Calculating Work Done by Gravity on a 21N Book Falling and Moving on a Shelf

[CanadianEh]

Homework Statement

- A book has a weight of 21N. It falls off the shelf on to the floor, 3 m below. The work done by the force of gravity is what?

- A book has a weight of 21N. You pick it up off the floor, and return it to the shelf, 3 m above. The work done by the force of gravity is what?

- A book has a weight of 21N. You slide it horizontally along the shelf, a distance of 3 m. The work done by the force of gravity is what?

Homework Equations

Work = Force x Distance

The Attempt at a Solution

I don't get the differences, please help!

 

[Coreyh7988]

Hello, Well you understand that Work=Force*distance. well let's look at this problem in this way, If a force is applied to an object, and that object slides in the direction of the displacement then the work is positive. so the first one should be W=63 Joules downward. because 21N*3m. Since the displacement (downward) is in the same direction as the force (gravity downward) then work is positive. The person picking up the book is different because the work is the same however since the displacement (upward) is in the opposite direction as the force (gravity downward) then work is negative. One important thing about work is that if there is no component of force in the direction of displacement then the Work is zero. So for the work of the guy sliding the book horizontally is zero. Hope that helps

 

[CanadianEh]

Thanks so much, that's much more clear. So basically the answer for the first question is 63 J, the answer for the second is -63 J and the answer for the last one is 0?

 

[Coreyh7988]

Yessir! Glad it makes sense