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I lift a block of mass $m$ by a height $h$ in gravitational field. I have done a work of $mgh$ on the block. This energy is stored in the block when it is at height $h$. At height $h$, the block has got some energy that was given to it, by the virtue of which it has a potential to do work. Hence that energy is called potential energy.

Now consider that the block moves with some velocity $v$. When the block is moving it has some energy and by the virtue of that energy the block could still do work (it may collide with a spring and compress it). Then why do we not call this type of energy which is possessed during motion potential energy as well?

I mean 'kinetic energy' has also got the potential to do work, then why it's called kinetic energy?

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  • $\begingroup$ Because it is the energy associated with the kinematics of the object? Not being explicitly named potential energy does not preclude it from being able to do work. $\endgroup$
    – Marius Ladegård Meyer
    Commented May 3, 2022 at 7:38
  • $\begingroup$ @MariusLadegårdMeyer Does that mean kinetic energy is a kind of potential energy? $\endgroup$
    – Harshit Rajput
    Commented May 3, 2022 at 7:46
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    $\begingroup$ see hyperphysics.phy-astr.gsu.edu/hbase/enecon.html $\endgroup$
    – anna v
    Commented May 3, 2022 at 7:48
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    $\begingroup$ Similar- Why potential energy is called potential energy and not kinetic one, even though it can be translated to kinetic ? (falling snow from the roof for example). We name things as it is, not what it can be. $\endgroup$
    – Agnius Vasiliauskas
    Commented May 3, 2022 at 9:05

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I mean 'kinetic energy' has also got the potential to do work, then why it's called kinetic energy?

Because kinetic energy (KE) and potential energy (PE) are the two possible different forms of all energy. KE is the energy of motion. PE is the energy of position.

Both forms have the "potential" (or capacity) for doing work, where the word "potential" is used in the general dictionary sense. The difference is kinetic energy is created when the energy stored as potential energy is released.

For example, when you raised the object and brought to rest at the height $h$ you did positive work of $+mgh$ (the direction of your force being the same as the displacement of the object). At the same time, gravity did an equal amount of negative work $-mgh$ (since its force is opposite the direction of displacement of the object), for a net work of zero and change in kinetic energy of zero. When something does negative work on an object, it takes energy aways from the object. In this case, gravity took the energy you gave the object and stored it all as gravitational potential energy in the object-earth system.

If the object is subsequently released, then gravity does positive work on the object giving the object kinetic energy of $\frac{1}{2}mv^{2}=mgh$ prior to impact with the ground.

Hope this helps.

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  • $\begingroup$ "..stored it all as gravitational potential energy in the object-earth system", this was thought-provoking. Ever since I was a kid I was always taught that Potential energy is stored in the body itself. But what you said makes more sense to me. Can we also think of this energy to be stored in the gravitational field? $\endgroup$
    – Harshit Rajput
    Commented May 3, 2022 at 13:01
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    $\begingroup$ @HarshitRajput It is common to refer to potential energy stored in an object because it is simpler than constantly referring to the object-earth system. But if you look deeper you find it is technically incorrect since PE of all forms (gravitational, electromagnetic, elastic) is a system property. GPE is due to the relative position of the two objects. It's probably better to think of it as energy "associated" with the gravitational field because there is no energy stored in the gravitational field of the earth alone or the object alone, only the combination. $\endgroup$
    – Bob D
    Commented May 3, 2022 at 13:17
  • $\begingroup$ Got it, thank you so much. Just last I would like to know what you think about this - When a bar is elongated by applying a tensile load on it, the energy that I expended in doing this work is stored in the internal resistive force field (which develops as a result of deformation), and when I remove the load this force field does the work and expends the same amount of energy that I gave it, in bringing the bar back to its original length (assuming it behaved elastically). $\endgroup$
    – Harshit Rajput
    Commented May 3, 2022 at 16:21
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    $\begingroup$ @HarshitRajput Yes, if the bar obeys Hooke's law (region of linear elastic behavior). $\endgroup$
    – Bob D
    Commented May 3, 2022 at 16:36
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Potential energy describes the interaction, while kinetic energy is the property that describes itself

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"Potential" is All energy, it has the potential to do work, Kinetic is energy applied energy of it's motion.

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Kinetic energy is manifested in the form of movement. Potential energy is not manifested, that's why its called potential. For example, a body at height h from the ground has a gravitational potential energy mgh, meaning that it could manifest itself through the process of conversion to kinetic energy, in other words by movement which happens when you let the body fall from the height h. For example, let's have one body on the ground and another body on top of the building. Let's say that these two bodies are the same; same mass, same shape, same material etc. They appear the same and you wouldn't say that one body is more "energetic" than the other, but a body on top of a building has the potential of converting its potential energy to the kinetic energy, by free falling, while a body on a ground doesn't have that potential.

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