Gravitaional potential energy/ absoute potential energy Watch

tom.draper95
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Im currently studying in year 13 A2 level physics, and need extra info on a certain topic.

Would I be able to get a definition of absolute potential energy, and maybe a brief explanation of why at infinite distance, this absolute potential is supposed to be equal to zero and how it becomes negatively larger as an object moves towards the Earth, for example. I thought abit of outside explanation might make me look at it from a different perspective.

Thank you.
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Stonebridge
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(Original post by tom.draper95)
Im currently studying in year 13 A2 level physics, and need extra info on a certain topic.

Would I be able to get a definition of absolute potential energy, and maybe a brief explanation of why at infinite distance, this absolute potential is supposed to be equal to zero and how it becomes negatively larger as an object moves towards the Earth, for example. I thought abit of outside explanation might make me look at it from a different perspective.

Thank you.

Welcome to TSR and thanks for the interesting question.

Potential energy is defined as being zero at infinity.

In physics, energy is something that can move from place to place and be transformed to other types. eg kinetic, heat, light etc.
In many cases, it's the change in energy, the amount of energy that has been transformed or moved, that is important; not the absolute amount.

Potential energy in a force field is something that changes if the object moves. This potential energy will be converted into some other form. For example, it is converted into kinetic energy when an object "falls".
What's actually important is the transfer or change in energy.
You could actually put the zero of potential energy anywhere you wanted, but it seems logical to place it at infinity where the force is also zero. (This is the case for the inverse square law).
It's by definition that the potential energy an object has in a force field is equal to the work done moving it there from infinity. (Where it was zero)
In an attractive field (eg gravitational field or two opposite charges) you actually need to do work to move the object away to infinity. If you define the zero at infinity, and have to give the object energy to get it there, then its potential energy has to be negative to begin with.
This is all just a consequence of how potential energy is defined in physics.
In a repulsive field you have to do work to move the object from infinity.

Potential energy has been "invented" in physics to enable the law of conservation of energy to work in a force field. You have to do work (against the field) to move the object. If you do work you transfer energy. Energy is transferred between kinetic and potential. The result is energy is conserved.
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tom.draper95
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(Original post by Stonebridge)
Welcome to TSR and thanks for the interesting question.

Potential energy is defined as being zero at infinity.

In physics, energy is something that can move from place to place and be transformed to other types. eg kinetic, heat, light etc.
In many cases, it's the change in energy, the amount of energy that has been transformed or moved, that is important; not the absolute amount.

Potential energy in a force field is something that changes if the object moves. This potential energy will be converted into some other form. For example, it is converted into kinetic energy when an object "falls".
What's actually important is the transfer or change in energy.
You could actually put the zero of potential energy anywhere you wanted, but it seems logical to place it at infinity where the force is also zero. (This is the case for the inverse square law).
It's by definition that the potential energy an object has in a force field is equal to the work done moving it there from infinity. (Where it was zero)
In an attractive field (eg gravitational field or two opposite charges) you actually need to do work to move the object away to infinity. If you define the zero at infinity, and have to give the object energy to get it there, then its potential energy has to be negative to begin with.
This is all just a consequence of how potential energy is defined in physics.
In a repulsive field you have to do work to move the object from infinity.

Potential energy has been "invented" in physics to enable the law of conservation of energy to work in a force field. You have to do work (against the field) to move the object. If you do work you transfer energy. Energy is transferred between kinetic and potential. The result is energy is conserved.
Thanks for the reply, i appreciate it, youve given me alot more information to work with now than the book im working from.

A few questions to ask:

  • If gravitaional potential energy increases with increasing distance from Earth, will it reach infinity at infinite distance?
  • If the gravitational field strength is zero at infinite distance from the Earth, then surely it tends towards infinity as you reach the centre of the Earth?
  • When g is zero at infinite distance, and it will get negatively larger as it moves from infinity towards the Earth, is the value -9.81N/kg at the surface of the Earth.

Sorry if these questions are wierd, but i just need clarification on a few things so i can understand it. The concept is a bit difficult.
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Stonebridge
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(Original post by tom.draper95)
Thanks for the reply, i appreciate it, youve given me alot more information to work with now than the book im working from.

A few questions to ask:

  • If gravitaional potential energy increases with increasing distance from Earth, will it reach infinity at infinite distance?
  • If the gravitational field strength is zero at infinite distance from the Earth, then surely it tends towards infinity as you reach the centre of the Earth?
  • When g is zero at infinite distance, and it will get negatively larger as it moves from infinity towards the Earth, is the value -9.81N/kg at the surface of the Earth.

Sorry if these questions are wierd, but i just need clarification on a few things so i can understand it. The concept is a bit difficult.
1) No, it reaches zero at an infinite distance. It's defined as being zero there.
It is negative near the Earth. As you give it energy to move it away from earth you increase its PE to zero. This is the case for an attractive field.
For a repulsive field the PE reduces to zero at infinity.
2) Yes. The equation has the potential energy depending on 1/r, where r is the distance from the centre. This would make it infinity when r = 0. (And zero when r=infinity) However, inside the Earth things are different and the 1/r formula no longer applies.
3) "g" is a measure of gravitational field strength (force per unit mass) not potential energy. It's important not to confuse these two, though they are related.
But yes, g is zero at infinity. The value can be - or + 9.8 depending on which direction you take as positive. If you take distance as being positive measured moving away from the Earth, then g is in the direction pointing towards the Earth so would be negative. However, as I said before, for calculation purposes using g in formulas (eg SUVAT equations), you decide at the start which direction is positive.
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tom.draper95
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Brilliant thankyou, this has really helped me understand the concept. So PE starts negative at the surface of the Earth, and increases to zero at infinity. So what value does it have at the surface of the earth? or does it have a value? because there shouldnt be any potential energy at the surface of the Earth?
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Stonebridge
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(Original post by tom.draper95)
Brilliant thankyou, this has really helped me understand the concept. So PE starts negative at the surface of the Earth, and increases to zero at infinity. So what value does it have at the surface of the earth? or does it have a value? because there shouldnt be any potential energy at the surface of the Earth?
You should have the formula for gravitational PE in your books somewhere.
PE = -GM/r
Where M is the mass of the Earth and r is the distance from the centre to the
place in question. So if you put r = radius of Earth you get the potential on the surface. This is the "absolute" value taking the zero at r= infinity.
When you do calculations using PE = mgh, this only applies "locally" on the Earth's surface where the field is more or less uniform. In these cases you are only interested in changes in PE. In these questions we arbitrarily set the zero of PE on the surface.
Using PE = mgh is actually a simplification that only works over a small region near the Earth's surface where g doesn't change significantly. If you move too far upwards g will get less and the formula doesn't work. You need to see the difference between simplified "mgh" calculations near the surface where you can put the zero of PE at the surface (or anywhere else near it) and g is constant; and more general calculations using PE = -GM/r where g varies with distance and the zero is placed at infinity.
You will no doubt study this in due course.
Remember, in most cases it's the change in PE that you are interested in, not an absolute value.
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zubairrashid997
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Thankyou very much!
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