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If we draw a box around the two trailers they can be considered a closed system or unit. When we look at the forces on this closed system the following forces will apply (we assume drag forces are absent):
These forces are called external forces to the system.
The following forces will not apply:
We can also draw a box around trailer A or B, in which case the forces will be different.
If we consider trailer A as a system, the following external forces will apply:
If we look at the example above and draw a force diagram of all the forces acting on the two-trailer-unit, the diagram would look like this:
It is important to keep the following in mind when you draw force diagrams:
Draw a labeled force diagram to indicate all the forces acting on trailer A in the example above.
In a free-body diagram, the object of interest is drawn as a dot and all the forces acting on it are drawn as arrows pointing away from the dot. A free body diagram for the two-trailer-system will therefore look like this:
Draw a free body diagram of all the forces acting on trailer A in the example above.
The easiest way to determine a resultant force is to draw a free body diagram. Remember from Chapter [link] that we use the length of the arrow to indicate the vector's magnitude and the direction of the arrow to show which direction it acts in.
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