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Confused about P.E.

  1. Jan 20, 2013 #1


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    Potential Energy is something that i have failed to understand since a long time. But now i want learn about it so can someone actually give a clear idea about what it really is and which is applicable to most general cases.
    1. What it really means?
    2. Why does a body posses it?
    3. What is the role of Work and Force on it?
    4. Why is it Negative?
    5. What do i visualize in my mind when someone says that a body has potential energy
    6. Other things that i wanted to know about it but have forgotten right now

    Like in case of kinetic energy when someone says that a body posses kinetic energy, i am able to understand its related to a body in motion. But when someone asks about P.E. i am often left thinking.

    I mean after scouting the web for info i have completely lost whatever little understanding i had about it often being confused by the various sources.
    I am sorry if i have posted a wrong question but please help me understand this.
    Thanks a lot.
  2. jcsd
  3. Jan 20, 2013 #2


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    Hi san203. A stretched rubber band has P.E., and upon release, the elastic band can deliver this stored energy by giving K.E. to a small projectile. (You have to then perform work on the band to stretch it back to its stretched state.)

    A rock balanced on the edge of your roof has P.E. due to gravity, and it can convert this gravitational P.E. to energy of motion by speeding up as it falls. At the end of its fall, the rock can convert this K.E. to work in deforming the concrete path it lands on. (If you wish to return the rock to the roof, you must do work on it by lifting it against gravity.)

    A battery has P.E. in the form of chemical energy, and can convert this to electrical energy and then into heat energy when lighting a light globe. (Eventually the battery energy is exhausted, and you must do work on it ❲in the form of electrical energy input❳ to recharge it.)
  4. Jan 20, 2013 #3
    A1) it means work is stored in a body..like if u raise a block by 1 newton force by one meter displacement then 1 joule of work is stored in it..now if u drop the block it will fall down..and come back to its intial state..
    A2) a body posses it when this body is under influence of any potential field for example
    for charges field is electric,for magnets field is magnetic,for masses field is gravitational
    another this is related to elastic bodies which i will u later.
    A3) as i told u in answer numbr one..when force is applied on a body under the influence of such fields work is stored in body..it is also called that potential energy is stored in it..
    A4) it is negative because work is always done AGAINST the field ..like earth is pulling us in..and if we jump we do a work against earth's gravity storing potential energy in us..thats is why it is negative..
    A5)first u need to know what type of field the body is in..for example: let say u have two magnets attracting each other..u try to separate them..now visualize that magnets are still attracting each other..trying to get back where they were initially..what u did by separating them is stored potential energy in them..
    i think i cleared ur concepts..sorry if could not
  5. Jan 20, 2013 #4
    1.) It means that a body is inside a force field like a gravitational field. If we know a body's initial speed, we can associate a total energy (potential energy plus kinetic energy) with this this body. This total energy allows us to predict the speed of this body for any point in the field without thinking about the actual movement of the body.

    2.) The potential energy is due to the nature of the force field.It is not "located in the object"

    3.) Forces express how much potential energy is converted to kinetic energy per distance.

    4.) Because only forces are important to calculate how a body moves only differences of potential energy between two locations matter. If we add the same number everywhere everything stays the same. As a consequence of the mathematical model for small distances force fields make objects infinitely fast. In reality this doesn't matter because particles have some width so they never get that close, but to make the model work we are forced to "subtract infinity" to make the formulas work out. This why all the potential energies of bound particles are negative. We could also work without this trick, but it is more elegant this way, and there is really no correct solution, we can add any number we want, because only Energy differences matter.

    5.) Just ignore 2.) and imagine it is carrying some energy that it can convert into speed (or some other form of energy). This model works very well, and that's why it was invented. As far as we know there is no location where this energy is stored or released, or something that exists outside of the models. It's a mathematical consequence of the force fields and another way to look at the system.

    The nature of energy is still a matter of debate in theoretical thermodynamics, but it won't really help to think about energy in another way unless one is deeply involved in the mathematical description of thermodynamics.
  6. Jan 20, 2013 #5

    jim hardy

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    Sometimes the simplest works best. And vivid mental images.

    What's in a name? Potential energy?

    I tell beginners "..it means that an object has the potential to do some work.
    Work and energy are, after all, equivalent. Think of Roadrunner's Acme anvil waiting to drop, or a mousetrap that's set.. '
  7. Jan 21, 2013 #6


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    Okay. Thanks everyone for your answer and yes, my understanding about P.E. has definitely gotten better.:smile:

    But i have few questions question

    1. according to most answers above, Potential Energy is nothing but Potential of Body to do work because that work has been stored in them.
      But where does this Energy actually come from?And how come this is stored in Body or field.Even if the body did do work in separating the Bodies, Wouldn't the Work done be used to change the Kinetic Energy, so how did this P.E. pop up?

    2. Also what will happen if the body were to enter the Force Field From outside. I believe was no work done in bringing separating it from the Zero Potential Position? Is this true?

    By Potential, you guys mean the... tendency(not sure if its the right word) to go back to its initial Position And Work done in this case refers to Change or Transfer of energy,right?

    Thanks again as always.:smile:
    Last edited: Jan 21, 2013
  8. Jan 21, 2013 #7
    To answer your first question just think about where most energy comes from.
    The sun provides energy to the plants which are harvested and you consume them in your salad (or perhaps you had a hamburger and which case the cow would have consumed the plants, either way the energy is changing form) the energy is now converted to chemical energy in your body and allows you to function, do work, move other objects... you now burn some fraction of calorie to pull back a rubber band or lift up a bowling ball onto a counter (giving the object potential energy). The object now has the ability to transfer potential energy into kinetic, but it's all possible due to the energy transformations that occurred before.

    To answer your second question I think you need to understand the formula for potential energy.
    Potential energy=mass x gravity x height
    So if you're on the moon and far less work is required to lift a heavy object to give it potential energy, it does not have the same amount of potential energy as if it were lifted on earth (gravity is taken into account).
  9. Jan 21, 2013 #8
    It didn't pop up. Whenever you move body in potential field some work is done. If extra work you did on body is, for example equal to the work done by the field force, but of oposit sign, then Kinetic energy of the body doesn't change. All the work done has gone into increasing P.E.
    If that work is greater than the field work, then both K.E and P.E will increase.
  10. Jan 21, 2013 #9
    It seems to me potential energy is just a convenient mathematical device. The thing we really want to know about is the velocity and position. What I am getting at is the work energy theorem, which says if you apply a force to something over a distance, force times distance (or the integral of force over distance if the force is not constant) equals the change in kinetic energy. Kinetic energy is just an expression that you can calculate the velocity from, essentially, just an expression of work done in terms of velocity. Potential energy tells you how much work the field is going to do when you move something from one point to another. It's just terminology. You shouldn't take it too seriously, I think. Nature doesn't use potential energy or any other terminology. Nature just moves stuff around this way and that way. We just create terminology and models to describe that.
  11. Jan 22, 2013 #10
    Potential energy is always stationary. We have to let things move to convert that energy to kinetic forms.

    So a stretched rubber band has strain distortions at the molecular level that store energy. Releasing the stretched band allows that stored energy to be discharged into kinetic energy.

    Or a rock dropped on the earth. The stone has a different gravitational energy at each height above ground in falling from infinity. This is its potential energy. So the potential energy in dropping a stone is the potential energy at the end point (the ground) less the potential energy at the release point. This is converted into kinetic energy.

    Or an electron and a positron separated by an infinite distance. As they accelerate towards each other their electric fields (classical electromagnetism) cancel more and more. This field cancellation reduces the energy in the overlapping electric fields and creates attractive forces. So the electric field energy is exchanged for kinetic energy. Hence the potential energy is the energy of the separated electric fields.
  12. Jan 22, 2013 #11
    Energy is the transfer electromagnetic radiation; specifically, kinetic energy. According to QED, kinetic energy is the transfer of 'virtual particles' of photons from one atom to the next (this transfer of virtual photons is called the electromagnetic force, and is what binds atoms to one another).

    When a body (bowling ball) is lifted from ground to the rooftop, a massive transfer of these virtual photons is taking place between the atoms in the lifting apparatus (muscle tissue in a man lifting the ball, for example), indirectly lifting the ball to the roof. Thus, the lifting of the ball by means of the transfer of virtual photons is the expression of kinetic energy.

    Once the ball is on the roof, it has potential energy. But is potential energy an actual thing or process, as it is with kinetic energy? No! In fact, saying that something (such as the bowling ball situated atop the roof) has potential energy is just a way of saying that something has the potential to do work from kinetic energy. To come full circle, what then is potential energy? Potential energy is nothing. It does not exist, nor is it a process. Potential energy is just a way of saying we have a bowling ball on a rooftop that has the potential to perform kinetic energy. That's it. Nothing more and nothing less.
  13. Jan 23, 2013 #12
    For classical physics the conversion of potential energy into kinetic energy is controlled by the equation F=dE/dx, where F is the force that causes the acceleration and hence the kinetic energy, and the right hand side of the equation defines the rate of change of energy with distance. So in these terms, there must be different energies associated with different positions of the object in order to generate force and acceleration. This energy difference is termed potential energy.
  14. Jan 30, 2013 #13


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    Sorry for the late reply. Mom locked down my PC. Anyways thanks for all the replies. Helped me a lot.
  15. Jan 30, 2013 #14
    It helps to know that PE is relative to the system you are talking about. For example dropping a marble onto a train floor when the train is moving at 50m/s. Relative to the train, the PE is simple gravitational potential, but the PE of the marble relative to the earth is much larger. If the train crashed during the drop this would become obvious as the marble would seem to shoot forward at 50m/s relative to the train.
  16. Jan 30, 2013 #15


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    A potentially annoying situation?
  17. Jan 30, 2013 #16


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    Would that not be Kinetic Energy?
  18. Jan 30, 2013 #17
    This is completely false. Potential energy is energy itself, not just the potential to have kinetic energy. While its absolute value is arbitrary, differences in potential energy are explicitly measurable. For example, potential energy can be put into the left side of ##E=mc^2## just as well as kinetic energy can. If you have an uncoiled spring and you compress it, the spring will have more mass when it's in its compressed state. This is not just a matter of convention: that extra mass couples to gravity, so if you had an extremely sensitive balance and put a coiled spring on one side and an identical uncoiled spring on the other, the balance would tip.
  19. Jan 30, 2013 #18
    Yeah, the example with dropping the ball sure is. Although it posseses PE until the ball has hit the ground. If the ball lost all it PE at halfway through the fall it would stop in mid air. There would be no potential for it to fall further.
  20. Jan 31, 2013 #19


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    You still seem to be under the misapprehension that KE is relevant here because you are not using the terms in an accepted way. Gravitational Potential Energy is defined as Weight times Height above a reference level. This is not an appropriate use of the word "potential". If the ball is arrested in mid air, it will still have the PE, associated with its new height relative to the ground. You have to stick to the same reference height throughout your description of the situation. The only way of changing PE is to alter the vertical position.

    The speed at which this ball is moving down is irrelevant to the energy situation. What counts is the work done on the way down, which is the Force times the Vertical Distance. Of course, in order to get to the bottom, it must be travelling (and still possess a finite amount of KE) but its vertical velocity when it finally makes contact with the ground can be arbitrarily low. The weight inside a grandfather clock will have transferred virtually all of its original PE in driving the mechanism, once it hits the bottom of its (very slow) movement - compare this with the same weight, dropped through the air from the same height, where its KE will be almost exactly equal to the PE value at the top and virtually no work will have been done on the way down.
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