Natural Science Forum / Physics / General Physics / December 2007

Question: A waiter carries a tray full of meals across the room. Is work
being done?

My first response: We can't answer the question yet, because the question is
flawed! Questions about work only make sense when they ask: Does (something)
do work on (some object)?  Also, questions only make sense when the motion
is precisely described. So let's restate the question. In fact, let's ask
three different questions.

A. Does the waiter do work on the tray, as he starts from rest and then
reaches some final velocity? YES

B. Does the waiter do work on the tray, as he slows down to a stop? YES

C. Does the waiter do work on the tray, as he moves at constant velocity?
NO. To be more precise, NO NET WORK is done by the waiter on the tray.

But here is what I want to examine: "While moving at a constant velocity,
isn't friction between the waiter and the floor doing work on the waiter?"

I want to say "Yes, the friction force between the waiter and the floor
constantly retards the motion of the waiter (and thus also would retard the
motion of the tray.) The friction force acts over the distance that the
waiter is traveling;
the friction force thus does work on the waiter."

Is this correct to say? Can I say that the friction force effectively doing
work on the waiter's muscles?

I also want to say "However, the waiter constantly uses his muscles to
overcome this friction force, and continue moving forward at constant speed.
The waiter thus does work that is equal to the work of the friction force,
but in the opposite direction."

Can I say that he does work on, or within, his own muscles?

As you can see, I wish to explain something like: The work of A on B is
cancelled out by the work of B on A, hence the NET work on the TRAY comes
out to zero.

But how specifically should I describe this? Is my description ok? I have
the feeling that it is almost correct, but I just want to be as precise in
my wording as possible. (I've seen too many questions that were worded, or
answered vaguely, even in textbooks.)

Robert