Re: positive and negative work

[kowalskil <kowalskil@MAIL.MONTCLAIR.EDU>]

You asked a very good question Justin. I also
want to know how to answer it correctly.

Justin Parke wrote:
> I am assuming that the mass is on *top* of the vertical spring and the spring is pushing up on the mass.  Is this a correct assumption?
>
> If so then I am confused as to how this is an accurate model of the situation.  What is the "spring" in the person's body?  Is it their muscles?
>
> Is the problem that some muscles push up (like the spring) and some pull down (like the agent compressing the spring?  Then I would ask, to which muscles does the problem refer, the "spring" muscles or the "agent" muscles?
>
> Sorry if I am being dense but I do want to understand this.
>
> Justin
>
> Justin Parke asked:
>  >
>  > A conceptual question from the fifth edition of Serway and Faughn
>  > College physics  (ch. 5 number 2):
>  > "Discuss whether any work is being done by each of the following agents
>  > and, if so, whether the work is positive or negative:
>  > d)  the leg muscles of a person in the act of sitting down."
>  >
>  > I say the muscles are doing negative work since they exert an upward
>  > force (to prevent the person from simply falling into the chair) while
>  > the motion of the person is down.  The answer in the book is positive,
>  > with no explanation given. Could someone explain this for me?
>
> Ludwik replied:
>
>  Let me try. Model the situation with a vertical spring
>  supporting a mass m; the system is in equilibrium. You
>  are the "agent" inside the spring (muscle). You have
>  to pull the ends of the spring toward each other. By
>  compressing the spring your two forces do positive work.
>  Ludwik Kowalski