Eastern Mediterranean University
Department of Mechanical Engineering
Laboratory Handout
COURSE: RIGID BODY Dynamics (MENG233)
Semester: Fall (2016-2017)
Name of Experiment: Measurement of Static and Kinetic Coefficients of Friction
Assistants: Poorya Ghafoorpoor Yazdi
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: Poorya Ghafoorpoor Yazdi
1. OBJECTIVES
The aim of the experiment is to measure the coefficients of static and dynamic friction and to investigate the laws that govern friction.
2. APPARATUS
Data collector, force sensor, wooden block, weights.
Figure 1: Friction set-up
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:
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.: μ³(Frictional Force) / (Normal Force).
The coefficient of friction is independent of the area of contact.
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. 1 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: F – μ N = 0.
Therefore, μ = F/N.
Next, we explore if there is a substantial change in m 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 cosq. Balancing forces along the inclined plane when the block is about to move up the plane, we obtain:
F – mN – Mg sinq = 0.
Here
N = Mg cosq
Substituting for N, we obtain:
m = (F – Mg sinq)/N.
(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.)
Kinetic Friction:
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 accelerate to the right and you will erroneously measure a higher kinetic frictional force. How does mk compare with ms.
Using the smaller of the two surfaces, determine the mk (and time permitting static friction) how does it compare with mk using the larger surface?
4. WORK TO BE CARRIED OUT
Set up the equipment as shown in the figure 1. Place the track on a flat, horizontal surface.
• Place the felt friction accessory tray on the track with the felt down.
• Tie a short piece of string between the friction tray and the hook on the Force Sensor.
Make sure you can see the graph labeled Friction Forces vs. Time. Hold the Force Sensor horizontally a few inches above the track. With slack in the string, press the zero button on the sensor. To zero the Force Sensor when measuring horizontally, with the Force
Sensor oriented horizontally, press the zero button. For a second run, add a 250-gram mass bar to the friction tray and perform the above procedure again. Don’t forget to zero the sensor before recording data. 6. For a third run, add a second 250-gram mass bar to the friction tray and perform the above procedure again.
5. EXPERIMENTAL DATA
Mass / Normal force / Fk / Fs / Static FrictionCoefficient / Kinetic Friction Coefficient
Table1: Observed and recorded data
6. Vocabulary
Use available resources to find the definitions of the following terms:
Friction:
Kinetic (sliding) friction:
Normal force:
Static friction:
7. Predict
1. Does is take more force to make a stationary object start sliding or to make a sliding object continue sliding?
2. How do you expect the friction between felt and aluminum to compare to the friction between cork and aluminum?
3. How do you expect the friction between the trays and the track to change as the trays and track are pressed more firmly against each other (that is, as the normal force increases)?
8. DATA ANALYSIS
Determine the coefficients. Uncertainties should be taken into account to have the preciseness and error analysis in your reports. Write the error analysis.
9. GRAPH
Plot the graphs of friction forces versus time.
10. ANSWER THE FOLLOWING QUESTIONS
- What are the units of μs and μk? Describe in words the meaning of μs and the meaning of μk?
- Compare the values of μs to the corresponding values of μk. Evaluate your prediction #1 in light of this comparison?
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- Compare the friction coefficients for felt on aluminum to the coefficients for cork on aluminum. Evaluate your prediction #2 in light of this comparison?
- What happens to the friction forces as the normal force between the tray and track is increased? Evaluate your prediction #3?
11. DISCUSSION, CONCLUSION & ANSWER TO QUESTIONS
State your observation during the experiment. Support your memo by mentioning the error sources which affect the accuracy of results and analysis.