Why doesn't energy exhaust itself?

Why doesn't energy exhaust itself over time? For example, isn't kinetic energy required for an object to move, but won't the kinetic energy decrease as the object moves since it uses that energy? $${{}}$$

• Jun 25, 2021 at 17:38
• Be careful not to confuse energy in general with the specific case of energy as fuel. Yes, you get tired over time because your body spends energy. Yes, a fuel engine stops eventually because it expend fuel. But these do not represent all types of energy involvement. Jun 26, 2021 at 10:03
• Because Aristotle was wrong, impetus isn't how energy works. :P Jun 28, 2021 at 0:24
• Too small of for an answer, but energy is something one has while moving, not something used up in order to move. This gets confusing because most things deal with friction, and friction does consume energy of the object while it is moving. Jun 28, 2021 at 2:50
• You seem to have mixed up fuel with energy. Jun 28, 2021 at 11:08

Kinetic energy is a property of moving objects, not a thing that moving objects consume. A moving object can no more exhaust its kinetic energy by moving than a red object can exhaust its redness by being red.

• This answer could be much more useful if you mentioned friction and heat. Jun 26, 2021 at 8:57
• @EricDuminil I agree, although there is also merit in conciseness. Jun 26, 2021 at 11:36
• The point is that you don't exhaust the KE just by the process by moving alone. Friction does not change this statement. If work is done on the object then the KE may change. Same as exposure to light, radiation or chemicals may alter the red color. But this is besides the point.
– nasu
Jun 26, 2021 at 15:36
• (pedantically, red objects do in fact lose their redness by being red as being red means they absorb more of the shorter, more energetic wavelengths of light, destroying their pigment faster than other colours do) Jun 26, 2021 at 18:48
• @PeteKirkham that raises an interesting question - is the object red if no light hits it? (In which case, it wouldn’t lose its pigment).
– Tim
Jun 26, 2021 at 20:39

Energy is conserved, that is not created or destroyed but only transfered between forms.

It requires energy to accelerate an object, this is called doing work on the object. That energy is transfered to kinetic energy. The energy is stored in the objects speed, it is not that the object needs to use the energy to keep moving.

However, in real life there are frictional forces that act to slow an object down. The forces dissipate the energy of the object as heat and so to maintain a constant speed more work must be done on the object.

Energy is only useful when we can use it to do work, e.g. accelerate an object. To do work we must have a store of energy that is higher in energy than the surroundings. The dissipation of energy over time is linked to the increase in entropy and so it would be true to say the amount of useful energy decreases with time.

Think about what happens to the depleted kinetic energy. If the law of conservation of energy holds true, it must go somewhere, right? In most cases, friction causes the object to lose its kinetic energy in the form of heat. However, if there is no friction, or some other way for the object to lose its kinetic energy, the kinetic energy will not decrease. For example, a rocket in deep space does not lose its kinetic energy and continues to move on forever.

One analogy that I've found to help with this comes from Alice in Quantumland, which compares energy to money.

Money comes in different forms: Cash, savings in bank accounts, shares, etc. When we buy something with money, the money is not exhausted or destroyed, simply given to someone else. In general we can say that a person has a certain amount of money, and we can say how much of it is in each different form. A person can transform some money from one type to another by withdrawing money from a bank account or selling some shares.

Likewise with energy a system has a given total energy in different forms: heat, potential energy, kinetic energy, etc. Energy can be transformed between these forms within a system and the total remains the same. e.g When an object slides down a slope, potential energy is transformed into kinetic energy, but the total remains the same. In theory an object in motion with no external forces continues in motion at the same speed, and has the same kinetic energy with no losses (Newton's First Law), but in the real world moving objects are nearly always subject to friction, which causes some of the kinetic energy to transform into heat.

You can observe this in a way quite simply. If you wave your hand though the air, it gets a certain speed. If you let it swipe across your other hand, you will notice it slows down and your hands get hot. This is a transfer of kinetic energy to heat energy through friction. (apologies if I haven't described the experiment very well).

• the analogy works better than you think. Money is inflationary since many governments like to print it out to fund their fiscal deficits. The universe is not that capricious at our time scales, but even the universe inflates and breaks time translation symmetry at cosmological scales Jun 26, 2021 at 15:51
• @lurscher Not... really. When a government prints new money, it doesn't create more value out of nothing - it decreases the value of all the other money. The total value is unchanged, only the distribution changes. Cosmic inflation might or might not be conservative in the same sense. Mind, neither value nor money are conserved quantities anyway. The analogy works fine from the point of view of individuals, just don't bring governments and banks into it :) Jun 26, 2021 at 16:57
• "it doesn't create more value out of nothing" indeed @Luaan, but the analogy was not between energy and value, but between energy and money Jun 26, 2021 at 17:53
• "a system has a given total energy in different forms: heat, potential energy, kinetic energy, etc." No. A system cannot "have heat". You're talking about the internal energy. Jun 26, 2021 at 20:13
• Does this mean that there is always a small part of our energy lost down the back of the sofa?
– Grim
Jun 27, 2021 at 15:24

Remember when we say, some system loses or exhausts energy, essentially we mean that system is doing some positive work on some other system, such that, the amount of work done is the amount of energy transferred to the other system.

Thus for an object to lose out some of its kinetic energy, it has to do some positive work on something, in most of the cases, it does to overcome friction.

• You should mention heat in the first paragraph. And as written, I really don't think the second paragraph is correct. Jun 26, 2021 at 8:55

It does.

The laws of thermodynamics basically state that any interaction between objects results in a portion of the energy of that interaction being wasted as waste heat. As a result, the amount of usable energy is the universe is continuously declining, while the amount of waste heat is slowly climbing. The point at which all of the energy of the universe has been converted into waste heat is referred to as the "heat death of the universe".

Fortunately, the rate at which this occurs is very small, and the universe has a lot of energy in it, so this won't occur for billions and billions of years.

Due to symmetry by translation in time, energy is conservated. May I remind, from Noether, that energy is not a thing, it's a quantity.

But the intuition we build inside ourself on Earth, is that the energy is used in a certain way. Actually the energy is not consumed but its nature changes.

The energy of an isolated system is conservated. If your moving object is isolated, the energy is conservated.

If the object is submitted to air resistance, fluid viscosity, then the energy is partly changed into fluid movement and fluid heating, and at the same time the speed changes. In $$\sum\vec F = m \vec a$$ on the left the forces are from the environment (fluids), and on the right, the object accelerates (negatively or positively). In your case (explicitly an isolated moving object), there is no force, hence no acceleration, hence constant kinetic energy.