266 Chapter 10 Force and Force-Related Parameters
forcesare created inside the material to hold the material and the components together. Examples
of internal forces are shown in Figure 10.15. Moreover, reaction forcesare developed at the sup-
porting boundaries to keep the object held in position as planned. Table 10.2 shows simpleF (external force)
F (external force)
F
f (internal force)
F
f (internal force)
■Figure 10.15 An example of internal force. When you try to pull apart the bar,
inside the bar material, internal forces develop that keep the bar together as one piece.TABLE 10.2 Simple Examples to Demonstrate Various Support Conditions and How They Influence the Behavior of an Object.
Examples of Supports Explanation Reaction Forces
Fixed Support
Pin Support
Pin and Roller Supports
Counter-clockwise
momentup
Right
F
F
F
For a fixed support, the object cannot move up and down,
nor can it move sideways, or be bent at the support location
point. Therefore, there are support (reaction) forces in the
vertical and horizontal directions and a reaction moment to
prevent rotation at the support location. For the example
shown, the external force will have the tendency to move the
object up and to the right and also bend it in a counter-
clockwise direction. Therefore, to prevent these tendencies,
the support reaction forces should act downward and to the
left, and the reaction moment is in a clockwise direction.At the support location, the object can not move up and
down, nor can it move sideways, however, it could rotate
about the pin. Therefore, there are only two support (reac-
tion) forces in the vertical and horizontal directions. Nothing
prevents the object from rotating. For the example shown,
the external force will have the tendency to move the object
up and sideways to the right and also rotate it in a counter-
clockwise direction. The support reaction shown can only
prevent the translational tendencies.In this example, we have added a roller to prevent the bar
from rotating. Note the change in the direction of the verti-
cal force at pin support and addition of the downward verti-
cal force at the roller location. Ask students to explain the
difference.M 1 F
R 2
R 1
F
R 2
R 1
F
R 2
R 3
R 1
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