Meta data: Data logging, for the teacher, English, friction, energy, work
Friction forces – static friction and sliding friction
(for the teacher)
Level of difficulty: easy
Summary
This experiment demonstrates the friction between a small wooden block and a horizontal surface – both
static and kinetic friction. The experiment employs a force sensor and a data logger to demonstrate the
frictional force as a function of time. It is also possible to add a position sensor for a closer view of the
transition between static and kinetic friction.
Theory (Theoretical background)
In this experiment a wooden block is pulled along a horizontal surface by means of a force working
horizontally. Contact forces operating between the block and the underlying surface will offer resistance.
These forces are termed friction forces or just friction. For as long as the block is lying still (non-moving)
this friction is known as static friction. While the block is moving, the friction between the moving surfaces
is known as kinetic friction, also known as sliding friction or dynamic friction. The magnitude of the friction
depends to a great extent on the nature of the materials moving against one another.
F = pulling force
R = friction
As long as the block moves at constant speed, the sum of the forces acting upon it equals zero, i.e. F = 0.
This is an expression of Newton’s first law. Only two forces act upon the block: the pulling force (F) and the
friction force (R). If we apply an even pulling force (F) and maintain a slow, even speed, the pulling force
will be equal to the friction (R), both while the block is stationary as well as when it is moving. According to
Newton’s 1st law: F = F + R = 0  R = - F. The two forces R and F are equal in size, but they work in
opposite directions.
Just when the block starts moving, its speed does change; a small acceleration (a) occurs. At the
commencement of the movement, and according to Newton’s 2nd law there will be a small excess of force
in the same direction as the movement, since F = ma. (F = F + R = ma > 0  F > - R). When the speed is
low the acceleration will be small enough that we may assume F = - R throughout the experiment.
This experiment is set up to measure the force F and observe how the friction may change as a function of
time and the movement of the block.
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Meta data: Data logging, for the teacher, English, friction, energy, work
Technical considerations
This experiment only requires a force sensor for the data logger, but a position sensor may also be used. As
the pulling force (and the position) will change rapidly over time, the data logger should be adjusted to read
data 25 – 50 times per second. The logger should also be adjusted to ensure that the force sensor registers
the pulling force as positive (i.e. not pushing).
Equipment
- Wooden block with a small eye screw. NB! It is also possible to use a block from material other than
wood.
- Cotton thread and scissors
- A horizontal surface (may be an ordinary table)
- Data logger with force sensor, possibly a position sensor as well.
- Possibly a computer with software compatible with the data logger.
Arranging the equipment
The equipment should be arranged as shown in the photo:
The cotton thread connects the block (via the eye screw) to the force sensor.
Important: The force sensor will only return correct data if it works parallel to the surface and
in line with the cotton thread.
The force sensor must be reset to zero before each set of measurements. The sensor
has a reset button.
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Procedure
Start recordings while the cotton thread is still slightly slack. Move the sensor slowly and at constant speed
(1 – 3 cm/s) until the thread is tense, and then for a further 10 – 20 cm at the same, constant speed. Stop
recordings. Examine the graph on the data logger set to show the friction (= the pulling force) as a function
of time.
Arrangement and procedure if the position sensor is also used
Arrange the equipment as shown in the photo:
In this arrangement the position sensor measures the distance to the rear surface to the block only.
Important: The logger must be adjusted to record data from both sensors with equal frequency!
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Meta data: Data logging, for the teacher, English, friction, energy, work
In the computer the two graphs may look like the graphs below:
Questions for discussion
- Discuss these graphs section by section. In the illustrations above, section 1 would cover the period from
0- about 2 seconds, section 2: from 2-4 seconds, section 3: from 4-9 seconds, and section 4: after 9
seconds. What was taking place in each of these time sections?
- The red graph is not quite flat between 4.5 – 9 seconds. Discuss what might be the reason(s).
- Calculate the average sliding friction. The data logger will do this very easily, just remember to mark the
relevant area of the graph (between 4.5 – 9 seconds in the graph above) for calculating this average.
Hint: Select Tools  Statistics
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Meta data: Data logging, for the teacher, English, friction, energy, work
- Discuss why the red graph has a very obvious maximum value immediately before the block starts
moving. Read the value for maximum friction at rest, and compare it with the average sliding friction.
- If a position sensor was used: Is it possible to observe from the (green) graph whether the block was
moving at constant speed? Is it possible to see evidence of acceleration?
Variations on the experiment
- Repeat the experiment a number of times as identically as possible. Compare the results of recordings
of maximum friction at rest and average sliding frictions between the different experiments.
- Repeat the experiment, but pull the block at different speeds. The speed still needs to be as constant as
possible every time. Compare the results of average sliding frictions at different speeds.
- Repeat the experiment with a view to record an average sliding friction, but apply varying loads to the
block. First record the mass (m) of the block with different loads by weighing on electronic scales, and
then work out the gravity (G) using the formula G = mg, in which g = 9.81 N/kg. Experiment with at least
3 different loads. Before you start, invite the students to agree on a hypothesis for predicting results.
Draw a graph to demonstrate how the gravity (G) relates to the sliding friction (R).
- Look up the term friction coefficient () in a text book, or an encyclopedia. Check whether the formula
R =  ∙ N agrees with your findings.
But first, what is N? When measuring friction, it is customary to use the force between the body and
the surface, called the normal force (N), in calculations. This force acts at right angles (perpendicular) to
the surface, pointing upward. On a horizontal surface, N will be equal to the gravity G. (This follows
from Newton’s first law, as seen when only the forces G and N are at work.)
- Try to calculate the friction (R) and the friction coefficient () when using surface materials of different
quality.
- If you used a position sensor:
In this experiment, the position sensor was used only to measure the distance to the block over time.
The data logger is also able to calculate the average speed (v) for each experiment. Request a v-t graph
for each set of data which are already stored in the data logger.
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