For example, when you stand on your little bridge, your weight is a force, pushing downwards, tending to alter the shape of the beam . Inside the beam the material deforms elastically just the right amount to produce an upward force of the same amount, opposing the tendency to change. These two tendencies are always in precise balance when everything is still. The art of the engineer is the solution of problems. That is the whole purpose of his training and education.
Problems never dismay an engineer.
We start by saying again that materials under load change their shape elastically at first, and then, if overloaded, plastically. In all the bridges still standing you will see that the forces never overload the materials, so the only forces you will meet are elastic ones. They are the ones that the bridge can manage, without permanent change of shape that is. If you should see a collapsed bridge you will be able to notice the plastic changes in the distorted shapes of the members, bent and sometimes twisted, and unable to spring back into their original form. Before we can understand how the forces act in the members of a bridge we must remind you again, because it is so very important, that every force is opposed by an exactly equal one.