Name: ___________________________________
DLesson11: Friction
Date: ___________
Period: _________
DLesson11: Friction
Notice the situation right before the shoe starts moving and right after.
a) Fill in the table that follows by constructing a force diagram for each team (two systems) for
these five situations.
Teacher has not
started pulling on
the shoe
Teacher starts
pulling on the
shoe, but the
shoe does not
start moving.
Teacher pulls
harder, but the
shoe still does not
move.
Teacher pulls
harder, and the
shoe just begins to
move.
Teacher pulls the
shoe with a slow
constant velocity.
b) Describe in words how the magnitude of the force that the floor’s surface exerts on the shoe
varies with the force exerted by teacher on the shoe.
c) Compare the magnitude of the force exerted by Teacher on the shoe (just before shoe starts
moving) to the magnitude when it is moving at a constant velocity. What do you observe?
d) What object is exerting this friction force for the scenarios given above?
e) Summarize your findings for the friction force exerted on an object at rest and on the same object
moving at a constant velocity.
Qualitative Observational Experiment 1
Instead of the block in the previous activity, you have rectangular blocks with different surface areas and
different types of surfaces on which the block slides horizontally.
The force that the string exerts on the block (as measured by the spring scale reading) when the block just
starts to slide is recorded in the table that follows. This force is equal in magnitude to the maximum static
friction force (as we discovered in the previous activity). Examine the data in the table that follows.
Mass of the block
1 kg
1 kg
1 kg
1 kg
1 kg
1 kg
Surface area
0.1 m2
0.2 m2
0.3 m2
0.1 m2
0.1 m2
0.1 m2
Quality of
surfaces
Medium smooth
Medium smooth
Medium smooth
A little rougher
Even rougher
Roughest
Maximum
static friction force
3.1 N
3.0 N
3.1 N
4.2 N
5.1 N
7.0 N
Draw a force diagram for the block at the moment just before it starts to move:
How does the maximum static friction force that the surface exerts on the block depend on the surface
area of the block?
How does the maximum static friction force that the surface exerts on the block depend on the roughness
of the two surfaces?
Adapted from A. Van Heuvelen and E. Etkina, PUM Dynamics Lesson 14
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Qualitative Observational Experiment 2
A spring scale pulls a 1 kg block over a medium smooth surface. The reading of the scale can be used to
determine the magnitude of the maximum static friction force—in this instance, the force when the block
starts to slide. In some experiments, a compressible spring also pushes vertically down on the block (see
the second block).
Spring pushes down
1 kg
1 kg
Use the data in the table to draw a graph of the maximum static friction force versus the normal force the
surface exerts on the block.
Extra downward
force exerted on the
1-kg block
0N
5N
10 N
20 N
Normal force
exerted upward by
the board on the
block
10 N
15 N
20 N
30 N
Maximum
static friction
force
3N
4.5 N
6N
9N
Static friction force
Force exerted upward by the Board on the Block
Adapted from A. Van Heuvelen and E. Etkina, PUM Dynamics Lesson 14
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14.6 Design an Experiment (On a separate sheet of paper)
Use PhET simulations to apply your understanding of friction forces. Go to Forces in 1
Dimension. Do not turn off Friction. Choose three objects and design experiments that will allow
you to determine the relationship between static and kinetic friction in the program.
a) Write a report in which you will describe the experiments and explain how you used the
results to determine the Maximum Static Friction Force and the Kinetic Friction
Force for a particular object (make sure to state what object you are exerting forces on).
b) Which one is greater? The Static Friction Force or the Kinetic Friction Force?
c) What is different about the Static Friction Force?
Did You Know?
When two objects touch each other, they exert a normal force on each other. In physics it is customary to
break the force that each exerts on the other into two forces – a force perpendicular to the surfaces and the
force parallel to the surfaces.
Normal force: This is the perpendicular component of the total force that one object exerts on the other
object. it points perpendicular to the surface of contact. Often one symbol N is used to denote this force
(do not confuse with the Newton, N). There is no equation for calculating the normal force. Its magnitude
must be determined for each situation by some other method.
Static friction force: The parallel component of the force that two objects exert on each other is called
a friction force. The friction force of the one object on the other object points parallel to the surfaces of
contact. If the objects are not moving with respect to each other, the friction force that they exert on
each other is static. The static friction force between two surfaces opposes the tendency of one surface
to move across the other and provides flexible resistance (as much as is needed) to prevent motion—up
to some maximum value. This maximum static friction force depends on the relative roughness of the
surfaces (on the coefficient of static friction µs between the surfaces) and on the magnitude of the
normal force N between the surfaces. The magnitude of the static friction force is always less than or
equal to the product of these two quantities:
Fs surface on object  s N
Kinetic friction force: The kinetic friction force between two surfaces is exerted parallel to the
surfaces and opposes the motion of one surface relative to the other surface. The kinetic friction force
depends on the relative roughness of the surfaces (on the coefficient of kinetic friction µk) and on the
magnitude of the normal force N between the surfaces:
Fk surface on object  k N
Adapted from A. Van Heuvelen and E. Etkina, PUM Dynamics Lesson 14
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