Re: positive and negative work

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

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?

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