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Gravity always seemed a bit off to me, even on macro-scale.

How can a resting object like earth have so much (infinite) energy to attract objects?

Does this energy ever fades away, assuming no other external factors?

In a practical example, if I want to fly a drone, why does the drone need to be constantly battling gravity? If my drone spends, let's say, 1000kJ to fly for 10 minutes, earth will spend the same amount of energy to attract it, right?

Where does that energy come from? If gravity is not "free" energy, why do we need an infinite amount of energy to keep an object floating above the ground for an infinite amount of time?

Qmechanic
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PedroD
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    Just a hint: a chandelier hanging down from the ceiling does not fall down for a long period of time, but it doesn't need an energy supply for that. – DoeJohn Mar 24 '20 at 08:53
  • Are you familiar with the concept of work done or the dot product of two vectors? – bemjanim Mar 24 '20 at 08:55
  • Related (kind of the inverse question): https://physics.stackexchange.com/q/1984/50583 – ACuriousMind Mar 24 '20 at 16:20

2 Answers2

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In physics, you only need energy if you have a force and a displacement. Just exerting a force alone does not require energy. This comes from the definition of work. Therefore gravity is not free energy; it's just a force.

Your drone does not need to spend energy battling gravity; it needs to spend energy to propel air molecules downwards because that's how it generates lift (it cannot fly in a vacuum). If you have some other form of repulsive force, then you can levitate without needing energy. See e.g. magnetic levitation.

Allure
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  • "Just exerting a force alone does not require energy." ---> THIS, is the thing that I cannot wrap my mind around... How is this possible? How can I understand this? For me, I can only exert a force by spending energy (to create tension, for eg. exerting a force against a wall, despite it not moving). – PedroD Mar 24 '20 at 10:47
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    @PedroD Consider instead: you are also exerting a force on your chair when you sit on it. How is it that you don't need energy to do that? Human experience is not how physics works. That said, when you exert a force against a wall, your muscles are doing internal work (this is in turn possible only because human bodies are not rigid bodies), hence it is tiring. – Allure Mar 24 '20 at 10:52
  • @PedroD, Re, "I can only exert a force by spending energy." That's a limitation of how your muscles work. You might think that you are exerting a constant force on some object, but at a microscopic level, your individual muscle fibers are continually firing and relaxing and firing again, continually consuming chemical energy the whole time. – Solomon Slow Mar 24 '20 at 12:11
  • @Allure "you are also exerting a force on your chair when you sit on it. How is it that you don't need energy to do that?" In my head this works because gravity is doing the work for me, the same way I can push a rigid block against a wall. The block is exerting a force against the wall, but I am the one spending energy to exert a force on the block. – PedroD Mar 24 '20 at 12:41
  • @SolomonSlow I still don't get it... If that's the case, than the energy I spend because I am soft and meaty, should not be spent if instead of me we have a machine (eg. hydraulic press). If forces do not require energy, in my mental model that would mean that hydraulic presses would be able to exert an infinite amount of pressure/force without spending any energy. What am I missing? This concept seems so assymetric. – PedroD Mar 24 '20 at 12:43
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    @PedroD first things first stop your mental model because it's wrong. I repeat, it only takes energy if there is movement to go with the force. Yes you can exert a near-infinite amount of pressure/force without spending any energy, but you would also need to a lot of work to get the hydraulic press to move. – Allure Mar 24 '20 at 12:50
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    @PedroD the hydraulic press increases pressure only if it moves at least a little - it needs to squeeze the object more to produce higher pressure Then you have force applied over some distance which requires energy. The movement is hard to see for usual materials, but if the hydraulic press would act on a balloon full of air, then you could easily see that pressure rises only if the balloon gets compressed more. – Umaxo Mar 24 '20 at 13:26
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    @Allure "In physics, you only need energy if you have a force and a displacement". Minor point, but energy transfer can be in the form of heat as well as work (force x displacement). – Bob D Mar 24 '20 at 16:14
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How can a resting object like earth have so much (infinite) energy to attract objects?

Does this energy ever fades away, assuming no other external factors?

As @Alure explained to you, gravity is a force not energy. Energy can be stored in and retrieved from the gravitational field. When you raise an object the energy you expend is stored as gravitational potential energy. When the object drops gravity does work and the same energy is extracted from the gravitational field. For example, when your drone went from the ground to the height where it hovers, work was done to raise it and give it gravitational potential energy.

In a practical example, if I want to fly a drone, why does the drone need to be constantly battling gravity? If my drone spends, let's say, 1000kJ to fly for 10 minutes, earth will spend the same amount of energy to attract it, right?

The drone is constantly battling gravity in the sense that the force of gravity is trying to pull it down and the drone requires an equal upward force to keep it in the air. But the Earth expends no energy to attract it because it is doing no work (downward displacement of the drone) as long as the drone hovers. The Earth only does work if the drone falls in which case the work done by gravity will equal the loss in gravitational potential energy of the drone-Earth system.

So why does the drone need to continuously expend energy to keep it hovering?

Again as @Alure points out the drone does work (uses energy) to exert a force that propels air molecules downward (Work = Force x displacement of the air). According to Newton's third law the air molecules will then exert an equal and opposite (upward) force on the drone. If the upward force the air exerts on the drone equals the downward gravitational force, the drone hovers. If it exceeds it, it rises. If it is less, it falls.

Hope this helps.

Bob D
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