Does Every Object in the Universe Have Absolute Zero Potential Energy?

In summary: Therefore, the potential energy of the rock only depends on its height relative to the reference point chosen. The concept of zero potential energy on Earth's surface is just a convenient reference point, and it is relative to that reference point that potential energy is calculated. This also means that potential energy is a relative term and its value can be positive or negative, depending on the chosen reference point. In summary, Rosenblum and Kuttner discuss potential energy in their book "Quantum Enigma" and use the example of a rock held in the air to explain it. They state that when the rock hits the ground, it transfers all of its potential energy to the ground and has no remaining potential energy. The concept of zero potential energy on
  • #1
daisey
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I am now reading Rosenblum's and Kuttner's book "Quantum Enigma". In one of the first chapters they talk about the different forms of energy. Regarding Potential Energy, they give the example of a rock held in the air having potential energy because of the force of gravity. They go on to state that when rock hits the ground it transfers ALL of its potential energy to the ground, and at that point is has no remaining potential energy.

I am confused with the difference with being held in the air, and resting on the ground. In reality, for the rock on the ground, if an earthquake were to occur and the ground were to separate under the rock, it could actually fall further.

So, is it really correct to state a rock lying on flat ground really has zero potential energy?

Daisey
 
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  • #2
You can set the zero for your potential energy anywhere that is convenient. Usually that is the lowest available position, but it doesn't have to be. Potential energy can be positive or negative, and only differences matter. For a rock in a constant gravitational field, the potential energy is [tex]mg(h-h_0)[/tex] where [tex]h_0[/tex] is your arbitrarily chosen zero point for potential energy. (Or, equivalently, one can choose to place the origin of their coordinate system anywhere that is convenient.) Only differences will matter, and when you take the difference in potential energy between two positions, then the h_0's will cancel. They have chosen a particular origin for the convenience of their illustration where the PE of a rock on the ground is zero. If the earthquake were to occur and the rock fell into the earth, then its PE would become negative.
 
  • #3
In reality , the only way an object on the Earth can have zero potential energy is when it is at the centre of the Earth , for then it has no tendency to fall further. However , the concept of zero potential energy on the Earth's surface is a misleading one. Potential energy is a relative term that physicist use for convenience in describing physical systems , i.e , when Newton's laws prove to be just too complicated. Potential energy is relative . In the given situation , the ground has been assumed to be the reference point , i.e , it has arbitrarily been assigned zero potential energy , and with respect to that , the potential energy of different heights has been calculated .
 
  • #4
siddharth5129...In reality , the only way an object on the Earth can have zero potential energy is when it is at the centre of the earth...

Agreed. Actually can't one say no object in the universe has ABSOLUTELY zero potential energy? That rock at the center of the Earth is still being gravitationally pulled by every other object in the universe, although in a VERY small way.

siddharth5129...Potential energy is a relative term that physicist use for convenience in describing physical systems,...

Now that makes sense.

Thanks!
 
  • #5
daisey said:
I am now reading Rosenblum's and Kuttner's book "Quantum Enigma". In one of the first chapters they talk about the different forms of energy. Regarding Potential Energy, they give the example of a rock held in the air having potential energy because of the force of gravity. They go on to state that when rock hits the ground it transfers ALL of its potential energy to the ground, and at that point is has no remaining potential energy.
I am confused with the difference with being held in the air, and resting on the ground.
1) Get a rock; heavier is better.
2) Stick your foot out forward; right foot if your are right footed; otherwise left.
3) Rest the rock on your toe.
4) If it does not hurt, then lift the rock above your head.
5) Drop the rock on your toe.
6) Calculate potential energy mgh for the rock.
 
  • #6
Bob S said:
2) Stick your foot out forward; right foot if your are right footed; otherwise left.
How do you determine if you are right- or left-footed?
 
  • #7
DaleSpam said:
How do you determine if you are right- or left-footed?

If you write with your left foot you're left-footed, and vice-versa.
 
  • #8
daisey said:
Actually can't one say no object in the universe has ABSOLUTELY zero potential energy? That rock at the center of the Earth is still being gravitationally pulled by every other object in the universe, although in a VERY small way.

This statement is not quite right. You can define any object in the universe to have ABSOLUTELY zero potential energy. The only thing you need to do is define that object as the zero potential.

On earth, the zero potential is usually taken at ground level, just because it's convenient. You could take the zero potential on the moon, and every calculation would yield the same answer. The only problem is that you would then need to know the exact distance between your object and the moon, and that distance varies.

Here is an example. Suppose you hold a rock at a height h above the ground. The potential energy of this rock, relative to the ground (which means, the ground is the zero potential), is just:
[tex]P_1 = mgh[/tex]

The potential energy of the same rock, relative to the center of the Earth (which means, the ground potential is the center of the earth), is:
[tex]P_2 = mg(r+h)[/tex]
where r is the radius of the Earth (or the distance between the center of the Earth and the ground under your feet).

Yes, those two potential energies are very different, and they do describe the same rock.

For example, let's calculate how much kinetic energy the rock gains when you drop it from your hand onto the ground. This energy is the difference between the potential energy where you release the rock and the potential energy where it lands.

In the first case, we have:
[tex]E_1 = P_1(y = h) - P_1(y = 0) = mgh - 0 = mgh[/tex]

In the second case, we have:
[tex]E_2 = P_2(y=h) - P_1(y = 0) = mg(r+h) - mg(r+0) = mgr + mgh - mgr = mgh = E_1[/tex]

The answers are the same!

This example shows that it does not matter where you take the zero potential, as it will always cancel if you look at any potential difference. And you always look at potential differences in physics. A single value of the potential energy doesn't say much, and is indeed arbitrary if you don't define the zero potential.
 

FAQ: Does Every Object in the Universe Have Absolute Zero Potential Energy?

What is potential energy?

Potential energy is the energy possessed by an object due to its position or state. It is stored energy that has the potential to do work when released.

How is potential energy related to gravity?

Potential energy and gravity are directly related. The higher an object is placed in a gravitational field, the more potential energy it has. This is because the object has the potential to fall and release its stored energy due to the force of gravity.

How is potential energy calculated?

The formula for calculating potential energy is PE = mgh, where m is the mass of the object, g is the acceleration due to gravity, and h is the height of the object above a chosen reference point.

Can potential energy be negative?

Yes, potential energy can be negative. This occurs when the reference point used to calculate potential energy is chosen below the object's position. In this case, the object's potential energy will be negative, indicating that it has less energy than it would at the chosen reference point.

How does potential energy change when an object moves?

As an object moves, its potential energy can change. This is because potential energy is dependent on the object's position. When an object moves higher, its potential energy increases, and when it moves lower, its potential energy decreases. However, the total amount of potential energy in a closed system remains constant due to the law of conservation of energy.

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