I need help understanding the answer (in plain talk) that I got to a physics question I posted.?

Question

My question was about "joules per second".

Anyways, the answerer said

"

However if a person were to be holding that rope they would be expending energy to resist that force, in joules per second .

So the potential energy is being resisted for a certain time - this is the power that the person needs in joules per second or watts to prevent the masses from falling."

I guess what I need clarifying is the following:

If a person is holding a rope looped through a pulley attached to a 1 kg object at 1 meter above the earth's surface. The 1 kg object has potential energy (U = mgh).

Does the person convert biological energy in order to oppose the 9.8 Newton force?

Is he responsible for the tension that exists in the (massless frictionless) rope? I am confused as I dont see any work being done, and how do I relate it all

Here is the original question (hope the link works for you) http://answers.yahoo.com/question/index;_ylt=Ai2I812hzIT9iUd02LkTMdvsy6IX;_ylv=3?qid=20080724030017AAVbSs

Answer 1

Hi there...

Well, if the person is holding the 9.8N object then he needs to spend energy.

The object hanging in the rope causes a tension in the rope. That tension must have a "pair" that, in this case, is located in the hand of the person holding the rope. That tension is not cause by the person holding the rope, but by the object.

The tension created in the hand of the person is the one that requires him to spend some energy in order to hold  the rope.

Hope i helped...

Answer 2

You're correct in that no mechanical work is done.  However, you've made teh problem a bit more complex by putting a biological system in the picture.  Muscles stay contracted only if teh body is willing to expend energy to cause the contraction.  So person has to consume energy to keep his muscles contracted and maintain the tension on the rope.  But no mechanical energy is changing so from a viewpoint of wokrk, kinetic energy and potential energy, everything is ok.  The energy consumed by the person is internal to him, so it does not enter into the conservation law.