Download Lab 2

Eastern Mediterranean University
Department of Mechanical Engineering
Laboratory Handout
COURSE: Dynamics ME 233
Semester: Spring (2006-2007)
Name of Experiment: Measurement of the Coefficients of Static and
Dynamic Friction
Instructor: Assoc. Prof. Dr. Fuat Egelioglu
Assistant: EHSAN KIANI
Submitted by:
Student No:
Group No:
Date of experiment:
Date of submission:
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EVALUATION
Activity During Experiment & Procedure
30 %
Data , Results & Graphs
35 %
Discussion, Conclusion & Answer to Questions
30 %
Neat and tidy report writing
5%
Overall Mark
Name of evaluator: EHSAN KIANI
1.
OBJECTIVES
The aim of the experiment is to measure coefficient of static and dynamic friction and to
investigate the laws that govern friction.
2.
APPARATUS
Wooden block A with a hook attached, a plane piece of wood B with a grooved wheel C
at one end, scale-pan S, light string, weights, boxes of weights, spring balance.
Fig. 1
3.
THEORY
We encounter friction at almost all times during the day. Friction between our foot and
the floor helps us walk. In spite of its importance, friction is still not well understood. However,
empirical laws describe the friction between two surfaces. These laws are as follows:
1.The ratio of the maximum frictional force and the normal force is a constant and equals the
coefficient of friction, μ, and depends only on the nature of the two surfaces in contact.
I.e.: μ(Frictional Force) / (Normal Force).
Fig. 2
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2.The coefficient of friction is independent of the area of contact.
3.The coefficient of kinetic friction μ k (the object is in motion) is lower than the coefficient of
static friction μ s(the object is stationary.)
We will first use the configuration shown in Fig. 2 to determine the coefficient of static and
kinetic friction between a few surfaces. Here, the normal force N = Mg, obtained by balancing
forces in the vertical direction on the block. Recall that the pulley only changes the direction of
force but does not change its magnitude. Balancing forces in the horizontal direction, we
obtain:
mg – μ N = 0.
Therefore,
μ = m/M.
Next, we explore if there is a substantial change in  if the surface on which the block is
sliding is at an angle to the horizontal. In this case the normal force N is not equal to Mg,
but rather to Mg cos. Balancing forces along the inclined plane when the block is about to
move up the plane, we obtain:
mg - N – Mg sin = 0 .
Substituting for N, we obtain:
 = (m/M – sin)/cos .
(Note: When the block is about to move downwards, the direction of the frictional force
is in opposite direction and therefore you will have to modify the formula appropriately.)
Fig. 2
Kinetic Friction:
6. Next, we determine the coefficient of kinetic friction (you may use either the
wooden or felt side.) The procedure is the same as before, except that after adding an
incremental mass to the hanger, give a gentle push to the block. If the block moves away
with a constant speed, then the tension in the string corresponds to the kinetic frictional
force. Note, you should take care not to add too much mass in which case the block will
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accelerate to the right and you will erroneously easure a higher kinetic frictional force.
How does k compare with s.
7. Using the smaller of the two surfaces, determine the k (and time permitting
static friction) how does it compare with k using the larger surface?
Friction on an inclined plane:
8. Tilt the Aluminum track through approximately 30 o (you may use the angle
indicator to approximately set this angle but, measure the height and length of an
appropriate angle to determine the angle more accurately.) You may use the larger side of
the wooden block for these measurements. (Does the block move up or down the slope
with just the hanger in place? If the block moves up the incline, chose a larger angle of
inclination such that the block moves downwards.) Add masses in small increments to the
hanger so that the block stops sliding down. Which direction is frictional force acting?
Determine the coefficient of friction.
9. (Optional) Next, add small increments of masses on the hanger, the block
will be stationary, and then at a critical mass m, the block will move up the slope. Does the
frictional force change as you add masses? Which direction is the frictional force pointing?
Determine the coefficient of friction and compare to the values you obtained earlier for the
same surfaces. What can you conclude from your experiments regarding the nature of the
coefficient of friction and the its dependence on the type of materials and conditions.
Questions:
1. In which direction does the frictional force act under your foot as you are
walking forward?
2. Can the coefficient of friction be greater than 1.0?
3. How does your measurement of static and coefficient of friction explain the
superiority of anti-lock brakes (as opposed to regular brakes?)
4.
WORK TO BE CARRIED OUT
Weight the block A and the scale-pan S on the spring balance. Attach the scale-
pan to the hook of A by light string passing round the wheel C. Mark the position of A on
board B with pencil. Then gently add increasing weighs to S until A, and by adding
increasing weights to S, again record the total weight in S when A begins to slip. Repeat
for two more increasing weights on A, returning the block A to its original place on B
each time.
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In Dynamic friction with the apparatus shown in the Fig.1 place a weight on S and give
A a slight push towards C. Add increase weight to S, giving A a slight push each time. At
some stage, A some stage A, will be found to continue moving with a steady, small
velocity. Record the corresponding weight in the scale-pan S. Now increase the reaction
of B by adding weight to A, and repeat for two more weights on A, returning the block to
its original place on B each time.
5.
EXPERIMENTAL DATA
Normal
reaction R/gf
6.
Weight in scale-pan
on slipping /gf
Limiting
frictional
force, F/ gf
DATA ANALYSIS
Fill out third to sixth column of the above table.
7.
GRAPH
Plot F’ v. R.
The gradient, a/b=  ’=….
8.
DISCUSSION AND CONCLUSION
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Dynamic
Normal
reaction
R/gf
Weight in
scale-pan on
moving A
/gf
Friction
force, F’
/gf