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Chapter 1: Mechanical Adjustments · Page 27
Figure 1-8 also shows a close-up of the contact surfaces, where little components of the
force you apply to the block and the frictional forces are opposing each other. While
some of the frictional forces actually do come from the surface's roughness, there is also
interaction between the molecules in the two surfaces that govern frictional forces.
Free body diagrams like the one in Figure 1-9 are used to analyze the forces at work for
both static (non moving) and kinetic (moving) objects. There are two tricks to analyzing
a free body diagram. The first is to set a convention for positive directions. For example,
in Figure 1-9 the x axis is positive to the right, and the y axis is positive pointing up. The
second trick is to add the forces up in each axis direction, and set the sum equal to zero.
If a force vector is pointing in the negative direction, you are adding a negative value, in
effect subtracting.
Figure 1-9
Free Body Diagram
with Positive x and y
Axes Shown
Here is how that analysis would work for Figure 1-9. Start by setting the sum of the
forces in the x direction equal to zero:
Ff
fF
0fF
0Fx
S
S
S
=
=
=−
=
∑
The Greek letter sigma Σ denotes the sum of a list of values. So, ΣFx = 0 is can be read,
"the sum of the forces in the x axis direction equals zero."
f
s
= F essentially states that the force of static friction is equal and opposite to the force
applied on the object. The same rule can then be applied to the force in the y axis
direction: