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`        Gravitational potential energy can be +ve , -ve or zero. Can anyone explain me this sentence with eg.? Also potential energy of a spring is always +ve. Is it right i mean we can stretch as well as can compress a sprinp.`
7 years ago

68 Points
```										Dear  RAjAT ,
The important point to understand is that potential energy is always  relative to some point.  What the potential energy is at  a point really  doesn't matter- it is the change between to points that is important.   We can always choose a reference point at which the potential energy is  0.  It happens that to be simplest, for gravitational problems, to take  potential energy to be 0 "at infinity".  Since potential energy  increases as you move away from the center of a gravitating system, the  potential energy is negative at any finite distance.  it should be pointed out that "gravitational potential energy  is negative" is only true for problems with distances large enough to  require the inverse square law.  Consider the problem, "A 1 kg mass is  dropped from the top of a 100 m cliff.  What is its speed just before it  hits the ground?"  A fairly standard way to do that is to say that the potential energy  is mgh= 1(9.8)(100)= 980 Joules (positive!) while the kinetic energy is 0  so the total energy is 980 Joules at the top of the building.  At the  bottom of the building the potential energy is 0 while the kinetic  energy is (1/2)v2 and so the total energy is (1/2)v2.  Neglecting air  resistance, by "conservation of energy" we have (1/2)mv2= 980 so v2=  1960 and v= 44.2 m/s.  We have, implicitely, taken the "reference"  point, for 0 potential energy, to be at the bottom of the building.  But we could just as easily take the "reference" point to be at the  top of the building.  Then, initially, we have both kinetic and  potential energy to be 0: the total energy is 0 Joules.  At the base of  the building, the potential energy is -mgh= -980 Joules.  Conservation  of energy now give (1/2)mv2- 980= 0 giving exactly the same answer.

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```
7 years ago
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