HOUSTON COMMUNITY COLLEGE
SYSTEMS SOUTHWEST COLLEGE
COLLEGE PHYSICS I – PHYS 1401
Experiment Three
Coefficient of Friction
Professor: Robert Hage
PRE LAB QUESTIONS
Due before lab begins.
1) Explain briefly the different types of frictional forces.
2) How does the coefficient of friction depend upon the area of contact? Explain.
3) Give some examples in which the force of friction causes the object to accelerate.
4) If you push on a heavy box that is at rest, you must exert some force to start its motion.
However, once the box is in motion you need a smaller force to maintain that motion. Why?
5) Find the force needed to pull a mass of 30 kg at constant velocity on a rough incline
making an angle of 400 with horizontal and with µk = 0.3
OBJECTIVE
The purpose of this experiment is to calculate the coefficients of friction between a wooden block and
a board as they slide relative to each other. We use the board in a horizontal position and then as an
inclined plane.
MATERIALS
Wooden Block
Wooden Board
Balance
Protractor
Weight hanger
Slotted weights
Pulley
INTRODUCTION
In general, friction is the force that slows down the motion of an object. The force of friction is directed
along the surface of contact between the object and surface and directed opposite to the direction of
motion of object. We deal with:
a) Static friction ( fs)
This exists when the object is at rest relative to the surface. This force must be overcome in order to
make the object start moving. It is given by fs = µs n.
b) Kinetic friction ( fk)
This exists when the object is in motion and is given by fk = µk n.
where µs and µk are the coefficient of static and kinetic friction and
n is the normal force which presses the two surfaces together. In
general, µs > µk because it takes a larger force to start an object
sliding (static friction) than to keep it sliding (kinetic friction).
They depend only on the materials in contact and independent of the
area of contact.
.
In this experiment, using the board in a horizontal position, we
measure the frictional force fk and fs as they vary with respect to the
normal force n. Using the second law of motion, we can calculate the coefficients of friction between
block and board.
Another way to find µs is to set up the board as an incline plane.
The coefficient of friction µs is related to the maximum angle θm to
which the board can be elevated before the block starts its motion is:
θm
µs = tan θm
(1)
EXPERIMENTAL PROCEDURE
1) Weigh the wooden block on the triple beam balance and record its weight
2) Set up the block on the board as shown. Make sure you
wipe away any dust for the surfaces. Put a mass of 200
g on the surface of the block. Slowly increase the load
on the hanger until the block starts slowly moving with
constant speed after given a small push. Don’t forget to
include the mass of the hanger.
Repeat the above procedure for masses of 400 g, 600 g, 800 g and 1000 g.
With this data we can find the coefficient of kinetic friction.
3) Set up the block again as in procedure 2 and place a mass of 600 g on it. Place weights gently on
the hanger and increase them slowly until the block just starts its motion WITHOUT any push.
Do a total of 3 independent trials and record your data.
With this data we can find the coefficient of static friction.
4) Remove the pulley and set up the board as an inclined plane. With the block on it, gently and
smoothly tip the board until block starts to move. Measure the angle θm with a protractor and
record it in the table. Do a total of 3 independent trials.
REPORT FORM
Weight of block _________
Procedure 2 Normal and Friction force for kinetic friction
Mass on block
200 g
400 g
600 g
800 g
1000 g
Total Normal Force, N Friction Force, N
µk from graph ________
Procedure 3
Mass on block
600 g
600 g
600 g
Averages
Normal and Friction force for static friction
Total Normal Force, N
Friction Force, N
µs
Deviation
Procedure 4 Inclined plane for static friction
Measurement
1
2
3
Averages
Angle θm
Percent difference ( from procedure 3 and 4)
Between the two values for µs
______________
µs
Deviation
CALCULATION
1) From procedure 2, plot the points and draw the best graph of the friction force versus the normal
and from the graph find the coefficient of kinetic friction between block and board.
2) From procedure 3, calculate the value for µs between block and board, find their average,
deviation and average deviation.
3) From procedure 4, calculate the value for µs between block and board, find their average,
deviation and average deviation. Compare the two values for µs for wood on wood by
calculating their percent difference which is given by:
Percent difference = [difference of the two values/ average] x 100 %
POST LAB QUESTIONS
1) Explain why the block should move at constant speed in procedure 2.
2) Derive equation 1, µs = tan θm
.
( apply 2nd law of motion for constant velocity)
3)
In procedure 4 , find the acceleration of the block as it breaks away if µk = 0.2 and θm = 200
4)
Compare your experimental value with the book value for the coefficient of kinetic friction for
wood( µk = 0.2). Find the percent error and give reasons for it.
5)
Is the frictional force in this experiment only due to the surface of contact between block and
board? Explain.