Question

Concepts and Static Structures 3. (3 marks) The forces are shown in the members of the given truss. Identify the members of this truss which experience tension or compression; note that the truss members are labelled with small case letters. Identify any normal forces that present in this system. Explain the difference between forces of tension and compression. Use examples to illustrate your answer. Note that the forces shown on the diagram are forces exerted on the joints. b a A g e E

Answer 1

The truss which experiences compression: a b f g

The truss which experiences tension: e c d

There are four normal forces acting on the system. The vertically downward force acting on the joint A & B and the vertically upward force acting on the joint C & D.

Tension refers to the force which tries to elongate the object. For example, if we hang a mass by a string which is attached to a hook, then the weight of the mass will try to pull it down whereas the normal reaction by the hook will pull it up. So, the two forces acting on the string are away from each other, hence they will try to elongate it. This force is tension.

On the other hand, the force that tries to shorten the length of an object is called compression. For example, if you keep some weight on a beam which is resting on the ground, the weight will push it down and the normal reaction from the ground will push it up. The two forces act into each other and will try to shorten the length. This is compression.