Homework Help: Gravitational potential energy of a retaining wall

1. Nov 27, 2014

Nithya115

1. The problem statement, all variables and given/known data
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.

2. Relevant equations
Eg=mgh

3. 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]

Last edited by a moderator: Nov 27, 2014
2. Nov 27, 2014

Orodruin

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

3. Nov 27, 2014

Maged Saeed

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

4. Nov 27, 2014

Nithya115

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

Last edited: Nov 27, 2014
5. Nov 27, 2014

Orodruin

Staff Emeritus
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.

6. Nov 27, 2014

Nithya115

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

7. Nov 27, 2014

haruspex

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