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? ${{}}$
 A: 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.
A: 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.
A: 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).
A: 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.
A: 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.
A: 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.
A: 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.
