gPE個 definition係咁:
the gPE of the mass m which is at a position with distance r from M(fixed point) is defined as"the work done by an external force required in moving the mass m from infinity to r slowly "
明明個m都已經距離M r咁遠,點解仲要有external force去move 佢from infinity to r slowly?仲乜會有infinity 的出現?
請回答詳細d,,Thank you!
有關gravitational potential energy
2008-12-05 5:54 am
回答 (2)
2008-12-05 7:33 am
It is becasue there is a reference points at infinity, where gPE is 0 . When two mass is at infinity far, they have no influence to each other and the gPE is 0.
The external force is the graviational force, F , governed by Newten's law of universal gravitation F = G mM/R^2 where G is the gravitational constant, R is the distance between two Mass in the system.
gPE= - GmM/r , the -ve sign indicate that gPE is always less when m get closer to M.
The work done to bring m to r from inifinty is -ve indicated that it done need to input enought to do it, actually it give out energy.
Then, where is the energy? During the process, m accerlates to certain speed and consists Kinetic energy.
2008-12-05 12:04:51 補充：
" -ve indicates that is don't need to input energy to do it "
The external force is the graviational force, F , governed by Newten's law of universal gravitation F = G mM/R^2 where G is the gravitational constant, R is the distance between two Mass in the system.
gPE= - GmM/r , the -ve sign indicate that gPE is always less when m get closer to M.
The work done to bring m to r from inifinty is -ve indicated that it done need to input enought to do it, actually it give out energy.
Then, where is the energy? During the process, m accerlates to certain speed and consists Kinetic energy.
2008-12-05 12:04:51 補充：
" -ve indicates that is don't need to input energy to do it "
2008-12-05 6:23 am
因為其實mgh=mgh1-mgh2,
除非oject係正地深吸力來源既中心.
其實佢咁樣講係想講拎起oject既力*距離地面既距離=work done.(W=FS)
你拉得愈遠mgh就愈大.infinity姐係無限遠.
除非oject係正地深吸力來源既中心.
其實佢咁樣講係想講拎起oject既力*距離地面既距離=work done.(W=FS)
你拉得愈遠mgh就愈大.infinity姐係無限遠.
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