Experiment
12
Static and Kinetic Friction
If you try to slide a heavy box resting on the floor, you may find it difficult to get the box
moving. Static friction is the force that is counters your force on the box. If you apply a light
horizontal push that does not move the box, the static friction force is also small and directly
opposite to your push. If you push harder, the friction force increases to match the magnitude of
your push. There is a limit to the magnitude of static friction, so eventually you may be able to
apply a force larger than the maximum static force, and the box will move. The maximum static
friction force is sometimes referred to as starting friction. We model static friction, fs, with the
inequality
fs  s FN where s is the MAXIMUM coefficient of static friction and FN the normal force
exerted by a surface on the object. The normal force is defined as the perpendicular component
of the force exerted by the surface. In this case, the normal force is equal to the weight of the
object.
Once the box starts to slide, you must continue to exert a force to keep the object moving, or
friction will slow it to a stop. The friction acting on the box while it is moving is called kinetic
friction. In order to slide the box with a constant velocity, a force equivalent to the force of
kinetic friction must be applied. Kinetic friction is sometimes referred to as sliding friction. Both
static and kinetic friction depend on the surfaces of the box and the floor, and on how hard the
box and floor are pressed together. We model kinetic friction with fk = k FN, where k is the
coefficient of kinetic friction.
In this experiment, we used a Force Sensor to study static friction and kinetic friction on a
wooden block.
OBJECTIVES


Observe the difference between static and kinetic friction graphically
Measure the coefficients of static and kinetic friction for a particular block and track.
PRELIMINARY QUESTIONS
1. In pushing a heavy box across the floor, is the force you need to apply to start the box moving
greater than, less than, or the same as the force needed to keep the box moving?
2. Draw a free body diagram of a block being pulled at a constant velocity across a frictional
surface.
3. We used a force sensor to measure the applied force you use in pulling the block at a constant
speed. Use the free body diagram to develop an equation to show how you can determine the
value of the frictional force.
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Experiment 12
4. Use the free body diagram to develop an equation to show how to calculate normal force
(which is part of the friction equation).
PROCEDURE
1. The following set up was used:
Mass
Wooden block
Dual-Range
Force Sensor
Pull
2. During the first set of data, the wood block was pulled without the mass and for the
subsequent sets of data, a mass was placed on top of the block. The surface used was a plank
of wood.
3. When the block was pulled, it made a graph similar to the one pictured in #1 of the Analysis
questions. You can see graphically where the static friction force is growing, then it peaks
and the block begins to move. You can see that the kinetic friction is lower than the peak
static friction as we pull the block at a constant velocity the rest of the way down the board.
4. Various values of peak static friction and kinetic friction were obtained in placed in the data
table for you.
5. Use the equation you determined in preliminary question #4 to calculate the normal force and
enter the values in the data table.
6. You can see in the data table that 3 trials were run with the block by itself, then with a 1 kg
mass attached, then with a 2 kg mass attached. Determine the average friction force for each
trial.
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Physics with Computers
Static and Kinetic Friction
DATA TABLE
Mass of block
Total
mass
(kg)
Normal
force
(N)
Trial 1
0.342 kg
Peak static friction
Trial 2
Trial 3
0.342
1.542
1.369
1.469
1.342
3.946
3.554
3.828
2.342
5.433
5.102
5.112
Trial 1
Kinetic friction
Trial 2
Trial 3
0.342
0.869
0.944
0.905
1.342
2.998
3.015
3.027
2.342
4.865
4.389
4.807
Total
mass
(kg)
Normal
force
(N)
Average
maximum
static
friction
(N)
Average
kinetic
friction
(N)
ANALYSIS
1. Here is an actual graph that was made using the procedures above.
Inspect the force vs. time graph. Label the portion of the graph corresponding to the block at
rest, the time when the block just started to move, and the time when the block was moving at
constant speed.
2. Still using the force vs. time graph you created in Part I, compare the force necessary to keep
the block sliding compared to the force necessary to start the slide. How does your answer
compare to your answer to question 1 in the Preliminary Questions section?
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Experiment 12
3. If you look at the equation fs = µsFN, you should notice that it has the form y = mx, where y is
represented by fs, x is represented by FN, and m (the slope) is represented by µs. If you graph
the average maximum static friction force (vertical axis) vs. the normal force (horizontal
axis), and draw a best fit line, then the slope of this graph is the coefficient of static friction,
s.
Get some graph paper and plot your points and determine your line of best fit. Pick two good
points and find the numeric value of the slope, µs, including any units. Show your
calculations here.
4. Now make another graph to find the coefficient of kinetic friction k. (Repeat question 3—
just use kinetic numbers instead of static.) Show your calculations here. (Make sure to attach
your graphs when you submit your lab report.)
For #3 and 4, there are a few options available to make things easier. Instead of calculating the
slope, you can put the data into a calculator to get a best fit line. Or you can use my
computers and use Logger Pro to create the graphs and use a line of best fit to determine the
slope.
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Physics with Computers