Gravitational potential energy of a retaining wall

[Nithya115]

Homework Statement

Forty 2.0kg blocks 20.0cm thick are used to make a retaining wall in the backyard. Each row of the wall will contain 10 blocks. You may assume that the first block is placed at the reference level. How much gravitational potential energy is stored in the wall when the blocks are set in place.

Homework Equations

Eg=mgh

The Attempt at a Solution

To find m (mass):
(40)(2.0kg) = 80kg
To find height (h):
20.0cm = 0.2m
(0.2m)(4)= 0.8m I used 4 because the wall will be four rows high.
Final...
Eg= (80)(0.8)(9.8)
= 627 J
The answer in my book says 240J

[Mentor's note: Thread title changed to reflect actual subject matter of post]

 

[Orodruin]

You need to use the location of the center of mass of the wall, not its total height.

 

[Maged Saeed]

What is the location of the center of mass ?
Is it 0.3m or 0.4m ?

 

[Nithya115]

You need to use the location of the center of mass of the wall, not its total height.

so...
Eg=(80kg)(0.4m)(9.8)
= 313.6 J
Im still not getting the answer which is 240J.

Can you please show your calculations? It would be really helpful!

 

[Orodruin]

Is the centre of mass really at 0.4 m? Remember that the bottom layer is placed in such a way that it does not have potential energy. Also, I very much suspect the answer is using g ≈ 10 m/s².

 

[Nithya115]

Is the centre of mass really at 0.4 m? Remember that the bottom layer is placed in such a way that it does not have potential energy. Also, I very much suspect the answer is using g ≈ 10 m/s².

I believe the height is 0.3m but when I do the calculations, I am still not getting the answer.

Eg= (60kg)(0.3)(9.8)
= 176.4 J

 

[haruspex]

Eg= (60kg)...

The 0.3m is an average over all blocks, so you must count all the blocks in the mass.