The key objective of this laboratory experiment is to help the students understand the concept of shear stress and normal stress on a cross sectional area of a surface or rocks, and to be capable of constructing failure envelops. As we spread or stacked the pennies on a piece of paper placed on the table, we inflict a normal stress on the paper. We then created the shear stress by putting pennies into the cup that was connected to the paper on the table by a strain, and the number of pennies that cause the paper to slide instantly determined the shear stress at failure. From the graph of the shear stress (σs) vs the normal stress (σN), the frictional coefficient (μ) can be calculated as the slope of the graph. The aspect of spreading and stacking the pennies on the paper placed on the table gives us an idea about how stresses act perpendicular and parallel to the cross sectional area of the surface these stresses act upon. With all these ideas in this experiment, we can relate to how different rocks beneath the earth surface fracture because of the principal stresses applied.

5. Errors

- Error due to dropping pennies in the cup: An error can be introduced depending on the way students drop the pennies into the cup. Without careful dropping of pennies into the cup, we may add an extra force on it that can cause the paper to slide eventually before the shear stress at failure is reached.

- Irregularity in spreading the pennies: this event can lead to an…