Conservative and non conservative forces

[chiddler]

I don't understand conservative and non conservative forces very well.

I know their definitions, but i'm looking for examples to apply the definitions to.

Some examples from a question that I got wrong:

A feather falls to earth in a vacuum
A bonding pair of electrons are attracted to the nuclei of the bonding atoms.
A spring returns to its original shape after being compressed.
An astronaut on the moon picks up a rock.

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[Pisiform]

I might take a guess:

A feather falls to earth in a vacuum (definately conservative)
A bonding pair of electrons are attracted to the nuclei of the bonding atoms. (def conservative)
A spring returns to its original shape after being compressed.(if no friction or air-resistance then conservative)
An astronaut on the moon picks up a rock.(if no air resistance, then conservative)

seems kinda ambiguous question though

 

[Pisiform]

The main idea is that "Conservative" forces conserve the total energy of an object and non-conservative forces do not. Also conservative depends on the displacement while non-conservative depends on the path taken. For e.g. if u go from point A to point B in any direction, the WD would be same so force is conservative but WD by friction would be diff (longer distance, more WD by friction) so non-conservative force.

I hope this helps and correct me if i am wrong. bed time for me ... biostats final tmrw lol

 

[MT Headed]

Pisiform describes it well.

For example 3 you'd need more information. Did the spring reurn to original size and stop due to friction? Nonconservative. Does the spring return to its original shape periodically as it oscillates forever? Conservative.

For example 4, one could argue the the (rock+astronaut) system is nonconservative. The astronaut burned off a Milky Way candy bar to lift the rock, and some of those energy joules were converted to heat joules and can never be recovered. If he picked the rock up and then ran around a crater once, he would have the same displacement but probably would have burned off a second candy bar. Definitely not path independent. Definitely nonconservative.

 

[SaintJude]

All of these examples you originally mentioned are of "conservative" forces:
A. Due to gravitational force
B. Due to Electric force
C. Due to spring force
D. Also related to gravitational force.

A conservative force is a force where the work done in moving an object is completely independent of the path taken. In physics problems, this is what enables you to calculate a quantity without having to take into account the exact path the object took. For example, when you measure the gravitational potential energy (mgh) of an object on a building, you just measure the height--you don't really care how it got there (i.e. it's PE is "path independent") That's because its conservative.

As the previous replies aptly noted, work done by non-conservative forces will cause the object to lose mechanical energy. That's why all your noted examples are conditional upon the absence of non-conservatives forces.

Non-conservative forces:
Applied forces, air resistance, friction forces, normal force and tension

If you really want to test you concept on these, I recommend trying this short question set on this page: http://www.physicsclassroom.com/reviews/energy/energyans1.cfm#5

 

[chiddler]

Thanks for all the helpful responses.

I think that picking up a rock is non conservative because we're not just looking at the rock, we're looking at the person lifting the rock. Energy is expended in a non-conservative manner as mt headed described.

Isn't this false: If the sum of an object's KE and PE is remaining constant, then non-conservative forces are NOT doing work.

Given KE + PE + Wnc = KE + PE

If KE + PE = KE + PE, then it can be either conservative or non conservative. It's non conservative if the Wnc force is there as it is in the first equation.

Says it's true.

 

[MT Headed]

Isn't this false: If the sum of an object's KE and PE is remaining constant, then non-conservative forces are NOT doing work.

Wow, I think that's a quadrulple negative. You should be writing MCAT questions!

I couldn't possibly parse your sentence, but I konw this. If the sum of (KE and all of the various PE's) remains constant before, during, and after the operation, then the only forces operating are conservative forces.

Non conservative forces involve losing some of those energy joules to heat joules, usually through friction. You can't get those heat joules 100% back to energy joules, so energy joules are not conserved in this type of scenario.

 

[chiddler]

Wow, I think that's a quadrulple negative. You should be writing MCAT questions!

I couldn't possibly parse your sentence, but I konw this. If the sum of (KE and all of the various PE's) remains constant before, during, and after the operation, then the only forces operating are conservative forces.

Non conservative forces involve losing some of those energy joules to heat joules, usually through friction. You can't get those heat joules 100% back to energy joules, so energy joules are not conserved in this type of scenario.

Haha! Thanks for the flattering compliment, but I can't take all the credit. It was a question given by a website somebody else wrote a few posts up.