Conservative Forces

[lhenslee]

I imagine that this is a relatively simple concept, but as I keep hearing different things from different sources, I thought I might try asking you all:

My EK books tell me that a conservative force (i.e. gravity, Hooke's) is unable to do work, yet I have taken several practice exams that refer to "work done by gravity" or some other conservative force.

Which one is right?

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[TieuBachHo]

Hi,
I think they are both right. Gravity do work on a system but it doesn't change the internal energy of a system; and therefore, it's unable to do work. For example, gravity actually does work on a ball when you drop it from a certain high (causing it moves from one position to another) but its internal energy doesn't change. What it gain in its kinetic energy, it loses its potential energy (conservation of energy for conservative forces). Since the internal energy is unchanged, there is no work to be used by it. There is only a transform of energy. Make sense

 

[sv3]

I thought in simple terms W = FD
If a ball is dropped, gravity being the force and the height being the distance results in work being done. I am not sure I am understanding Ek properly when they say conservative forces are unable to do work?? I also thought the definition of energy was the ability to do work - so a ball sitting on the edge of a cliff has the ability to do work due to PE. This is a pretty important point so hopefully we can all clear it up...

 

[Chowdder]

The word is kind of ambiguous and not clearly stated, which is what I think is causing the problem. Conservative force is one that changes the potential energy of an object. Potential energy is a state function, so the changes in the energy is path independent (depends only on the end-points). Gravity, spring, electric, and magnetic forces are all conservative forces. Technically, they don't do work because work is defined as "Force times distance". Work is path dependent, hence it is not a state function. When you see the term "work done by gravity", what they are referring to is the change in potential energy. If the path the object takes is a straight line connecting the endpoints, then work done on the object is equal to the absolute change in potential energy. What those questions are usually referring to is for example, an object falling from point A to point B. The work done gravity is equal to the absolute change in potential energy. Hence the term "work done by gravity".

To answer your question:
EK is correct, conservative forces do not do work in the technical sense, it only changes potential energy.

 

[sv3]

Hi,

thanks for the reply...it was helpful in clearing the problem up. I still am caught up on one aspect though (i've bolded it in your reply below). If PE changes (i.e a ball on a cliff dropping to the ground), then something had to cause the PE to decrease, and wouldn't that be the work that was done by gravity? So even in a technical sense, work was done (the force being g and the distance being the height). I get that gravitational PE is a state function, but if it decreases, then it's because work was done....or so i thought? Can you help me clear up what i still seem to missing? Or is this just a matter of perspective? I realize sometimes we go way above and beyond on this board and I rather avoid that whereever I can! Thanks very much

steve

The word is kind of ambiguous and not clearly stated, which is what I think is causing the problem. Conservative force is one that changes the potential energy of an object. Potential energy is a state function, so the changes in the energy is path independent (depends only on the end-points). Gravity, spring, electric, and magnetic forces are all conservative forces. Technically, they don't do work because work is defined as "Force times distance". Work is path dependent, hence it is not a state function. When you see the term "work done by gravity", what they are referring to is the change in potential energy. If the path the object takes is a straight line connecting the endpoints, then work done on the object is equal to the absolute change in potential energy. What those questions are usually referring to is for example, an object falling from point A to point B. The work done gravity is equal to the absolute change in potential energy. Hence the term "work done by gravity".

To answer your question:
EK is correct, conservative forces do not do work in the technical sense, it only changes potential energy.

 

[Chowdder]

So let me put it this way. Work = F*D. The force that causes an object to fall from point A to point B is gravity, a conservative force. So in this case, work is being done by gravity, but only because work equals to the change in potential energy.

This might help clear things up for you if you're still having problems.
http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Energy/PEandForces.html

 

[sv3]

i was just at that site after using google. here's a quoe from it:
Conservative Forces are path independent, in that the work done by the force to move an object between any two points is independent of the path taken.

So it's a state function, but work is/can be done. Thats what i get from this. Ah well, I'll wait till i see this in practice before I bother with it more. but thanks very much!

steve