Register to reply 
Understanding Time and Work 
Share this thread: 
#19
Apr2414, 03:41 PM

Mentor
P: 21,397

What you wrote before, and what I was commenting on, was this: 


#20
Apr2514, 12:44 AM

P: 59




#21
Apr2514, 06:58 AM

P: 59

As far as formula goes I understand that
##\frac {M_1D_1H_1}{W_1}## = ##\frac {M_2D_2H_2}{W_2}## = ##\frac {M_3D_3H_3}{W_3}## = ##k## is constant. But I do not understand it intuitively. Please explain the intuitive meaning of ##\frac {MDH}{W}## to be constant. Explain it with following example 


#22
Apr2514, 10:34 AM

Mentor
P: 21,397

From the information in the problem here, 120 men have been working 64 days, and have completed 2/3 of the job. The work performed so far represents (120 men) * (64 days) = 120*64 mandays. From this information, how many mandays are required for the entire job? How many mandays have already been used in the first 64 days? How much time is left to complete the job? How many mandays are needed to complete the job? If you know how many mandays are needed to complete the job, and how many days are left, you should be able to figure out how many men are needed, and therefor, how many can be laid off. The reason this is so difficult for you, I believe, is that you are trying to pick the "right" formula to use, rather than trying to reason things out. Thinking is always harder than plugging numbers into a formula by rote, which part of the reason that we are 22 posts into this thread. The only "formula" I'm using here is that "work done" is in units of mandays (in this problem), and is calculated by (work done) = (number of men) * (number of days). If the units of time in the problem had been given in terms of hours, then (work done) would be in units of manhours. Don't use both hours and days, as in MDH. You'll just confuse yourself. In an example I gave earlier, "work done" was in units of "square feet that are painted". In this case MDH is meaningless. 


#23
May2414, 12:01 AM

P: 59

Sol: As we know Man hour/unit work is constant. Hence ## \frac{M_1H_1}{W_1} = \frac {M_2H_2}{W_2}## Now we can easily plug data in LHS of the above equation. But for RHS as we know, we have to calculate days required by 1 man working 2 hours/day to complete the 1/2 of that work. so we have to double the total Man hour in RHS i.e. ## \frac{2*(4*3)}{1} = \frac{2*(1*(X*2))}{1}## ## \frac{2*(4*3)}{1} = \frac{1*(X*2)}{1/2}## ##X=6## Correct If I did something wrong. 


Register to reply 
Related Discussions  
Understanding Work  Introductory Physics Homework  31  
Understanding Work Done  Classical Physics  4  
Understanding how the genes actually work  Biology, Chemistry & Other Homework  2  
Understanding Work  Introductory Physics Homework  1  
Need help understanding work equations :)  General Physics  9 