Real World Applications For Friction and Lubrication
Author:
Date Created:
Subject:
Level:
Standards:
Lucas J.T. Landherr
Fall 2009
Physics
9-12
New York State – Physics
(www.emsc.nysed.gov/ciai/)
STANDARD 1 – Analysis, Inquiry, and Design
STANDARD 4 – The Physical Setting
STANDARD 6 – Interconnectedness: Common Themes
STANDARD 7 – Interdisciplinary Problem Solving
Schedule: 3- 75 minute blocks
Objectives:
Determine the effect of load, contact
area, material, and sliding angle on
static friction. Investigate lubrication
properties and determine what
characteristics make a good lubricant.
Students will:
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Supplement their knowledge of
friction provided in their classes
with real-world applications and
examples.
Learn how and why friction can be
increased or reduced.
Test the effect of several parameters
on friction.
Test the lubrication capabilities of
various materials.
Calculate the friction force and
friction coefficient based on angled
and lateral movement.
Evaluate the properties of a good
lubrication system.
Vocabulary:
Friction
Lubrication
Force
Adhesion
Polymers
Contact Area
Load
Materials:
For Each Group of
Three Students:
Other Materials
Required:
String
Sliding Disc
Wood Block
Paper
Sandpaper
Pulley
Frictionless Surface
(hard smooth
plastic sheet with
low roughness)
Weight Stand
Weights (1g to 1kg)
Book
Various Lubricants
chosen by students
8 feet of tubing,
either glass or
smooth rubber
Water
Polyethylene Glycol
(purchasable from
Fisher Scientific or
another lab
equipment
company)
Rubber stoppers
Clipboard
5 strips of different
types of material
Safety:
Safety concerns with this activity depend on the
lubrication materials chosen by the students.
Science Content for the Teacher:
Basic Friction
For basic friction systems, the friction force (FF) is proportional to the normal load
(N)
.
FF = µ N
This law assumes that the static friction coefficient, µ, remains constant at an
interface between two objects regardless of the load.
In order for an object to be able to move along another surface, the driving
force on the object must be larger than the friction force. At the exact force that
an object begins to slide, the driving force will be equal to the friction force.
When an object is freely moving across a surface (the driving force is
much larger than the friction force), kinetic friction is said to be occurring. When
an object will begin to slide at a constant rate (the driving force is equal to the
friction force), static friction is taking place.
The force of gravity (Fg) on an object:
is related to Newton’s second law:
Here m is the mass of the object, g is the gravity
acceleration, and F is a general force.
The force of gravity on an object can be equal to the
rest if the object is on a flat planar surface.
Fg = mg
F = ma.
acceleration constant, a is
normal force of an object at
N = Fg
Inclined Plane
For friction on an inclined plane, the perpendicular and normal forces acting on a
sliding object need to be calculated from the angle of the plane (θ).
The parallel force (F║) on an object on an inclined plane is found by:
F║ = sin (θ) Fg
The perpendicular force (F┴) on an object on an inclined plane is found by:
F┴ = cos (θ) Fg
The normal force (FN) on the object is the opposite of the perpendicular force:
FN = - F┴
The friction force at the point of static friction is equal to the sum of the driving
force and the parallel force, assuming proper negative and positive directions for
the forces.
These forces are illustrated in Figure 1.
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θ
F┴
Fg
Driving force
F║
θ
Figure 1. Illustration of the forces at work for a body on an inclined plane.
Complex Friction
If a non-solid object is introduced as one of the two materials at an interface,
other parameters may begin to have effects on the friction that would not be seen
for basic systems. Specifically, the contact area at an interface may become
important if the two materials are highly adhesive, meaning they cannot be
separated easily in response to vertical forces.
Drag Reducing Polymers
When a fluid moves through a pipe at high velocities, eddies or turbulence with
the pipe can develop that increase the friction of the fluid. This turbulence
develops as a result of excess kinetic energy contained within the walls of the
flow. A phenomenon known as ‘drag reducing polymers’ exists where a dilute
polymer concentration can reduce the friction in the fluid by up to 80 percent.
This occurs as a result of the kinetic energy within the fluid being converted by
the polymer chains into elastic energy, thus elongating the polymers and
reducing the overall fluid friction.
A number of real-world applications exist in relation to this phenomenon.
Naval vessel travel and firefighting equipment are more common examples. The
use of biodegradable polymers to help increase blood flow rate is also currently
under investigation.
Lubrication
Lubrication can result in friction changing from simple sliding friction. Rolling
friction can occur when one of the surfaces will roll in response to horizontal
movement, like ball bearings underneath a large block. Plowing friction occurs
when a hard object will move through and disrupt a loose layer of material.
Chemical reactions can also occur to permit sliding to continue
The success of a liquid lubricant is directly related to its viscosity: too thick
will increase friction and prevent sliding, and too liquid-like would cause the
lubricant to spread too easily.
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The objective of this study is to reinforce students’ knowledge and understanding
of the concept of friction, to help them recognize the many practical applications
of friction that exist in their daily lives, and to help them take the concept of
friction and make a conceptual leap to lubrication effects and properties.
Preparation:
Day One: Setup the drag reducing polymer apparatus. Mix small concentrations
of polyethylene glycol with water (approximately 1g per L as a maximum solution
concentration, with lower concentrations to 0.01g per L also possible with
diminished effectiveness) the previous night to allow the polymer to completely
dissolve. Make sure that each group has a frictionless surface, a wood block,
string, a set of weights, paper, and sandpaper at their laboratory station for
experimentation.
Day Two: Have each group’s station prepared as the previous day, but also with
books or some other means of creating a slope with the frictionless surface.
Have examples of different household items that could be investigated as
lubricants (e.g. deodorant, baking powder, ketchup, toothpaste). Set up the
clipboard demonstration.
Day Three: Prepare for potential clean-up of the students’ lubricant systems.
Replace the wood blocks at each laboratory station with plastic discs.
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Classroom Procedure:
Day One:
Engage (Time: 20 min)
Remind students about their recent friction studies in the classroom. Ask
students to provide examples of friction at work in solid, liquid, and gaseous
systems. Provide visual examples either in the form of pictures or hand-held
items; for example, a system of mechanical gears or syringes represents solidsolid friction, a boat or swimsuit represents solid-liquid friction, and an airplane or
other flying craft represents solid-gaseous friction.
Ask students what the sources of friction are and how they were
represented or visualized in the examples discussed. Explained how friction can
be represented as forms of energy, such as the transition from kinetic to heat in
solids, or from kinetic into tension in rubbery materials.
Reinforce these examples discussed in class with a demonstration of drag
reduction in fluid flow. In this demonstration, water is poured into a beaker and
allowed to flow down a vertical pipe and out a horizontal pipe, as illustrated in
Figure 2. The distance that the water spouts out of the horizontal pipe is
measured and marked. Water with a very low concentration of polyethylene
glycol is then poured through the pipe; as fluid exiting the spout changes from
pure water to a dilute polymer solution, the flow rate will visibly grow, increasing
the distance of the water spout by several inches.
5-6 ft
Beaker open to air
Valve permitting
flow into the
Water outlet
2 ft
Outlet valve to remove trapped air (not
necessary if valve exists at bottom of beaker)
Figure 2. Set-up for in-class demonstration of drag reduction in fluid flow.
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Use the demonstration as a starting point for a brief discussion of
lubrication. Ask the students how lubricants could be used in solid, liquid, or
gaseous systems. Discuss how friction can be beneficial for systems needing
high friction (like vehicular tires), and how it can be detrimental. Ask the students
how friction can be reduced.
Explore (Time: 40 min)
Provide the students with the first laboratory worksheet. Permit the student to
start working (in teams of two or three) on the initial sliding friction laboratory
experiment as depicted in Figure 3. Wood blocks with a hook on one end will be
attached to a string, which lead over a pulley to a weight stand. Students then
add small weights that, when the stand was pulled vertically by gravity, increase
the horizontal driving force on the block. The block is then steadily pulled on a
frictionless surface of some type (or at least a smooth surface with minimal
roughness).
Sliding block
String
Pulley wheel
Frictionless surface
Weight
d
Figure 3. Set-up for the sliding friction laboratory experiment.
Students should be asked to measure the weight required to move the block,
conducting this measurement three times for each of a series of designs:
- The wooden block with a piece of sandpaper attached on one side in between
the block and the surface
- The wooden block covered by a sheet of plain paper
- The wooden block, covered by plain paper, turned on to its thinner side
- The wooden block, covered by plain paper, with a weight placed on top of the
block
These experiments show the students the effects of changing the surface of the
sliding object, of changing the contact area, and of changing the applied normal
force.
Explain (Time: 10 min)
Ask the students to calculate the friction coefficient for each of their data points,
and average out their values for each subset of information. Discuss what
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effects influenced the friction coefficient and which did not. Ask which sets of
experiments were more likely to have error.
Expand (Time: 5 min)
Explain to students how they will investigate friction on an inclined plane the
following day. Ask if they expect any effects to play an important role.
Day Two:
Explore (Time: 45 min)
Have students begin conducting the friction lab experiments with an inclined
platform in the same groups as the previous day. If students need to continue
working on the flat plane friction experiments from the previous day – this is likely
if the in-class discussion is lengthy - allow them to do so. Have students
complete related calculations after finished their measurements.
Engage (Time: 15 min)
Bring the groups together to analyze which changes to their set-up had an effect.
Summarize what they had seen during the experimentation so far. Ask how the
COF of the experiment could be either increased or reduced.
Prepare the students for their lubrication experimentation with a friction
demonstration. Place a clipboard on a table in front of the class with five different
strips of material (for example, glossy paper, plain paper, cardstock, saran wrap,
etc.) taped lengthwise on the board. Place five rubber stoppers on one end of
the clipboard and ask the students which stopper is likely to slide first. Steadily
elevate the end of the clipboard until the first stopper slides freely. Then replace
the five stoppers with discs of some other material, and ask the students what
they expect to happen this time – if the discs will slide earlier or later, if they will
slide freely in the same order, etc.
Explain (Time: 5 min)
Students will have been shown the demonstration for both rubber stoppers and
discs of some other material. Ask them how this relates to lubrication. Help
them make the connection between how the static friction coefficient changed not
just for different surface materials but also different sliding materials.
Expand (Time: 10 min)
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Having seen different aspects of lubrication, explain to the students that they will
be able to investigate several friction-changing lubricants the next day in an
inquiry-based atmosphere. Provide examples of household materials they might
exhibit good lubrication, such as liquid-like materials like ketchup and toothpaste,
as well as more solid chemicals such as deodorant and baking powder. Inform
each group they will investigate three different lubricants and ask them to bring in
household materials they choose to study. For the remaining period of class
time, each group should pick three materials they all agree on, and predict the
results they will obtain from each lubrication material.
Day Three:
Engage (Time: 10 min)
Ask the students to show the lubricants they will be using. Provide them with the
second lab experimentation worksheet. Have them write down how they expect
each lubricant to perform, and have them complete their laboratory objectives
before beginning their experiment.
Explore (Time: 45 min)
Each group will investigate a minimum of three lubricant systems. Encourage
them to maintain control throughout their experiments – try to limit contamination
between the different lubricants. Groups will be using solid plastic discs as
opposed to the wooden blocks from the previous day as a means of limiting error
or sliding block – make sure that they measure the friction of the solid plastic disc
with no added lubricant in order to reinforce the concept of a control experiment.
Explain (Time: 5 min)
If students have witnessed types of lubrication, discuss with the class what
occurred. Depending on the lubricants used, these could include plowing friction,
rolling friction, or chemical reactive lubrication.
Discuss with the students how more advanced lubrication systems may allow
some parameters to affect the friction when not witnessed before (for example,
contact area).
Expand (Time: 10 min)
With the students, summarize the friction experimentation and help them draw
conclusions from their lubrication studies. Have them write their COFs on the
blackboard in front of the class. As a class, evaluate which materials were
successes and which were not. Discuss how a balance needs to be achieved
based on the lubricant viscosity. Ask students which lubrication systems would
be most viable in real-life applications.
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Assessment:
The following rubric can be used to assess students during each part of the activity. The
term “expectations” here refers to the content, process and attitudinal goals for this
activity. Evidence for understanding may be in the form of oral as well as written
communication, both with the teacher as well as observed communication with other
students. Specifics are listed in the table below.
1= exceeds expectations
2= meets expectations consistently
3= meets expectations occasionally
4= not meeting expectations
Engage
Explore
Student 1
Student demonstrates interest in the assignment and helps other group members to craft theories as to the aspects of friction and lubrication they will observe. 2
3
Explain
Expand
Student is actively demonstrates Student helps involved in summarizing interest in the describe different the observations from experimentation, effects of the project. Student will taking careful notes lubrication that ask questions about why on the laboratory were witnessed certain lubricants were worksheet and and proposes more effective. Student completing the hypotheses on will propose applications related calculations. why various where various Student was engaged effects occurred. lubrication systems can with partners in be used. conducting the measurements. Student Student shows participates in interest in the filling out the laboratory, writes objectives of the down findings on the laboratory and laboratory making worksheet, and assumptions to conducts the the aspects of experiments and friction and calculations with lubrication they partners. will observe. Student Student participates in the demonstrates little group discussion interest in the with little interest. experimentation. Little effort is put Measurements and into the answers calculations may be for the objectives written down but -9–
Student participates in discussion of the effects that were witnessed. Student participates in summarizing the project observations. Student discusses the effectiveness of lubricants and their applications. Student participates in disussion with little to no interest. Students participates in the class discussion with little to no interest. Answers from the student are simple direction observations with no thought behind of the assignment. 4
Student does not participate in the group discussion, and little to no effort is put into filling out the laboratory objectives. without care. Student will asisst partners but with little interest. Student shows no interest in the laboratory. Student fills in little to no information on the laboratory worksheet and does not assist with the experiments. them. Student does not participate in the discussion. Student does not participate in the class discussion. Safety:
No safety concerns are expected with the project, except for the lubricants
students might bring in to study. While most household items are unlikely to
present any safety concerns, one example would be raw eggs and related
salmonella concerns.
Supplemental Information:
Project worksheets are also available with objectives and goals for the laboratory
experiments. Please see the additional worksheet forms.
Acknowledgments:
Karen Balbierer – Waterloo High School, Waterloo NY
Dr. Claude Cohen – Cornell University, Ithaca NY
Cornell BME NSF GK-12 program: DGE 0841291.
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