ANSWER and EXPLANATION
We want to classify the forces given as either external forces or internal forces.
An internal force is a force that does not change the mechanical energy of an object. In other words, these forces act between two different parts of an object.
An external force is a force that is due to an external agent acting outside of a body or system.
Therefore, going by these definitions, we see that the forces that are internal forces are:
- Shear
- Gravity
- Torsion
and the force that is an external force is:
- The force of the floor on your feet when you are standing.