Is any Work done when you convert KE to PE?

[johnwandering]

I am wondering exactly what Work really is.

I was under the impression that W was when E was added or taken away from the system.


But consider a ball being dropped from 100m to 1m. The E of the system (T=KE+PE) is identical throughout the system, yet W is being done by gravity on the ball.
So W is being done, although the E of the system remains unchanged.


So what exactly is W?
Is it when E is add/taken away AND when E is changed from KE to PE?

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[johnwandering]

whoops realized that my original didn't make any sense.

I fixed the question.

 

[MedPR]

I'm going to guess that work is only done when energy is added or removed. It's convention to say that gravity doesn't do work.

Work = change in energy, so converting between KE and PE doesn't require work (I think).

 

[Dasypus]

The 'potential energy' at a height is calculated with h, or delta, because that sort of energy isn't really in the object. Rather, it's in the system, in the vertical distance created over which gravity can then work on the object. So the energy lost in kinetic form is used, as work, to change the location of the object; because it's done against the force of gravity, some of that work is then recoverable as the work that will be done by gravity when it's allowed to fall.

 

[hellocubed]

I'm going to guess that work is only done when energy is added or removed. It's convention to say that gravity doesn't do work.

Is this right?

I was under the impression any Fx was Work. And Gravity clearly imparts acceleration...

 

[Dasypus]

Is this right?

I was under the impression any Fx was Work. And Gravity clearly imparts acceleration...

Gravity definitely does work.

But... not every force over a distance does work. Centripetal forces are active on an object as it moves through a distance, but they do no work, because they are always perpendicular to the motion.

 

[MedPR]

Is this right?

I was under the impression any Fx was Work. And Gravity clearly imparts acceleration...

I'm not sure if it's right, but I learned that Work = deltaEnergy = deltaKE + deltaPE and according to that, if energy is conserved no work is done... right?

 

[MedPR]

Gravity definitely does work.

But... not every force over a distance does work. Centripetal forces are active on an object as it moves through a distance, but they do no work, because they are always perpendicular to the motion.

If you consider W=PE+KE, then gravity does no work. If you use W=KE then gravity does do work. At least that's what I learned in physics :/

 

[hellocubed]

I am going to go out on a limb and say this is outside the scope of the MCAT?

Only because I am afraid to delve deeper into it haha.

 

[Dasypus]

Is this right?

I was under the impression any Fx was Work. And Gravity clearly imparts acceleration...

If you consider W=PE+KE, then gravity does no work. If you use W=KE then gravity does do work. At least that's what I learned in physics :/

There is no "W=PE+KE". Total mechanical energy = PE + KE. To change this sum, work must be done by an outside force, so W=Δ(PE+KE).

Gravity does work *changing* PE into KE. Gravity's not the only force that does this. Any force that stores energy, like that in an electric field, will do the same.

 

[hellocubed]

W=Δ(PE+KE).

In the sense of Working changing PE to KE, how is that relavent to this equation?

(PE+KE) is same before and after the fall of the ball (100+0)=(0+100)

So no work should be done according to this formula?

 

[Dasypus]

In the sense of Working changing PE to KE, how is that relavent to this equation?

(PE+KE) is same before and after the fall of the ball (100+0)=(0+100)

So no work should be done according to this formula?

Er, sorry, different works. W=Δ(PE+KE) is for work done by an outside agent, say by someone lifting a book onto a table, or giving a kid a shove on a skateboard.

Work as the transfer between PE and KE is work done just by the potential-energy-storing environment, the gravitational or electric fields for example.


Edit: quick argument from authority: check http://en.wikipedia.org/wiki/Work_(physics)#Force_and_displacement "A most common example is the work done by gravity – see diagram. The object descends along a curved path, but the work is calculated from dcos(&#952=h, which gives the familiar result mgh."

 

[MedPR]

There is no "W=PE+KE". Total mechanical energy = PE + KE. To change this sum, work must be done by an outside force, so W=Δ(PE+KE).

Gravity does work *changing* PE into KE. Gravity's not the only force that does this. Any force that stores energy, like that in an electric field, will do the same.

I meant deltaKE+deltaPE.

 

[DrRichand1]

E-initial + netW-nonconservative forces= E-final
work is only done when the forces is parralel to displacement.

 

[gettheleadout]

milski for the final word?