work to get them into that position. ...

19

Does gravity break physics? No. Physics is the study of how reality works. The "description" or "modelling" of however everything technical around us behaves. Thus, reality can't break physics anymore than reality can break e.g. language. Reality can be surprising (because we didn't expect it) or unintuitive (because we aren't used to it) etc. but it can't ...

18

So, why there is a concept of potential energy when it can be understood as the field is doing the work and supplying the kinetic energy rather than potential energy is getting converted to kinetic energy? Work (and heat) is energy transfer. Work does not create energy. It transfers energy. That includes the work done by gravity. When something does ...

10

Your statement I have given [gravitational] potential energy to the mass. is incorrect. It is the mass and Earth system which gains the gravitational potential energy not just the mass. When the separation between the mass and the Earth decreases gravitational potential energy is converted to kinetic energy. How then might you switch off gravity? ...

8

Your question has a few problems. The definition of potential energy as the work done to move the object against the force is not a very good definition (although it is often repeated). A better definition is potential energy is the negative of the work done by the internal (conservative) forces $$\Delta U = -W_\mathrm{internal}$$ As pointed out by @...

6

I prefer slightly different point of view on this problem. We have some field and some object. There is some force acting on the object in the field. In some situations the nature of the force is such that: the force depends on position of the object only when we move the object from some arbitrary point A to arbitrary point B the work done by this force ...

4

They are complementary and equivalent. When we state these principles for thermal equilibrium, entropy is maximized at constant energy, and energy is minimized at constant entropy. They give the same answer but correspond to two very different behaviors for a system. EDIT: First let's see why they are equivalent. Consider a system with energy $U$ and ...

3

So, why there is a concept of potential energy when it can be understood as the field is doing the work and supplying the kinetic energy rather than potential energy is getting converted to kinetic energy? It is perfectly possible to describe the situation in term of work done by the field supplying kinetic energy. That's the content of work-energy theorem. ...

3

Gabriel Golfetti's clear answer stresses the mathematical equivalence of the two extremum principles. Probably it could be helpful to discuss more in details the physical meaning of the two principles. First of all, let's make explicit which variables the two functions (entropy $S$ and internal energy $U$) are functions of. Maximum entropy principle ...

3

If there was a superdense particle of mass M/2 ...... the superdense material falls down so as to keep the centre of mass at rest. Yes it does. Newton's law of gravitation states that : $$\mathbf F_{12}=-\mathbf F_{21}=G \frac {m_1m_2}{r^2} \hat r$$ Therefore the planet of mass $M$ moves towards the superdense particle with acceleration $a$ and the ...

3

For this subject I can recommend the following book by P W Atkins 'The second law' (1984) That book is written to be accessible to a large audience. Let me first describe a particular demonstration that is in that book. Take a grid of cells, 5 by 10 is large enough. Place a colored marker on the cells of a 5 by 5 square at one end of the grid, and a ...

3

In a plain old Newtonian context, there are some things in your question that show some incorrect understanding. Kinetic energy isn't conserved, it's total energy that's conserved. It's not true in general, for motion under the influence of a central force, that conservation of energy forbids a collision with the origin. That depends on how the force varies ...

3

The reason you can't do that is as follows, which really doesn't have anything directly to do with energy conservation. We apply a voltage to the wire, which pushes current through it which then produces a magnetic field that acts against the magnetic field produced by the permanent magnet, then that reaction force acts on the wire, causing it to rotate. ...

2

As long as the object is outside of the event horizon it will not lose kinetic energy (disregarding any gravitational wave phenomena). You can think of this case as an elastic scattering of two bodies. If the object falls inside the event horizon, it will become a part of black hole and will not go out. Let's assume that the object has no charge and falls ...

2

Well as Steeven answered, the force just exists there and is how the Universe behaves regardless of our intervention with the systems. But I believe a more intuitive answer to your question might be provided by General Relativity which eliminates this problem of the origin of gravitational force. According to Einstein's General Theory of Relativity, Gravity ...

2

Yes, since positions $1$ and $3$ are the positions where the velocity of the pendulum is zero, these are the positions where all of the kinetic energy has been converted into potential energy. Thus, these are the positions where the pendulum achieves a maximum height, or, a maximum deviation from the equilibrium.

2

Your overall question is largely a pedagogical rather than physics one: you don't understand the concept of potential energy, and with regard to that issue, we can only make guesses as to what explanations will make things click for you. You are making a distinction between "the field is doing the work and supplying the kinetic energy" versus "potential ...

2

There are at least 2 lessons to be learned from OP's set-up: Noether's theorem is not necessarily about strict symmetry of the action. It is enough if the action has an (infinitesimal) quasi-symmetry, i.e. symmetry up to boundary terms. There's no free lunch. To prove energy conservation, one must use a non-trivial assumption: In this case, that the ...

2

Energy is conserved in inelastic collisions. Bulk kinetic energy is not conserved. The sources I learned from never introduced a "Law of conservation of Mechanical Energy". I assume it applies in a restricted mechanics where thermalization is disallowed and all energy must be expressed in terms of macroscopic coordinates. In that case the energy lost from ...

2

In most collisions heat is generated and/or work is done deforming material around the point of contact. (Consider the condition of a car after a collision with another car.)

2

The total energy you have is Kinetic + Potential. And for a spring system, the potential energy is minimum when the spring is not stretched or compressed. But since energy is conserved, when potential energy is zero, all the energy in the system is present as kinetic energy. So kinetic energy is maximum when the displacement from natural length of the spring ...

2

The reason one does not use permanent magnets alone for electromechanical devises, is because permanent magnets would rapidly lose their magnetization if energy is taken from them. The law of conservation of energy is always at work, and the potential energy turning into kinetic energy will deplete the magnetization of the permanent magnet. Permanent ...

1

In a perfectly elastic collision (which is not possible in a macroscopic setting), in an ideal situation (since in reality B would have to move at least somewhat), then A would rebound with the same kinetic energy it had before. In reality, A and B would both have some kinetic energy after collision, and some kinetic energy would be lost as other forms of ...

1

When we talk about a false vacuum decay we mean there is some field that is stuck in a metastable state and then decays to its ground state i.e. the true vacuum. The energy of the field in the metastable state cannot just disappea,r so when the field escapes from the metastable state it will initially have the same energy density. We expect the field to ...

1

Let's assume you are on a surface with friction adjacent to the frictionless surface of the on coming object. The object makes contact with you and you exert a force opposite to the direction of the moving object in order to bring it to a stop. According to the work energy theorem the net work done on an object equals its change in kinetic energy. Or, in ...

1

All energy is conserved, but it is not all converted to kinetic energy. Some of the kinetic energy is conserved, some is converted to heat energy. If you apply a force to a moving object, it will apply an equal but opposite force on you. If you were on the same frictionless surface as the object you pushed on, then you would move in the opposite direction ...

1

No the energy is already there in the system (in case of asteroids, planets,etc.) or is provided by you or some external agency (like evaporation of water via rays from sun, lifting a ball from the ground,etc.)

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One definition of energy is the capability to perform work. But it only applies to entire systems and interactions therein (energy is only useful because there’s the conservation law, and that one only applies to systems). According to the principles of magnetism, the two wires will attract each other, now if I take the violet wire far away (all I mean is ...

1

I really wanted to make this a comment on lesnik's insightful answer, but I don't have enough reputation to do that, so I am just going to elaborate on my thought. When you say "why do we say that potential energy converts into kinetic energy when it is the force(the gravitational force) which is supplying the kinetic energy?" I think you are noticing the ...

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