Thursday 7 May 2020

newtonian gravity - Systems and Gravational PE


Can an object that is part of a system of objects have gravitational potential energy if the Earth is not part of the said system? For example say i have an incline and a block near the surface of the earth. If the block is at the top of the incline and i define my system to be the block and the incline, then does the block have any gravitational PE?


Sorry if this is a stupid question.




Answer



Gravitational potential energy is not "inside an object". It is shared between two objects that attract each other. It is stored in the system or field between them.


If you define your system as one of these objects alone, then it get's a bit difficult. Your system will not be isolated. Then you cannot rely on e.g. the energy conservation law, because you essentially are allowing energy to exit and enter the system; as you say, removing Earth will severally change the system you have defined, since the external force changes. And without this force there is no tendency of the object to move, thus no stored energy.


So, defining your system as only a part of a gravitational potential energy system is best to avoid.


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