PASCO ME-9502 Statics System - Manual (Page 45)

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M o d e l N o . M E - 9 5 0 2 E x p . 8 : S l i d i n g F r i c t i o n
0 1 2 - 1 2 8 7 6 B 41
Exp. 8: Sliding Friction
Equipment Needed
Theory
Friction is a force between two objects that resists motion between the two objects. Static friction (or sticking fric-
tion) is friction between objects that are not moving relative to each other. For example, static friction can prevent
an object from sliding down an inclined plane. If a net external force on an object is greater than the force of static
friction, the object begins to slide. Sliding friction (or kinetic friction) occurs when two objects are moving relative
to each other. In most cases, the static friction between two objects is greater than the sliding friction between the
two objects. Once sliding begins, if the sliding object’s motion is constant, then the external force equals the force
of sliding friction.
An explanation of friction assumes that surfaces are atomically close to each other over a small fraction of their
overall area. The surfaces that are atomically close to each other will exert retarding forces on each other. This
contact area is proportional to the normal force, and therefore the friction force is proportional to the normal force,
or f
 F
N
where f is the friction force and F
N
is the normal force. The ratio of the friction force to the normal force
is called the coefficient of friction,
. The friction force is f = F
N
. The coefficient of friction is a unitless number
that is between zero and one for several common surface-to-surface combinations and is determined empirically.
In this experiment you will investigate how the normal force, the contact materials, and the contact area affect the
sliding friction.
Procedure
1. Use the Spring Scale to measure the weight of the Friction Block and then record the value.
• weight, W, of friction block = _________________
2. Mount the Inclined Plane on the Statics Board and use the plumb bob to level the plane. Set the Friction Block
on the Inclined Plane and use thread to connect it over a pulley to a mass hanger as shown.
3. Adjust the pulley so that the thread is parallel to the Inclined Plane.
4. Add or subtract masses on the mass hanger until the Friction Block moves at a very slow, constant speed
when you give it a small push.
Item Item
Statics Board Inclined Plane and Friction Block
Pulley Mounted Spring Scale
Mass and Hanger Set Thread
Plumb bob
Figure 8.1: Equipment Setup
Friction
Block
The thread must
be parallel to the
Inclined Plane.
Level the
Inclined Plane
Mass
Pulley
Mass
Hanger