Jimmy20002012
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I am a bit confused on how gravitational potential works, I know that it is the work done to move a mass from infinity to that point. But I don't seem to get why gravitational potential increases as you move away from earth?


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Stonebridge
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(Original post by Jimmy20002012)
I am a bit confused on how gravitational potential works, I know that it is the work done to move a mass from infinity to that point. But I don't seem to get why gravitational potential increases as you move away from earth?


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Because you actually have to do work to move an object away from the Earth. This is because the force is attractive. If you do work against the force you increase the object's potential energy.

If the force was repulsive you would have to do work to move the object nearer the Earth. Then the opposite would be true. GP would increase as you got nearer the Earth.
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Kallisto
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(Original post by Jimmy20002012)
(...) But I don't seem to get why gravitational potential increases as you move away from earth?
The gravitational potential decreases the more a mass, a spaceship, is approaching the 'edge' of the gravitational potential to leave the earth. Its quasi the field of gravitation in which the gravitation of the earth works to every single mass. Thus a spaceship needs a certain escape speed to overcome this gravitational potential. In a certain distance to the earth, the gravitational potential cannot or hardly influence the mass of the spaceship. It increases when the spaceship is approaching the earth to come back.
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Stonebridge
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(Original post by Kallisto)
The gravitational potential decreases the more a mass, a spaceship, is approaching the 'edge' of the gravitational potential to leave the earth. Its quasi the field of gravitation in which the gravitation of the earth works to every single mass. Thus a spaceship needs a certain escape speed to overcome this gravitational potential. In a certain distance to the earth, the gravitational potential cannot or hardly influence the mass of the spaceship. It increases when the spaceship is approaching the earth to come back.
No, potential increases as you move away and decreases as you move nearer in a gravitational field.
It's zero at infinity and negative as you get nearer the mass.
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Kallisto
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(Original post by Stonebridge)
No, potential increases as you move away and decreases as you move nearer in a gravitational field.
It's zero at infinity and negative as you get nearer the mass.
Just to close some contradictions of myself: Say there is a spaceship which is trying to leave the earth and this gravitational field. By trying to leave potential, it increases as the spaceship achieves higher spheres in the meantime in which the potential is higher. So the increase of gravitational potential is caused by equipotential lines, where the zero potential is at the ground of the earth and the highest one is at the edge of the gravitational potential. That makes sense in my opinion. So far so good.

But when the edge is exceeded, does the gravitational potential not decrease in certain distances? or does the gravitational potential with the equipotential lines work up to a certain limit?
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Stonebridge
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(Original post by Kallisto)
Just to close some contradictions of myself: Say there is a spaceship which is trying to leave the earth and this gravitational field. By trying to leave potential, it increases as the spaceship achieves higher spheres in the meantime in which the potential is higher. So the increase of gravitational potential is caused by equipotential lines, where the zero potential is at the ground of the earth and the highest one is at the edge of the gravitational potential. That makes sense in my opinion. So far so good.

But when the edge is exceeded, does the gravitational potential not decrease in certain distances? or does the gravitational potential with the equipotential lines work up to a certain limit?
I think you may be mixing up a couple of different ideas.
First.
Gravitational potential and gravitational potential energy.
Both are about energy. Potential refers to the potential energy of a unit (1kg) mass.
Potential energy at a point is defined as the work done bringing a mass from infinity to that point. The convention is that if you have to do work to do that (the force is repulsive) then this work is positive. You, using your force, transfer energy to the mass and it gains energy. Similarly, if the field does the work for you (the force is attractive) then the work you do is negative and the object loses energy.
The convention is also that the value of the potential energy is zero at infinity where the force is also zero.
So in the case of gravitation you have to do positive work (you need to input energy) to move an object away from the central mass. This means you give (potential) energy to the mass as you move it away.
So moving away means an increase in potential energy. Moving closer means a decrease.

Equipotentials are lines drawn to help you visualise a field. They just show places where the potential is the same. For a spherical mass, they would be concentric circles going around the mass. Nothing is "caused by" equipotential lines. They are just imaginary lines drawn in space.

Near the surface of the Earth is a special case. You are confusing this with the general case I described above. Near the earth, if you don't go too far away, the gravitational field is very nearly uniform. For this reason you can use the simple formula mgh to calculate potential energy. This only works where g is constant, near the surface.
As you are normally only interested in changes in potential energy in these cases (near Earth) you can put the zero of potential energy at any convenient point. This is usually taken as ground level but doesn't need to be. It can be anywhere. Then changes in PE can be found simple from changes in h. Remember, this only works near the Earth's surface where the field is uniform.
There is no "edge". I'm not sure quite what you mean by this.
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Kallisto
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(Original post by Stonebridge)
(...)
There is no "edge". I'm not sure quite what you mean by this.
First thanks for your answer. You have brought light into darkness. And yes, I have mix some aspects with another ones up. The edge is just a word which I have used instead of Earth's surface. I did it to describe the influence of gravity in a gravity field of earth, and to explain where the (equi-)potential lines of the earth exist, not to explain the potential energy of a spaceship like you.
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