Name _______________________
Date ____________
COEFFICIENTS OF FRICTION
Pre-lab Questions
1. The direction of a frictional force is always in the (opposite / same) direction as the
motion or the tendency toward motion of an object?
2. What two kinds of coefficients of friction do you know? Describe whether the object is
moving or not moving for each.
3. A block of mass 12.0 kg rests on a horizontal plane. The coefficient of a static friction
between surfaces is 0.450. What is the maximum possible static frictional force that could
act on the block if a force is applied to it? Show your work
4. What are the units of the coefficient of friction?
5. How does the force of kinetic friction depend on the speed of a moving object?
1
Static and Kinetic Friction:
Friction is a resisting force that acts along the tangent to two surfaces in contact
when one body slides or attempts to slide across another. The direction of the frictional
force on each body is an opposite the body's motion. The frictional force is directly
proportional to the normal force. There are two different kinds of friction. When two
surfaces are still at rest, but attempt to slide one over another, static friction is present.
The frictional force f s, for static friction is f s = µ s FN. When two surfaces are moving
with respect to each other, the other kind of friction occurs. It is kinetic friction. The
frictional force, f k, for kinetic friction is f k = µk FN. It is also true that in general µ s 
µk. This means that when enough force is exerted to overcome static frictional force and
cause object to move, the kinetic frictional force is less than the applied force.
Aim: This exercise is designed to determine the
coefficient of static and kinetic friction and
compare them.
Materials: Whiteboard, wax paper,
aluminum foil, felt, quarters, and a measurement
device.
Prediction:
Which material will have the greatest static friction? _________________
Which material will have the least kinetic friction? _________________
Procedure:
1. To measure the static coefficient of friction between two materials, simply place an
object made of one material (in this lab, quarters) on a ramp made of another
material. (whiteboard, wax paper, felt, or aluminum foil).
a. Raise the angle until the object starts to slide unaided from standing still.
b. Measure the angle by using the height and hypotenuse of a right triangle
formed from the wooden ramp.
c. Run three trials and take an average.
d. Use the angle to mathematically determine the coefficient of friction.
2. To measure the kinetic coefficient of friction between two materials, simply place
an object made of one material (in this lab, quarters) on a ramp made of another
material. (whiteboard, wax paper, felt, or aluminum foil).
a. Raise the angle until the object slides at a constant velocity (Give it a
nudge to get it going!).
b. Measure the angle by using the height and hypotenuse of a right triangle
formed from the wooden ramp.
c. Run three trials and take an average.
d. Use the angle to mathematically determine the coefficient of friction.
Note: a quarter masses in at 5.68 g, on average.
3. For each material on the ramp, run three trials with the same object and then find
the µ for each angle using the shortcut. Finally, take an average to use in your
2
mathematical calculations.
4. Repeat steps 1-4 for different surfaces (whiteboard, wax paper, felt, or aluminum
foil).
5. Please use a free body diagram to calculate the value of the kinetic coefficient of
friction for an object moving at constant velocity down a ramp.
Coefficient of Static Friction
Whiteboard
Wax Paper
s


s
Trial 1
Trial 2
Trial 3
s =
s =
Aluminum Foil
s

Felt
s

Trial 1
Trial 2
Trial 3
s =
s =
Coefficient of Kinetic Friction
Whiteboard

Wax Paper
k
k

Trial 1
Trial 2
Trial 3
k =
k =
Aluminum Foil
k

Felt

Trial 1
3
k
Trial 2
Trial 3
k =
k =
4
Conclusions
1. Which material has the greatest static friction?
2. Which material has the least kinetic friction?
3. Do these results agree with your prediction? Explain.
4. Explain why some materials had a lot of friction and why the others had less.
5. What is the relationship, from this lab, between the coefficient of static friction
and the coefficient of kinetic friction for each of the materials? (i.e., use a greater
than or less than symbol)
6. Is this relationship found in #5 the expected relationship based on your notes?
Explain.
7. If the plane was positioned at 60° relative to the horizontal, what would be the
acceleration acting on your object on whiteboard? (Calculate Mathematically
showing ALL WORK using your coefficient of kinetic friction that you found
for your object on whiteboard)
5