Forces act in different places
Identify and analyse forces
The weight of the boy in Figure 4 presses down on the chair as the arrow shows.
When one object pushes against another object, we say that a force is exerted on the object. In this case, you can say that the boy exerts a downward force on the chair, or that there is a downward load on the chair.
In each of the pictures on this page and the next, make an arrow to show how the load acts on the structure.
Figure 4: The boy sits still on the chair. Figure 5: A man walking on a roof. Figure 6: A solar heating system on a roof. Is the load on the roof in Figure 5 always in the same place? Why do you say so?
Is the load on the roof in Figure 6 always in the same place? Why do you say so?
Are the loads on the bridge in Figure 7 always in the same place? Why do you say so?
As long as a person sits still on a chair, the force on the chair remains in the same place. This is called a stationary or a static force.
In the above picture, the truck and the car exert forces on the bridge. Can these forces also be called static? Explain why you say so.
When a moving object exerts a force on another object, you can say that the force is dynamic.
In each of the following cases, state whether the force exerted on the table is static or dynamic. Explain why you say so in each case.
A pot of flowers standing on the table.
A cat walking on the table.
A boy rolling a soccer ball over the table.
A man scrubbing the table.
What is the difference between the loads exerted on the two tables below?
Look at the different ways in which the two trucks below are loaded. On the one truck, the drums exert force everywhere on the cargo deck of the truck. On the other truck, the load is just one big drum. The single drum exerts force on a small part of the cargo deck.
A load that exerts an equal force over the whole structure that supports it, is called an even load. A load that mainly exerts a force on one part of the structure that supports it, is called an uneven load.
Think of a house with a zinc roof and the forces that the sheets exert on the roof structure.
Is the load even or uneven? Why do you say so?
Is the load static or dynamic? Why do you say so?
Think of people climbing up and down wooden steps.
Is the load even or uneven? Why do you say so?
Is the load static or dynamic? Why do you say so?
You have to design two wooden tables, and you are requested to use as little wood as possible. For the one table, the design brief states that the load on the table will always be static and even. The design brief for the other table states that it has to carry the same weight as the first table, but the load will sometimes be dynamic and uneven. Describe how your designs for the two tables will differ, and explain why.