The picture shows a two-dimensional free body diagram of a truss with a pin joint at one end.

The free body diagram at the pin joint should show:
a. no forces at the joint
b. only parallel force component
c. only perpendicular force component
d. both parallel and force components

(I think it is d?)

The answer is c

Yes, you are correct. The free body diagram at the pin joint should show both parallel and perpendicular force components. So the correct answer is d.

To determine the correct answer, let's analyze the situation step by step.

First, let's understand what a pin joint is. A pin joint is a type of joint that allows rotation but restricts translation along its axis. In the context of trusses, a pin joint enables the truss members to rotate freely around the joint.

In a two-dimensional free body diagram of a truss, each joint is typically represented by a point where the members of the truss meet. A free body diagram shows all of the forces acting on a particular body or joint without any other elements or constraints.

Now, let's consider the forces that might act on the pin joint.

Since the pin joint allows rotation but not translation along the axis, it means that there can be no parallel forces acting on the joint. This eliminates options b and d.

Next, we have to analyze if there could be perpendicular forces acting on the joint. In a truss, the perpendicular forces acting on a pin joint are usually caused by the tension or compression in the truss members meeting at that joint. These forces can be vertical or horizontal, depending on the orientation of the truss members.

Therefore, the correct answer is c: The free body diagram at the pin joint should show only perpendicular force components.

To summarize:
a. No forces at the joint - Incorrect, since trusses experience forces in the members that meet at the joint.
b. Only parallel force components - Incorrect, since the pin joint restricts translation along its axis.
c. Only perpendicular force components - Correct, since perpendicular forces are caused by the tension or compression in the truss members meeting at the joint.
d. Both parallel and force components - Incorrect, since no parallel forces act on a pin joint.

yep, but in reality, a pin joint can have torsion also. I assume you forgot to type parallel AND PERPENDICULAR force