Calculating Work for Stacking Books on a Table

[laxboi33]

Homework Statement

Six identical books, 4.0 cm thick and each with a mass of .80 Kg, lie individually on a flat table. How much work would be needed to stack the books one on top of the other.

Homework Equations

U= mgy

The Attempt at a Solution

total mass = .8kg x 6 = 4.8
G= 9.8
y= .004 * 6 = .024

(4.8)(9.8)(.024) = 1.12 J

the answer is 4.7 J. What am I doing wrong?

 

[Redbelly98]

y is not simply 6 x brick thickness.

The bricks are each raised a different height, in order to make them into a single stack of 6 bricks.

 

[Senjai]

Youre doing work against gravity right? You have to lift one book to put it on the other book, a certain height. When you add a third book, has the height you have to lift it changed?