hi, what is the definition of work column please?

Work = Duration * assignment units.

It is the amount of effort (work) expected for the resource assigned to the task.

so for instance i have 100 sf of wall to paint. when i want to calculate durations using the sf data and the productivity / worker / hour data, how can i translate this 100 sf of wall to work?

for example if i know that one worker can complete 100 sf of wall in 8 hours, then does the work become 8 hours?

in other words,

1-does the work mean the amount of time required for one (1) unit to finish that entire task?

2-in general, when i have sf data, and productivity data, and when i want to translate this into ms project to come up with my duration, is the work column where I start? if not which column i start with? if work column is the place to start, then i enter resources available, assign resources and then it calculates the duration? is that the sequence?

No, the amount of work is the amount of effort for that particular resource. Split your screen to show the Task form in the lower pane.

Assign the resource using the Task form and enter the work in the work column. Click OK and Project will recalculate the duration required for the assigned resource to accomplish that work given the assignment units.

In the case of you specifying work, project will calculate duration by:

Work/Assignment Units = Duration

thanks but what do you mean by assignment units exactly?

When you assign a resource to a task, you get to specify assignment units. Split the screen and you'll see "Units" in the task form at the bottom.

The assignment units will default to the resource's maximum units that you defined in the Resource Sheet view.

what i am trying to figure out is, how can i translate the real world info to ms project.

lets say i have a wall to paint, 400sf. and 1 worker takes 8 hours to paint 100 sf wall, and I am going to use two labors. lets say i know these two things. now i go to my ms project, and
1-enter an activity called "paint wall"
2-assign laborer#1 to it, and then assign laborer#2, each 100%
3-now what? do i enter work? if so do i enter 32 myself? i mean how can i let ms project know, i have 400sf to paint? if i am entering work here, it means i enter 32h? which means i do the calculation myself outside, to find out the amout of time required for one (1) unit and enter it as work? if not how?

You have several choices. Project does not "know" anything other than basic math. So if you say 1 painter can paint 100sf of wall in 1 day. You can:

1. Enter the task "Paint 400 sf of wall" and set the duration to 4 days.
2. Now assign painter 1 - Project will automatically calculate 32 hours of work based upon a 4 day duration task * 100% assignment units.

Now you realize you have two painters. The task "Paint 400sf of wall" task could be done by two painters -- the space is large enough and you have enough paint brushes. So, you ensure the task is effort driven -- this is in the Task Form at the bottom once you have split the screen.

3. You add Painter 2 to the task. Because the task is effort driven and you are adding another resource -- the total amount of work on the task (32 hours calculated from the first assignment) stays stable. Project drops the task duration to 2 days because now each resource can do 16 hours of work at the same time as the other resource does 16 hours of work.

There is no automatic way to tell Project that a painter can paint 100 sf of wall in 1 day, so 400sf should take 4 days.

I hope this helps.

based on your answer,

1-so by telling me to enter duration as 4 days, you are in a way telling me that, the first time I enter duration for an activity, I should first think of it as if it will be done by just one (1) full time resource? such as i should calculate "how long would it take for just one full time resource to finish this", and then i do the math myself outside, and come up with 4 days and enter that?

2-as soon as I assign 4 days, and then enter just one (1) full time resource, the project automatically multiplies 4 days by 8 hours (because lets say we had set a full day to 8 hours) to come up with 32 hours?

You may certainly work by either specifying duration or entering the work for each resource and letting Project calculate duration.

You expressed that you knew how long a task took one person. How long is duration. If you said you knew a painter could paint 100 sq feet of wall in 8 hours of work -- that is work.

Use what ever method makes the most sense for you. You are correct -- if you set duration, Project will calculate work based upon:

Duration * assignment units = Work

You can specify work and Project will recalculate duration based upon:
Work/Assignment units = Work.