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-   -   How to find the amount of work done on an object (http://www.physicsforums.com/showthread.php?t=279874)

itsmarasilly Dec15-08 04:45 PM

How to find the amount of work done on an object
 
1. The problem statement, all variables and given/known data

A student librarian picks up a 2.2 kg book from the floor to a height of 1.10 m. He carries the book 7.9 m to the stacks and places the book on a shelf that is 0.35 m above the floor. How much work does he do on the book?

2. Relevant equations

work=force(netdisplacement)=mg*(.35)

3. The attempt at a solution
work = ?(7.9-1.1)
work = ?(6.8) = 2.2(.35)

olgranpappy Dec15-08 05:01 PM

Re: How to find the amount of work done on an object
 
Quote:

Quote by itsmarasilly (Post 2003208)
1. The problem statement, all variables and given/known data

A student librarian picks up a 2.2 kg book from the floor to a height of 1.10 m. He carries the book 7.9 m to the stacks and places the book on a shelf that is 0.35 m above the floor. How much work does he do on the book?

2. Relevant equations

work=force(netdisplacement)=mg*(.35)

3. The attempt at a solution
work = ?(7.9-1.1)
work = ?(6.8) = 2.2(.35)

Only the displacement parallel to the force matters. Thus it was correct for you to use 0.35 meters in your equation. Now, tell us: what is m? what is g? what is mg?

itsmarasilly Dec15-08 05:15 PM

Re: How to find the amount of work done on an object
 
Quote:

Quote by olgranpappy (Post 2003230)
Only the displacement parallel to the force matters. Thus it was correct for you to use 0.35 meters in your equation. Now, tell us: what is m? what is g? what is mg?

m = 1.10 or 7.9
g = 2200
mg = 2,200,000

LowlyPion Dec15-08 05:35 PM

Re: How to find the amount of work done on an object
 
Quote:

Quote by itsmarasilly (Post 2003255)
m = 1.10 or 7.9
g = 2200
mg = 2,200,000

I think you've missed the point. g is not grams here, it's g gravity.

The work done on an object is the dot product of the Force acting on the object and the distance over which it acts.

What oldgranpappy was trying to convey is that you should be only considering the forces that acted on the book along the direction over which work was done against a force.

For instance work in picking up something against gravity is the product of its weight and the height that you pick it up to. How far you carry it, since that direction is perpendicular to the force of gravity, then does not affect the amount of work against gravity.

olgranpappy Dec15-08 05:56 PM

Re: How to find the amount of work done on an object
 
Quote:

Quote by itsmarasilly (Post 2003255)
m = 1.10 or 7.9

no.
m is not 1.10 or 7.9.

m is the mass of the book. what is the mass of the book?

itsmarasilly Dec15-08 06:37 PM

Re: How to find the amount of work done on an object
 
Quote:

Quote by olgranpappy (Post 2003338)
no.
m is not 1.10 or 7.9.

m is the mass of the book. what is the mass of the book?

2.2 kg

HallsofIvy Dec15-08 06:52 PM

Re: How to find the amount of work done on an object
 
And g is the acceleration due to gravity, 9.81 m/s2, so what is the weight of the book, mg?

And lifting it a tota vertical distance of 3.5 m (NOT 1.1+ 3.5 m. Do you see why?) what work is done on the book?


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