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Q & A: What's the difference between energy and work?

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Most recent answer: 07/06/2014
Q:
I am very much baffled by the concepts of work and energy. I have tried every possible resource but still don't get it. Why has been work defined the way it is. What can we know from it and how to interpret the message within it. I mean what predictions can we make if we think work is same as energy. Say I have 1000 J energy So does that mean I can apply any any amount of force and the displacement adjusts itself so that their product is constant? If yes, then consider that I have 1000J energy and I apply force of 10N on a very small object.So the displacement will be 100m. Consider a similar condition but now the object is very very heavy.In both cases force applied was same but time of application of force was different. The 2nd case I feel should require more energy. Or else it will mean that we can apply a force for any amount of time with a certain amount of energy. Or is it that if we have certain amount of energy we can chose any amount of force we like and seeing the mass we are displacing nature itself adjusts the time of application of force so that we can displace only by that value which came from work equation. Please explain thoroughly how the entire actual picture fits together and in a way that is intuitive. You must know that I have already read many resources but couldn't feel satisfied. Please go too, deep too simple and too informal. A lot of thanks.
- Viham (age 17)
India
A:

Hello Viham,

Energy is like a commodity; an economic analogy would be like money in your bank account.  You can convert one form of energy into another. For example if you have a heavy suspended mass it has potential energy equal to EPotential = mgh, where m is its mass, g is the acceleration due to gravity and h is the height above ground.  If you cut the cut the rope holding it up the mass will accelerate and potential energy will be converted into kinetic energy, EPotential(initial) =  EKinetic(final) = mv2/2   where v is it's velocity when it hits the ground.  In the economic model you can covert rupees into an equivalent amount of dollars or you can withdraw from the bank account and put the cash in your pocket.   The total amounts of energy and economic values remain the same.  

Work on the other hand is a specific form of energy transfer, one due to large-scale motions along the direction of an applied force.     Work can either change the kinetic energy (e.g. when you press on a sliding block) or change the potential energy (e.g. when you lift a block). 

LeeH and Mike W.


(published on 07/06/2014)

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