How Do You Calculate Potential Energy for Different Orientations of a Brick?

[SammyLP250]

Homework Statement

A 1.57 kg brick measures 20.0 cm x 8.00 cm x 5.50 cm. Taking the zero of potential energy when the brick lies on its broadest face. (a) What is the potential energy when the brick is standing on end? Note: You can treat the brick as though all its mass is concentrated at its center. (b) What is the potential energy when it's balanced on its 8 cm edge?

Homework Equations

U_g=mgh

The Attempt at a Solution

I could not figure out the first part of the question because I don't know what "standing on end" means.

As for B:

m = 1.57 kg
g = 9.81
h = 0.08 m

U_g = (1.57)(9.81)(0.08) = 1.23 J

That is the answer I get, but Mastering Physics tells me that there was a rounding error and the answer is 1.17 J and I cannot figure out where I was wrong. This is not supposed to be a hard problem. Maybe I'm thinking too hard about it??

 

[azizlwl]

Check the difference of CM between 20.0 cm x 8.00 cm surface facing the floor and that of
8.00 cm x 5.50 cm.

I guess it is between the broadest and the least broad. There are 3 dimensions of faces.

 

[SammyLP250]

Thanks for replying! Could you explain to me what you mean by check the difference? I'm a little slow...

 

[azizlwl]

If it is standing with height of 20cm, then the height of CM is 10cm.
Likewise if it is standing with height of 5.5cm, then the CM must be (5.5/2)cm