A force pushing/pulling on an object causes it to move; a moving object has a certain amount of movement energy. Gravity causes things to move/fall. Therefore, we should be able to get energy from gravity. Yes! In fact we do. In older days water wheels were commonly use to grind grain. Nowadays, hydroelectric dams are significant sources of electrical energy. Water behind the dam flows through turbines driving generators.
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Hydroelectric dams images
Note however, that such water power is made possible because of the natural water cycle. Solar heating takes water vapor into the atmosphere from where it comes down as rain. Thus, the water upstream and behind the dam is constantly replenished by solar heating.
Numerous inventors over the ages have tried to develop a machine that is powered by the force of gravity alone. The basic idea of such machines is that gravity pulling weights down on one side should provide more than enough force to push the the weights up the other side. The excess force can drive a generator to produce “free energy” for lighting, motors, etc. The following are some of the ideas that have been tried.
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The problem with all such machines, is that when tested they don’t work. This brings us to and reinforces the conclusion that you can’t get more energy out of a system than has been put into it. Energy cannot be created; it must come from somewhere else. Finding absolutely no exceptions to this, we now take it as a Natural Law. Energy cannot be Created of Destroyed.
In all modern technology, it may be forces that actually move things, but we invariably find that there is an expenditure of energy behind generating those forces.