Maxwell Wright
Physics 2215
Static and Kinetic Friction
Objectives
The objectives for the experiment are:
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Use a Dual Range sensor to measure the force of static friction.
Determine the relationship between the force of static friction and the weight of an
object.
Measure the coefficients of kinetic and static friction for a particular block and track.
Use a Motion Detector to independently measure the coefficient of kinetic friction and
compare it to the previously measured value.
Determine if the coefficient of kinetic friction depends on weight.
Equipment
The equipment needed for this experiment included: a computer, a Vernier computer interface, Logger
Pro software, Vernier Motion Detector, Vernier Force Sensor, string, a block of wood with a hook, a
scale, a mass set, and a large book.
Preliminary Questions
1. When pushing a heavy box across the floor, the force needed to start moving the box is greater
than the force needed to keep the box moving. This assumption is based on personal experience
and the concept of activation energy in chemistry.
2. The force of friction is proportional to the normal force acting on the box, which is related to the
weight of the box.
Procedure
Part 1
1. We measured the mass of the block with the scale.
2. We connected the Vernier Force Sensor to channel 1 of the computer interface and set the
recording range from 0 to 50 N.
3. We opened the “012 Static Kinetic Frict” file in the Physics with Vernier folder of the software.
4. We attached the Force Sensor to one end of a piece of string and the hook on the block of wood
to the other end. We put 1 kg of mass from the mass set on the block. We pulled the force
sensor in a horizontal line slowly on the table until the block began to move and then continued
pulling for one second at a constant velocity.
5. We sketched a graph of the force vs. time used to pull the block.
6. We held the Sensor with no tension in the string and zeroed the reading in the program.
7. We started the readings and pulled the block slowly across the table as in Step 4.
Part 2
8. We removed that masses that had been on the block.
9. We repeated step 7.
10. We used the statistics functions in the software to find the maximum force used to pull the
block, the maximum force of static friction.
11. We highlighted the part of the graph where the block was moving at constant velocity, and used
the statistics function to find the mean value of force used during that time, the force of kinetic
friction.
12. We repeated steps 9-11 two more times and averaged the results of these three runs.
13. We added 250 g from the mass set and repeated steps 9-12.
14. We repeated this process for masses of 500 g, 750 g, and 1000 g.
Part 3
15. We connected the Motion Detector, set to “track” to the computer with the interface.
16. We opened the “12b Static Kinetic Fric” file in the software folder.
17. We placed the detector about 1.5 m from the block of wood on the table inline with the
direction of motion..
18. We practiced sliding the block towards the detector so that it slid about 1 m before stopping.
19. We collected data form a run that produced a freely sliding block.
20. We used the linear fit capability of the program to find the slope of the velocity vs. time graph in
the area when the block was freely sliding, and recorded this value for acceleration.
21. We repeated steps 19 & 20 four more times.
22. We placed 500 g from the mass set on the block and repeated steps 19 & 20 five times.
Data
Part 1
Mass of Block 208
Part 2
Total
Mass (kg)
Normal
Force (N)
Peak Static Friction
Trial 1
Trial 2
Trial 3
Average Peak
Static Friction (N)
0.208
0.458
0.958
1.458
1.958
2.04
4.493
9.388
14.288
19.188
0.4517
0.9033
1.452
2.710
3.097
0.3872
0.9033
2.258
2.742
3.903
0.3997
0.9248
1.893
2.629
3.87
Total
Mass (kg)
Normal
Force (N)
Kinetic Friction
Trial 1
Trial 2
Trial 3
Average Peak
Static Friction (N)
0.208
0.458
0.958
1.458
1.958
2.04
4.493
9.388
14.288
19.188
0.2435
0.5297
0.9769
1.643
2.565
0.2322
0.5654
1.274
1.820
2.631
0.2349
0.5303
1.2073
1.761
2.601
Part 3
Kinetic Friction
0.3549
0.9678
1.968
2.936
3.871
0.2290
0.4957
1.38
1.819
2.603
Block with No Additional Mass
Trial
1
2
3
4
5
Acceleration
(m/s2)
4.456
4.001
3.496
3.573
3.680
Kinetic Friction Force
(N)
0.9456
0.8322
0.727
0.7432
0.765
Average coefficient of kinetic friction
μk
.454
.408
.364
.364
.375
.391
Block with 500 g Additional Mass
Trial
1
2
3
4
5
Acceleration
(m/s2)
3.954
4.619
4.116
4.607
4.5
Kinetic Friction Force
(N)
2.545
3.27
2.914
3.262
3.186
Average coefficient of kinetic friction
μk
.366
.471
.42
.469
.458
.437
Analysis
1. The block began to move at about 4 seconds, and moved at a constant speed from 54.2 second
on.
2. From the Force vs. Time graph, the force necessary to keep the block sliding is less than the
force necessary to start the block moving. It agrees with the supposition in the Preliminary
Question.
3. From the Force vs. Time graph, I would conclude that μs is greater than μk.
4. The normal forces exerted by the table on the block are listed in the data tables.
5. The maximum static friction force vs. normal force is given onstatic friction graph is .2141.
6. The graph of the maximum static friction force vs. normal force should pass through the origin,
because they are proportional and there is no friction with no weight (normal force).
7. The coefficient of kinetic friction from plotting average kinetic friction vs. normal force , graph
3, gives a coefficient of kinetic friction of .1204 . This value should also produce a graph that
go through the origin
8. A free body diagram for the block is given below the graphs.
9. The coefficient of kinetic friction for each trial is in the data tables.
10. The coefficient of kinetic friction does not depend on speed but it does depend on
acceleration..
11. The force of kinetic friction depends upon the weight of the block.
12. The coefficient of kinetic friction does not depend on the weight of the block.
13. The coefficient of kinetic friction from the part 3 is .391 for a block with no additional mass
and the value from part 2 is .1204 for a block with no mass.
The coefficient of kinetic friction from part 3 is .437 for a block with 500 g mass. We did not
use a block with 500 g mass in part 2, but an value from the slopes from a block with 250 g and
750 g is .2141.
Conclusions
Our vales for the coefficient of static friction and the coefficient of kinetic friction varied wildly between
the two methods of experimentation. This can be attributed to human error from inconsistencies in
pushing the block and the use of differing sensors and setups.
Extensions
1. We tried to test whether or not the surface area of the the block affected the coefficient of
static friction or the force of friction by putting the block on its side and moving it. The block
kept toppling over or moved too close to the sensor to get a reading.
2. To examine the force of static friction on an incline we held a large book up at one end and
measured the angle with a protractor when the block began to slide. We found this angle to be
15o and using a program on a laptop calculated a coefficient of friction of .268. The problem
with this experiment was that the book had a different surface than the table and so we cannot
truly compare the values with those of the earlier experiments.