Slip or Stick and the
Force of Friction
11
When the surfaces of two objects are I contact, molecules on the surface of one object are attracted to
molecules of the other. Ridges and valleys of one surface settle into the valleys and ridges of the other surface.
When one surface slides over another, a slip-and-stick sequence occurs as molecules cling and break away from
each other. Air between the surfaces tends to act as a lubricant. The more tightly the surfaces are pressed
together, the more air is squeezed out and the greater the electrical interaction between molecules on each
surface. So the force of friction between the surfaces depends on the nature and condition of the surfaces, and
on how hard the surfaces are pressed together.
For a block sliding on a horizontal surface, the force that presses the surfaces together is simply the weight of
the block. The ratio of friction force to normal force is called the coefficient of friction and is symbolized by the
Greek letter µ (mu):
µ=
Friction Force (Ff )
Normal (N)
It must be emphasized that this relationship holds true only on a flat surface when the force that presses the
surfaces together is only the weight. The coefficient of friction μ is greatest when the two surfaces are at rest,
just before motion starts. (Then the ridges and valleys have had time to sink into each other and are meshed.)
Once sliding begins, μ is slightly less. The coefficient of friction for sliding objects is called the coefficient of
sliding friction (or coefficient of kinetic friction). When friction holds an object at rest, we define the coefficient
of static friction as the greatest friction force than can act without motion divided by the normal force. A partial
list of coefficients of both sliding and static friction is shown in Figure 18-1.
Figure 18-1
Surfaces
Steel on Steel, dry
Steel on Wood, dry
Steel on Ice
Wood on Wood, dry
Metal on Metal, greased
µs
(Static)
0.60
0.40
0.10
0.35
0.15
µk
(Kinetic)
0.30
0.20
0.06
0.15
0.08
Friction always acts in a direction to oppose motion. For a ball moving upward in the air, the friction force is
downward. When the ball moves downward, the friction force is upward. For a block sliding along a surface to
the right, the friction force is to the left. Friction forces are always opposite to the direction of motion.
Learning Targets:

Students will analyze the types of friction and measure the coefficient of friction for each type.
Procedure
A. Computing the Coefficients of Static and Kinetic Friction
1. Obtain a lab setup from the front of the room with 3000g worth of
mass; all the
masses are 500g each. Obtain the weight of your friction block by
suspending it
from the spring scale. Record in Table 18-1.
2. Place the friction block on the wooden board at the left end. Put a
single mass on
the block. Attach the spring scale to the eye hook and while holding it parallel to the wooden board,
pull gently to the right. Once the block starts to move, you’ll notice the scale max out at some value
and then decrease. Your challenge is to observe what value the
max number is
worth. Once it moves, stop and do it again to verify your observed
max value.
Record this value as Force to Just Get Going in Table 18-1.
3. With the same mass still on the block, pull the block and mass
system at a
slow constant velocity. Since it is at C.V. we already learned that
the system is in
equilibrium (because it is not accelerating), the force you are
pulling it
forward has to be equal to the force of friction pulling it back. Record the value from your spring
scale during this process as Drag Force at Constant Velocity in Table 18-1.
4. Compute the Normal for your system for each trial by adding the weight of the block to the weight
of the added masses. Remember, on level surfaces the Normal and Weight vectors cancel out!
Record in Table 18-1.
5. Compute the static and kinetic coefficients of friction for the trial. Record in Table 18-1.
6. Repeat Steps 2-5 for each of the masses.
Table 18-1
Added mass
to block
(g)
500
1000
1500
2000
2500
3000
Ffs
Force to Just
Get Going
(static)
(N)
Weight of Block:
__________ N
Ffk
Drag Force at
Constant Velocity
(kinetic)
(N)
N
Total
Normal for
System
(N)
µstatic =
𝐅𝐟𝐬
𝐍
µkinetic =
𝐅𝐟𝐤
𝐍
7. Drag the block at differing constant speeds. Try changing the mass and repeating. Does the dragging
speed have any effect on the Force of Friction (the scale reading)? Using your previous response as a
guide, does the dragging speed affect the Coefficient of Friction?
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8. At each weight, how does µstatic compare to µkinetic?
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9. Does µkinetic depend on the weight of the block? Explain.
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B. The Effect of Surface Area on Friction
1. Using your friction block and 1500g of added mass, drag the block at constant speed on its side. The
smaller side is a smaller surface area and we will seek to determine if this has any effect. Record the
Drag Force at Constant Velocity from the spring scale in Table 18-2.
2. Find the data for the 1500g on the large surface area in Table 18-2 and transfer it here.
3. Fill in the rest of the table.
Table 18-2
Ffk
Area
Drag Force at
Total
Of
𝐅
Configuration Constant Velocity Normal for System
µkinetic = 𝐍𝐟𝐤
Contact
(kinetic)
(N)
(cm2)
(N)
Small Surface
Area
Large Surface
Area
Average
4. Does the area make a difference in the coefficient of friction? Explain.
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Interpretation Questions
1. Tables in Physics books rarely list coefficients of friction with more than two significant figures. From our
discussions or your notes, why are more than two sig. figs. not listed?
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2. If you press down on a sliding block, the force of friction increases but µ does not. Explain.
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3. Why are there no units for µ?
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