# Is the gravitational force higher than it should be?

• B

## Main Question or Discussion Point

does the mass on the surface of the planet (in state of rest ) has inside it the same gravitational energy to a similar mass after a free falling above its surface for one entire second?

## Answers and Replies

Related Other Physics Topics News on Phys.org
sophiecentaur
Science Advisor
Gold Member
Hi and welcome to PF
It's not clear exactly what you want to know but I can tell you that the Gravitational Energy just depends on the Position (height above the reference level) of the mass. If it has already fallen for a second, it will have gained Kinetic Energy and will have less Gravitational Potential Energy than where it started.
Ignoring energy lost by friction through the air, the sum of the GPE and the KE will be the same.

CWatters
Science Advisor
Homework Helper
Gold Member
does the mass on the surface of the planet (in state of rest ) has inside it the same gravitational energy...
Gravitational potential energy is relative so it depends where your reference is. If your reference is the surface of the earth then it will have zero gravitational potential energy. If your reference is the centre of the earth it will be non zero.

..to a similar mass after a free falling above its surface for one entire second?
While it falls its gravitational potential energy will reduce. The absolute value at any time depends on its height at that time. It will only have the same gravitational potential energy when it's also on the surface.

If the reference is the surface of the Earth, the mass will have zero gravitational energy ? so how the mass on the surface of the earth cause pressure to the interior? it must have the energy to cause pressure.

phinds
Science Advisor
Gold Member
2019 Award
If the reference is the surface of the Earth, the mass will have zero gravitational energy ? so how the mass on the surface of the earth cause pressure to the interior? it must have the energy to cause pressure.
Well, first you are taking the reference point to be the surface of the Earth and are correctly stating that it has zero potential energy relative to that position, THEN you are taking the reference point as being the center of the Earth and you are complaining that it has potential energy relative to THAT reference point, which of course it does. You need to make up your mind what your reference point is.

Last edited:
CWatters
Science Advisor
Homework Helper
Gold Member
If the reference is the surface of the Earth, the mass will have zero gravitational energy ? so how the mass on the surface of the earth cause pressure to the interior? it must have the energy to cause pressure.
I said that on the surface it has zero gravitational potential _energy_ relative to the surface. The gravitational _force_ is not zero.

Work (or energy) = force * distance

The distance between the surface and the surface is zero (obviously).

The force is not zero.

CWatters
Science Advisor
Homework Helper
Gold Member
If the reference was a cloud at 1000m the object on the surface would have negative gravitational potential energy.

LURCH
Science Advisor
There may be a bit of a language barrier at work here. I notice that the OP asked about “gravitational energy “, and all responses have been about “gravitational potential energy “. Nabil23, let me see if I understand your latest post correctly. You are observing that an object on the ground exerts pressure against the ground, and you are asking how it can exert a force if it has no energy, and the source of that energy is apparently gravity, right?

So the main questions to ask would be, “does an object with weight exert a force against the ground beneath it?”, “is energy required in order to exert a force?”, and “if so, what is the source of that energy?”. Does that sound right to you?