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Thread: Force and Energy. Difference?

  1. #1 Force and Energy. Difference? 
    Forum Freshman LotusTiger's Avatar
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    I'm sorry if this question is not correct but what is the differece between a force and an energy. Are some forces energy and others not? I mean are some forces not energy because though they can slow down the velocity of an object they can't move it? But then if this is the case would something like kinetic energy not be an energy because it's doesn't move something but only slows something down. When is something an energy?


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  3. #2  
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    There's potential energy, and then there's actual energy. A charged battery has potential energy, but actually energy only happens when that battery is depleting itself by expelling something.


    Energy is only considered to take place if something moves (and even then, it has to be changing speed, not just coasting at a fixed speed), so the force of gravity isn't considered energy (it's considered potential energy). Or consider a hydro-electric dam, that isn't allowing any water through it for the moment. The force of the pressure of the water against the dam isn't energy. (It is potential energy, however). It's only when the water actually passes through the dam that you get energy.

    Another kind of potential energy is kinetic energy. Any object that's moving has kinetic (potential) energy. Understanding the difference between "potential" and "actual" can sometimes be very complicated.

    .... and ... slowing something down is energy too. (It usually involves heat energy, and people don't really like that particular kind very much because it's hard to make electricity out of.)


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  4. #3  
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    I'm sorry if this question is not correct but what is the differece between a force and an energy. Are some forces energy and others not? I mean are some forces not energy because though they can slow down the velocity of an object they can't move it? But then if this is the case would something like kinetic energy not be an energy because it's doesn't move something but only slows something down. When is something an energy?
    Forces, you can say, are instances where energy is passed form one object to another. For example, when you push something, you are transfering energy from yourself to the object to make it move. When you try to slow something down, you are transfering energy from yourself to make the object slow down. You might wonder why you transfer energy from yourself to the body in both cases; this is because the effect of a force also depends on the direction in which it is applied.
    This is not, I should mention, the textbook explanation of a force; however, it does simplify things somewhat, which was my motive in using it.

    Energy, however, is the capability to do some work. If an object has no energy, it cannot do any work. Forces and energy are completely different, because energy is a component of force; a force is never part of energy. Kinetic energy is energy because it is used to make the object move; in other words make it do some work.

    Potential energy, however, is a different concept altogether. Potential energy is best explained with an example. Suppose you hold a ball over the ground. If you drop the ball, the ball will fall to the ground i.e. it is doing some work. However, we might ask ourselves where this energy is coming from, since we certainly haven't pushed it or anything. In this case, the energy comes from the force of gravity, and this kind of energy is called potential energy.

    Does that answer your questions? Or would you like some more clarification?
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  5. #4  
    Forum Freshman LotusTiger's Avatar
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    What I’m beginning to slowly acquire from all your saying is energy in simplicity is movement itself as per one definition. The hydro electric dam is one thing I find particularly helpful, you say energy, “movement” only happens when the dam is opened, the water (energy since it moves) is let out. While potential energy is the “potential” of a “thing” for that “thing” to “move something else. And then you say that kinetic has potential energy, because it can move something ealse, but like wise the only that which has “energy” (on its own) is what it moves not itself.

    Liongold your description of transfer of energy make’s and the fact you say you are transferring energy when something slows down makes it sound like energy is “speed of an object being determined “and changed”( as explained by Kojak)

    I’d say I’m not going to get this straight of the mat so a more help may help more in understanding. And I apologize if I don’t so far understand all of what you are explaining out. But keep going.
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  6. #5  
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    I can try. Maybe you can understand energy as the agent of effect while a force is the cause. Without energy, a force cannot be established. Energy is also just a property of a thing without a direction. A force originates from a thing and has usually the character of a direction. While the force acts, energy is transferred.

    Do you play billiards? Here is an example. You hit a ball and it moves. The ball now has kinetic and rotational energy that allows it to maintain its current state. Some energy is lost due to friction on the cloth, but let us forget about it for this example. So, without an additional force acting on this ball, it maintains its properties, i.e. its speed and direction. It keeps its energy. As soon as it hits another ball, it reveals its potential to exert a force on the second ball. The force causes the second ball to change its properties, i.e. its energetic state that is observable by its sudden movement. At the same time, the first ball reduces its speed, i.e. kinetic and rotational energy was lost, or better, transported to the second ball.
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    @LotusTiger:

    Energy is force times displacement:

    E = F*s

    A force exerted along a way yields energy.
    An object, without any force applied to it, will move in a linear motion with no change in velocity. If a force is applied, then this object will either change direction, or the value of the velocity will change, or both.

    A force is F = m*a (mass times acceleration)

    So if a force on a mass m is applied, then

    a = F/m

    Kinetic energy is E = 1/2 mv² .

    As said before, energy is the capability of an object to do work. Work is the transfer of energy, as for example heat.

    It is indeed so, if you consider each single particle, that a force simply changes velocity - the direction, value or both. So actually a force is indeed the effect of energy transfer. We perceive this transfer as a force.
    Why is there an energy transfer at all? Well, for some reason, the universe wants to reach a state of equilibrium - where all energy is distributed equally in the whole universe. So there will be energy transfers, thus forces, as long as this equilibrium hasn't been achieved.
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  8. #7  
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    Quote Originally Posted by mastermind
    @LotusTiger:

    Energy is force times displacement:

    E = F*s

    A force exerted along a way yields energy.
    An object, without any force applied to it, will move in a linear motion with no change in velocity. If a force is applied, then this object will either change direction, or the value of the velocity will change, or both.
    That's actually the most accurate. Energy = Force x Distance

    If a force is applied, but nothing moves, then Distance = 0

    So: Energy = Force x Zero = 0

    If something moves, but no force is applied, then Force = 0

    So: Energy = Zero x Distance = 0

    An example of a force acting and nothing moving would be like the string in a loaded cross bow pushing against the catch, but not moving the catch. (The energy in the string is potential energy.) Energy is only released when you pull the trigger, and the string is able to apply its force to the bolt and send it flying.

    An example of something moving, but no force being applied would be like a comet in empty space just coasting along from its own momentum. (It still has potential energy, however, because if it collided with something it would probably move it.)
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    Quote Originally Posted by kojax



    An example of something moving, but no force being applied would be like a comet in empty space just coasting along from its own momentum. (It still has potential energy, however, because if it collided with something it would probably move it.)
    Actually, it has kinetic energy due to



    The part comes from a force(f) applied over a distance(d) which causes the mass(m) to have a velocity(v).
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  10. #9  
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    Energy = Force x Distance
    Just want to correct that; it's not energy but work that is equal to the product of the force and the displacement.
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  11. #10  
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    Quote Originally Posted by kojax
    There's potential energy, and then there's actual energy. A charged battery has potential energy, but actually energy only happens when that battery is depleting itself by expelling something.
    This is an odd way of explaining it. You might be confusing work and energy. Potential energy is also energy.

    There are many different types of energy: electric, magnetic, kinetic, heat, gravitational, chemical...
    (there is overlap on a very small scale, as heat is actually kinetic energy of the particles and chemical is electromagnetic energy between particles)

    Quote Originally Posted by kojax
    Another kind of potential energy is kinetic energy. Any object that's moving has kinetic energy.
    It's the first time I've heard or read something like that. If you're making a big division in types of energy, you have kinetic energy (mass that moves) and potential energy (everything else). I would not say that kinetic energy is a kind of potential energy.

    Quote Originally Posted by Liongold
    Forces, you can say, are instances where energy is passed form one object to another. For example, when you push something, you are transfering energy from yourself to the object to make it move. When you try to slow something down, you are transfering energy from yourself to make the object slow down. You might wonder why you transfer energy from yourself to the body in both cases;
    This is not correct: when you are slowing something down, you are transferring energy from the object to yourself, not the other way around:

    It's vectorial, so if F and s have an opposite direction, the work you do by slowing the object down is negative. You don't actually gain energy and it all goes to heat, because our muscles aren't able to recuperate energy in this way.
    A spring slowing down an object does gain potential energy.

    Now for my own attempt at an explanation: every object has some kind(s) of potential energy, and they want to get rid of it. If the potential energy changes by changing position is space (which is often the case e.g. the potential gravitational energy of a ball increases as you lift it higher), then the object could decrease it's potential energy by changing position. This "urge" to change towards a lower energy position is what we feel as a force. The larger the possible change in potential energy (for a fixed, small displacement), the higher the force.
    Note that the potential energy doesn't necessarily have to change to cause a force, if there is potential for change, that's enough. In fact: the absolute value of the potential energy is not relevant, it's only the change that matters.

    A system of objects with all kinds of potential energies will search for a minimum of the sum of all potential energies. At that point, there is also an equilibrium in forces. e.g. a mass on a spring has both gravitational and spring potential energy, and will hang still in an equilibrium position where the potential change of both energies, and thus the forces, are equal.

    This is statics. In dynamics, you also have kinetic energy. There is no urge to get rid of it, and objects just continue on their way, until they come under the influence of other objects and the kinetic energy gets converted in potential energy, resulting in "collision forces".

    I hope this is somewhat clear.
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  12. #11  
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    I think you're right. I've confused energy with work, but it becomes very complicated. A force all by itself is not energy.

    The SI unit for energy is the Joule. It's 1 kg*(m^2)/(s^2) (For reference, a kilowatt hour is 3600 joules. )

    The SI unit for force is the Newton. It's 1 kg * m /(s^2)

    So: 1 Joule = 1 Newton * 1 meter

    Basically, if you multiply a Newton times a distance, you get joules. A Newton of force does actually have to move something, in order to become a Joule of energy.

    However, an object at some height above the ground has potential energy, because a force is acting on it now, and it already has been moved a certain distance in the past (which it is possible it could move again, by falling.) So, yeah: potential energy is energy in that sense.

    A Comet going through space has potential energy because it's moving now, and it already has been accelerated in the past (to get it to its present speed), and it's possible it could be decelerated again back to a lower speed by colliding with something. So again: potential energy is energy.

    Now I feel bad, because I might have confused the OP by suggesting that potential energy wasn't energy. I guess it's more accurate to say that the motion of the comet doesn't expend energy (just stores it), and the string of a loaded crossbow also doesn't expend energy (just stores it). But the comet, and crossbow string are examples of energy.
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  13. #12  
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    Quote Originally Posted by kojax
    (For reference, a kilowatt hour is 3600 joules. )
    Excuse me my nitpicking, you probably just made a typing error: a kilowatt hour is 3600 kilojoule. A watt hour equals 3600 joule.
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  14. #13  
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    Oh.... yeah. Sorry. I totally forgot that I added the KW part.
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