Download Static and Kinetic Friction Lab

Static and Kinetic Friction Lab
Introduction
The amount of friction force (Ffr) between two surfaces in contact depend on the nature of the
surfaces in contact () and the amount of compression between the surfaces (FN) according to
Ffr =  FN
Objectives
 Measure the coefficients of static and kinetic friction for cork and track.
 Determine if the coefficient of kinetic friction depends on weight.
Materials
computer
Vernier computer interface
Logger Pro
Vernier Force Sensor
Protractor
string
Wood block with cork base
balance or scale
mass set
metal ramp
Procedure
Part I Peak Static Friction and Kinetic Friction
1. Measure the mass of the block and record it in the data table.
2. Connect the Dual-Range Force Sensor to Channel 1 of the interface. Set the range switch on
the Force Sensor to 50 N.
3. Open the file “stat and kin friction” from the group share folder.
4. Tie one end of a string to the hook on the Force Sensor and the other end to the hook on the
wooden block. Practice pulling the block and masses with the Force Sensor using a straightline motion (see below): Slowly and gently pull horizontally with a small force. Very gradually,
increase the force until the block starts to slide, and then keep the block moving at a constant
speed.
Mass
Wooden block
Dual-Range
Force Sensor
Metal ramp
Pull
5. Hold the Force Sensor in position, ready to pull the block, but with no tension in the string.
Click
to set the Force Sensor to zero.
6. Click
to begin collecting data. Pull the block as before, taking care to increase the
force gradually. Repeat the process as needed until you have a graph that reflects the
desired motion, including pulling the block at constant speed once it begins moving.
7. Examine the data by clicking the Statistics button, . The maximum value of the force occurs
when the block started to slide. Read this value of the maximum force of static friction from
the floating box and record the number in your data table.
8. Drag across the region of the graph corresponding to the block moving at constant velocity.
Click on the Statistics button again and read the average (or mean) force during the time
interval. This force is the magnitude of the kinetic frictional force.
9. Repeat Steps 6-8 for two more measurements and average the results to determine the
reliability of your measurements. Record the values in the data table.
10. Add masses totaling 250 g to the block. Repeat Steps 6 – 9, recording values in the data
table.
11. Repeat for additional masses of 500, 750, and 1000 g. Record values in your data table.
Part 2 Peak Static Friction for an incline
1. Put the block on the horizontal ramp.
2. Slowly increase the ramp’s angle of inclination to the horizontal until the block just begins to
slide.
3. The largest angle at which the block does not slide is called the critical angle, c. Measure this
with a protractor and enter in the table.
4. Repeat two more times. Average the results.
5. Repeat 1 – 4 with 1000 g mass taped securely to the top .
DATA TABLE
Part I Peak Static and Kinetic Friction
Mass of block (kg)
Total
mass
(kg)
Normal
force
(N)
Total
mass
(kg)
Normal
force
(N)
Peak static friction (N)
Trial 1
Trial 2
Trial 3
Average
peak static
friction
(N)
Kinetic friction (N)
Trial 1
Trial 2
Trial 3
Average
kinetic
friction
(N)
Part 2 Peak Static Friction for an Incline
Total
mass
(kg)
Critical Angle (c)
Trial 1
Trial 2
Trial 3
Average
Critical Angle
(c)
Analysis
1. For Part I, calculate the normal force of the table on the block alone and with each
combination of added masses. Since the block is on a horizontal surface, the normal force
will be equal in magnitude and opposite in direction to the weight of the block and any
masses it carries. Fill in the Normal Force entries for both Part I data tables.
2. Plot a graph of the maximum static friction force (vertical axis) vs. the normal force (horizontal
axis). Use either Logger Pro or graph paper. Include full title, axis labels and units.
3. Since Fmaximum static = s FN, the slope of this graph is the coefficient of static friction s.
Find the numeric value of the slope, including any units. Should a line fitted to these data
pass through the origin?
4. In a similar graphical manner, find the coefficient of kinetic friction k. Create a plot of the
average kinetic friction forces vs. the normal force. Recall that Fkinetic = k FN. Should a line
fitted to these data pass through the origin?
5. Does the force of kinetic friction depend on the weight of the block? Explain.
6. Does the coefficient of kinetic friction depend on the weight of the block?
7. For Part 2, derive the maximum angle for which the block will not slide down the ramp
c = tan-1(µs)
where µs is the coefficient of static friction
8. Using your measured critical angle, determine µs
9. Compare (% difference) your coefficients of static friction determined in Part 1 to that
determined in Part 2. Discuss the values. Do you expect them to be the same or different?
Static and Kinetic Friction Lab
Scoring Rubric
Name(s) _____________________________________________ Class ______
1. Data Tables filled out correctly
____________ 15 pts
2. Graph of the maximum static friction force (vertical axis)
vs. the normal force. Title, fully labeled, linear fit with
slope.
____________ 10 pts
3. Stated coefficient of static friction s from graph
____________ 2 pts
4. Graph of the average kinetic friction forces
vs. the normal force. Title, fully labeled, linear fit with
slope.
____________ 10 pts
5. Stated coefficient of kinetic friction k from graph
____________ 2 pts
6. Explained whether the force of kinetic friction depends on the weight
of the block
____________ 2 pts
7. Explained whether coefficient of kinetic friction depends on the weight
of the block?
____________ 2 pts
8. Derived maximum angle before slipping
c = tan-1(µs)
____________ 4 pts
9. Show how used critical angle to determine the coefficient of
static friction
____________ 3 pts
10. Compared the two calculated values for the coefficient of
static friction
____________ 5 pts
Total
____________ 55 pts