Wikipedia:Reference desk/Archives/Science/2023 August 31

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August 31

Energy and gravitation

When a body gains kinetic energy through gravitational attraction, where does it come from? Malypaet (talk) 18:51, 31 August 2023 (UTC)

The energy comes from the conversion of gravitational potential energy to kinetic energy; the kinetic energy gained by the object is equal to the amount of potential energy lost. 136.54.106.120 (talk) 19:55, 31 August 2023 (UTC)

Lost to whom?

When gravity increases potential energy increases, so gravity is a form of energy? Malypaet (talk) 20:22, 31 August 2023 (UTC)

Gravity is a force. A force can impart energy. The energy comes FROM the force of gravity between bodies with mass.

The potential energy is a measure of the available energy from the gravitational field, so when an object gains kinetic energy from gravity, it loses the same amount of gravitational potential energy. PianoDan (talk) 20:59, 31 August 2023 (UTC)

The term "gravity" is not used by physicists to refer to a form of energy.

Our scientific understanding of natural phenomena is embodied in theories that give mathematical models for such phenomena, expressed as equations relating physical quantities. One of these laws can be expressed as

${\frac {\operatorname {d} }{\operatorname {d} \!t}}(E_{\text{kin}}+E_{\text{pot}})=0.$

It follows from this law that

${\frac {\operatorname {d} }{\operatorname {d} \!t}}E_{\text{kin}}=-{\frac {\operatorname {d} }{\operatorname {d} \!t}}E_{\text{pot}}.$

It is common to talk about the phenomena as described by these models using everyday words, such as gain and loss for an increase or decrease in time. It is meaningless to try to extend this beyond the immediate physical context and ask "Lost to whom"?  --Lambiam 11:54, 1 September 2023 (UTC)

• Try to think of it this way. Energy is a human defined term, it's an invention, wholly created out the mind and creativity of people, as a means of quantifying change; it isn't a substance or a thing or a material or anything like that; it's simply a number we use to quantify how changes happen over time. Energy is defined as the ability to do work, work = force * distance; thus one of the ways we can define changes in energy is that it can result from one of two things: a change of position (i.e. it can be moving over time) or a change in force (the forces exerted on an object can change). The part of an object's energy derived from its change in position over time is called kinetic energy and the part of an object's energy derived from the changes in the forces acting on an object is called potential energy. When an object is in a gravitational field, there are forces on the object, so when you move the object within that field (like lifting it up off the ground) you change its gravitational potential energy. Where does that energy come from? Well, from the kinetic energy of your arm moving through space. Where did your arm get that energy? From the chemical potential energy stored in the chemical bonds of adenosine triphosphate in your muscle cells. Where did that energy come from? Well, it came from the chemical potential energy of glucose in your blood? Where did... well, you get the idea. The infinite chain of energy conversions is expressed as the First law of thermodynamics, which is also called the law of conservation of energy; the idea that energy is a conserved quantity, and the universal value of energy remains constant. I hope that all makes sense.--Jayron32 12:06, 1 September 2023 (UTC)

  If you want to hold an object 1m from the ground you must apply an equal and opposite force to it. For example with a propulsion engine, which will consume an energy flow in Watts (Joules /s). How does the second law of thermodynamics come in here? Malypaet (talk) 20:52, 1 September 2023 (UTC)

  Sorry, first law ! Malypaet (talk) 20:53, 1 September 2023 (UTC)

  Applying a force to a stationary object does not require energy. The energy, also called work, is the product of force and displacement; in a formula,

  $W=Fs.$

  So if $s=0\,{\text{m}},$ $W=0\,{\text{J}}.$

  Using one's muscles to hold up a heavy object, one needs to burn calories to maintain the tension of the muscles; how much depends on the anatomical and physiological details of the specific organism, which are not described by the laws of physics. If the organism can rest the object on its head, shoulders or back, much less caloric energy needs to be expended.  --Lambiam 16:35, 2 September 2023 (UTC)

  But if you keep this object stationary in space with a rocket or a drone, to counter the gravitational attraction, the rocket or the drone will consume energy, precisely power. Here it is not a question of applying a force to overcome the inertia of a mass as in the definition of Joule for example, but to counter the force of attraction. In the first case we cause a movement, while in the second we prevent a movement. Malypaet (talk) 21:41, 2 September 2023 (UTC)

A drone spends a lot of energy making wind. PiusImpavidus (talk) 09:06, 3 September 2023 (UTC)

The energy consumed by the rocket motor burning fuel goes into the kinetic energy of the exhaust gases. The rocket does not perform work on the stationary object.  --Lambiam 10:24, 3 September 2023 (UTC)

Here the notion of work is ambiguous. If I take as a frame of reference an object in free fall and passing close to the rocket, he will see the force exerted by the rocket doing work equal to the potential gravitational energy, right ?

What I understand is that the closer a mass is to another, the more it gains, either in potential energy or in kinetic energy. According to the principle of conservation of energy, this acquired energy must come from somewhere? Or is there another way to understand it? Malypaet (talk) 22:59, 3 September 2023 (UTC)

When an object is moved away from the source of a gravitational field, work is done to overcome the gravitational force, and its potential energy increases. When the object is allowed to fall towards the source, the potential energy decreases and the kinetic energy increases, so that the total energy remains constant. It's very similar to a spring. When you compress a spring, you do work on it and its potential energy increases. When the spring is released and returns to its original shape, the potential energy decreases and the kinetic energy increase. I think you're confusing yourself by viewing the original situation as an object far away from a gravitational source, and asking where the kinetic energy comes from when it falls. The energy comes from whatever process moved it away from the source in the first place. If the objects were "always" separated, then it originally came from the Big Bang which put them in that separated state. CodeTalker (talk) 03:08, 4 September 2023 (UTC)

"the Big Bang", thank's. Malypaet (talk) 16:32, 4 September 2023 (UTC)

It comes from the energy of the gravitational field, which is by the way negative. Ruslik_Zero 19:57, 1 September 2023 (UTC)

@Ruslik0: Not necessarily. Potential energy of a mass in a gravitational field being negative is not an absolute truth, but rather a result of a conventional assumption that the energy in infinity is zero. Even though it is useful, it is just a convention. If one assumes the zero level at the ground surface, the potential energy above the ground would be positive. --CiaPan (talk) 20:30, 4 September 2023 (UTC)

Per CiaPan, the absolute energy of any system is not a definable quantity, but the change in energy is. One can arbitrarily define a "zero point" for the purpose of calculations, for example defining "the ground" as zero gravitational potential energy, but that's just a calculational convenience. It doesn't actually matter what you define as "zero" however, as all one needs to calculate is the changes in energy (ΔE) which doesn't depend on where zero is placed. --Jayron32 11:58, 5 September 2023 (UTC)

The problem is that the gravitational energy in the classical mechanics is not bounded from the below – it can become infinitely negative. So, you cannot fix this by simply redefining zero point. Ruslik_Zero 13:20, 5 September 2023 (UTC)

How so? --Jayron32 18:04, 5 September 2023 (UTC)

The potential energy becomes infinitely negative if the distance between two masses becomes infinitely small. Ruslik_Zero 20:55, 6 September 2023 (UTC)

That only tells you that the potential energy must be negative somewhere (and only if you're looking at point masses), not that it must be negative everywhere. Still, putting the potential to zero at infinity is convenient because then sign of the total energy $E_{\mathrm {tot} }=E_{\mathrm {kin} }+E_{\mathrm {pot} }$ tells you whether the system is bound (distance always less than some maximum distance) or unbound (distance can be arbitrarily large). --Wrongfilter (talk) 22:19, 6 September 2023 (UTC)

This is all fine, but my point was that the negativity of the gravitational energy has some quite real physical consequences, e.g. the inherent instability of gravitating systems. In this respect the gravity is different from for example electrostatics where the energy can be made always positive. Ruslik_Zero 20:41, 7 September 2023 (UTC)

Oh, the negative heat capacity of gravitating systems? --Wrongfilter (talk) 20:48, 7 September 2023 (UTC)