Static loads differ from dynamic loads in the fact that the force exerted by the static load remains constant. With a dynamic load, the forces associated with the load change according to outside circumstances.
Static Load vs. Dynamic Load The main difference between a static and dynamic load lies in the forces produced by the weight of an object. When static, the load remains constant and doesn't change over time. With a dynamic load, some outside factor causes the forces of the weight of the load to change. Some of the factors that can affect a load and make it dynamic include:
Examples of a Static and Dynamic Load A good example of a static load is a truck with cargo inside sitting still in one spot. The force of the weight of the load has little chance of changing as long as the truck remains still. Once the truck begins to move, the load becomes dynamic, as the force of the movement can cause the load to shift, changing the effect of the force of the weight of the cargo. If the truck goes too fast, it could even cause the forces of the load to shift greatly, causing it to fall or to at least make it harder to drive the truck on the road's surface. Also, when stopping, the force of the weight of the load can shift forward, making it harder to stop the vehicle as quickly.
A bridge represents another example of static and dynamic forces in play. The weight of the bridge is a static load, as it doesn't change over time, as long as nothing moves across it or outside forces, such as the wind, don't move against it. A truck moving across the bridge places a dynamic load on the bridge by increasing the weight of the bridge as it crosses. A wind blowing against the bridge can also change the forces of the weight of the bridge, as it moves it from side to side, creating a dynamic load on the bridge. That is why it is important that engineers take all of the forces that might apply to a particular bridge in order to design a stable and safe structure. Another important force to keep in mind is torsion, with any twisting of the bridge in the wind causing additional tension on the structure, which in turn can affect how much of a load the bridge can handle.