Datasheet

Chapter 1: Mechanical Adjustments · Page 31
Coefficients of friction do not have units. A friction force is always equal to the
coefficient of friction, multiplied by the normal force. There are no units that need to cancel
each other out. For example, if you start with a normal force that's in newtons, and multiply
by a coefficient of friction, the result will be a frictional force in newtons.
Problem: It took a 700 g mass (W
1
) hanging from this pulley system shown in Figure 1-
11 to make a 2100 g mass (W
2
) start to slide. Find the coefficient of static friction, µ
S
.
Figure 1-11
Pulley Problem
Tension in the string
applies force equal to
weight W1 to the
block on the surface.
Solution: If the system is exactly at T = f
S,Max
, nothing is moving yet. From Figure 1-11,
we know that T and T
1
are equal and opposite:
1
TT =
The next step is to draw separate free body diagrams for each object (see Figure 1-12).
The sum of the forces for the hanging block should add up to zero:
11
11
WT
0WT
0Fy
=
=−
=
∑