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Vocabulary
Grade Level: 8

Friction
Overview:

Momentum
This lesson integrates the LIMBS prosthetic knee, the 8th Grade
Physics (Newton’s laws of Motion), and discussions of friction in our
everyday lives.
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Static friction
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Dynamic friction
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Resistance, force

Synovial fluid
Big Idea: Friction, Force, Motion - Investigating Newton’s laws of
motion and understanding how friction affects our daily lives
 Cartilage
Friction affects us all on a daily basis. We have protection against
friction within our bodies, to enhance our movements. We
appreciate the benefits of friction in every day life like when we step on the brakes in our car. Our shoes are designed
to provide adequate friction between our feet and the surfaces we traverse, so we don’t slip and fall.
The knowledge of friction is important for biomechanics engineering prosthetic limbs for amputee patients. Lubricants
are needed to prevent wear and tear, or to enhance movement. If there is too much friction, the patient won’t be able
to bend their knee. The LIMBS International M1 knee is a polycentric four-bar mechanism designed to mimic the
movements of a human knee, but has the amazing benefit of being very low-cost. The use of low-cost manufacturing
techniques means they can be built inexpensively, and help thousands of patients in developing countries lead more
productive lives.
In this lesson students will:

Develop 21st century skills of team work, problem solving and communication
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Perform a science experiment on that demonstrates the force of friction
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Read an article (expository text), and analyze, make inferences and draw conclusions about the text
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View a few short video clips from the Discovery Channel of the popular show Mythbusters
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Create a visual presentation
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Provide evidence from text to support their understanding of expository text
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View a video about LIMBS International
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Students learn:
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There are physical laws affecting motion - A force is something that tends to cause motion. Simply stated, it
is a push or pull exerted by one object on another.
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Friction is one of those outside forces that halts inertia
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Force causes motion to change. An object’s motion changes in response to force. A force has a size and
direction – both are important in determining an objects motion
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There are two kinds of friction: static and dynamic (kinetic)

The understanding about the interaction of static and dynamic friction allows engineers to make the world
a safer place, and allows biomechanics to create better medical parts for humans.
Students will be able to:
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Explain how friction affects our daily lives
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Describe the effect of force on the knee joint and how Newton’s laws of motion affect body movement

Appreciate the work of charitable organizations which provide help around the world

Understand how the laws of physics and friction work in a sport, like figure skating

Compare the lubricants in the human body that help movement to other forms of lubricants in the real
world.
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Explain the difference between static and dynamic (kinetic) friction

Give examples of static and kinetic friction
Levels of Thinking:
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Synthesis (Create, design, discuss)
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Analysis (Compare, examine, contrast),
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Application (Illustrate and demonstrate)
Science:
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Using critical thinking, scientific reasoning, and problem solving to make informed decisions
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Analyze relationships among force, motion and energy
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Identify Newton’s Laws of Motion

Apply Newton’s Laws of Motion
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Materials:
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Website www.Limbs.org Video titled: Project 4 Awesome ( Time: 2:53)

Possible videos - Promotional from homepage (Time – 3:04) and Founder’s Story (Time: 3:50)

Cardstock, Index cards, student journals, paper, art supplies, poster boards

Friction in the Real World – The Physics of Figure Skating Hand-outs – one per student or one per pair

Soft cover books (around 100 pages each) or phone books, two books per pair of students

Computer and presentation station to show a video clips – Discovery Channel.com
Day(s) 1-2
This lesson should be used after, or in-conjunction-with, the unit on Physics and Newton’s Laws of Motion. Please
refer to the 8th Grade Science text for this background information.
Big Idea: Friction - Investigating Newton’s three laws of motion and explaining how friction occurs in our everyday life
Teacher Introduction:
What is friction? Lead a short class discussion over these main points.
Without getting too technical, friction is simply a force that reduces the motion between objects that are in contact
with each other. So friction stops or slows down movement.
In general, the smoother an object, the less friction it will create. The rougher the surface of an object is, the more
friction will be produced. The angle of the surface and the weight of the object can also affect friction.
There are times when more friction is best, like when you are pressing the brakes in a car. And there are times when
you want less friction, like when you are going down a water slide.
Basically, you have friction, or the lack of friction, to blame when you slip on something like a rotten banana peel or a
slab of bacon fat. No surface is perfectly smooth, and whenever we come into contact with one, such as sitting in a
chair or stepping on and icy sidewalk, two different surfaces interact. The less friction between an object, the more
slippery it is.
Teacher:
Show the video clip from Mythbusters http://www.youtube.com/watch?v=YZRq3XxCZXo Time - 2:00
Let students chat with each other for a few minutes after the video or have a quick class discussion to let the students
respond/reflect.
Teacher Explains or reads this information to students:
The original myth that was tested on Mythbusters was the banana peel fall: One banana peel, one person = one
person on their butt!
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
It is a myth that got it’s start in the early 1900’s when bananas became the most popular fruit in the United States.
People would toss their scrap skins on the sidewalks, where they rotted and became slippery. Enough people slipped
on the aging peels and were injured that many places enacted anti-littering laws. They myth has been perpetuated in
slapstick comedy over the years.
Mythbusters found that – although the slick underside of a fresh banana skin does have some friction-reducing properties – a single peel isn’t a guaranteed fall magnet. If you are determined to see some major slippage, try running on
lots of peels! Putting peel upon peel reduces static friction, the force that keeps an object from moving when it goes
from “stop” to “start”. And the older the peels, the more slippery they will be, because the solid material decomposes
into a soft, slimy texture.
The one peel, one person myth was busted, but as we can see, banana peels CAN cause some major slippage. For our
study’s purpose, we can say that there is a lot of physics involved in the action of slipping and falling on a banana peel!
Teacher - Give class notes and discuss:
Newton and Bananas
Though best known for his apple, Sir Isaac Newton can also help explain the physics of slipping on a banana peel.
Newton’s first law of motion states that any object in motion will stay in motion unless acted on by an outside force.
Friction is one of those outside forces that stops inertia. Newton’s third law holds that for every action, there is an
equal and opposite reaction. When we run into a banana peel while walking, the forward force of our intended
velocity overcomes the peel’s static friction and sets it in motion. In reaction to that accelerated forward force, we
experience an equal, backward force on our bodies that can send us to the ground with a little assistance from gravity!
Two kinds of friction are at work in our slip-up scenario: static and dynamic (aka kinetic).
Static friction refers to the resistant force that prevents something from moving forward when external force is
applied. Think about the difference between pushing a heavy box on a smooth marble floor vs. a gravel road. The
static friction, a downward force, is exerted less on the smooth marble than on the rocks.
Dynamic force is the resistant force acting on a moving object. Imagine the difference between sledding down a hill
after a snowstorm and trying to do so in the middle of summer. The kinetic friction between the moving sled and the
grass is greater than that between the sled and the snow.
In order to slip on a banana peel, or any other obstacle on the ground, the force of forward linear velocity must
overcome downward static friction. Once the banana peel is in motion, it requires less force to continue moving since
kinetic friction is always less than static friction. That explains why after you step on something slippery and begin to
slip, it can be difficult to regain your footing.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Beneficial Friction
Understanding the interaction of static and dynamic friction allows engineers to make the world a safer place. By
measuring the friction between a tire and a wet road, for example, the manufacturers know how much tread is needed
to prevent skidding. The bottoms of our shoes are also designed to provide adequate friction between our feet and the
surfaces we traverse.
Student Activity: Day 1
Now that we have talked about what friction is, let's investigate how it works with the following experiment.
Book Friction
Friction: Stronger than glue!
**Give the students simple instructions, and challenge them to get the books apart without telling them that it will be
impossible!
Idea: Interwoven pages of books will stick together so tightly by the friction they can't be pulled apart. The books will
stick together like glue because each page is exerting friction against the page from the other book. When the
combined friction from each of the pages is added up, it equals a big force.
Materials:
2 soft covered books about the same size - per pair of students – the best type of books are like workbooks or student
atlases, with about 75 -140 pages each. Phone books are what is usually used for this experiment, but any books with
lots of pages (100+) and soft covers will work. The main thing you want is two books of about equal size.
Notes:
This experiment was tested on the show Mythbusters. http://www.youtube.com/watch?v=AX_lCOjLCTo Time – 2:39
The phone books wouldn't come apart using all the Mythbusters' gadgets and even attaching the phone books
between two cars. It took two tanks to pull the books apart. That's how strong friction can be.
Experiment:
Before the students begin lacing the pages, just half the books and put them together to show how easily they will
come apart that way.
Now, have the students with their lab partners interlace the pages by alternating a page from each book. The pages of
the books will be woven and the spines will be facing out. Make sure several inches of the pages overlap. So you'll lay
down the cover of book A, then the cover of book B over it. Then the back page of book A and the back page of book B.
Then the next page and so on until the books' pages are woven together.
You don't actually have to do every page for the experiment to work. You can weave a few pages at a time so it won't
be an all-day process.
After the books are woven together, have each partner grab one of the book spines and have a friend grab the other
spine. Pull on the books to see if you can pull them apart. The students can have a game of tug of war with the books.
(Note: Ensure that the students are in an area that is free from objects that they could potentially fall on or into!)
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Reflection/extension:
Do some tests with interlacing the pages. What if you only weave every ten pages or every twenty? Is the force still as
strong? What is the least amount of weaving necessary to produce the glue-like effect?
When you are done with the experiment, just unlace the pages one by one to release the books. They don't have to
stay stuck forever!
Teacher leads class discussion: Day 2
Teacher ask students:
When can friction be negative? Friction stops movement in some cases – when can this be a bad thing? When do you
want less friction?
Answers may vary, take some guesses…possible answer - Friction is responsible for the wear and tear on bike gears
and other mechanical parts. That's why lubricants, or liquids, are often used to reduce the friction — and wear and tear
— between moving parts.
Teacher asks students How about when we need to walk? We will use the example of our knee joint to demonstrate the need to eliminate
friction in some cases. If our bones at the knee joint were stuck together, without any kind of lubricant between the
bones, how do you think that would go? Would we move freely?
Key Points:
The knee is one of the most complex joints in the body. It must bend, extend and rotate in addition to supporting the
full weight of the body. The knee has only three bones but many tendons and ligaments that control the movements.
Friction between bones during movement is prevented with pads in between the bones called meniscus.
The knee joint, the body's largest joint, is unusual because it can swivel on its axis, allowing the foot to turn from side
to side. Thus, the knee is constantly rolling and gliding during walking.
Examples of lubricants in the human body: These lubricants lower the friction
Moveable joints have cartilage and capsules of fluid that keep hard bone surfaces from rubbing together and causing
damage. What do we call the fluid that protects moveable joints? Synovial fluid. Synovial fluid functions with cartilage
to cushion joints that move, which are appropriately termed synovial joints. Types of synovial joints include hinge
joints, ball-and-socket joints, saddle joints and pivot joints. Osteoarthritis occurs when synovial fluid loses its
consistency with age, contributing to the breakdown of cartilage, which in turn causes bones to rub against each other
and become damaged. Some joints, such as the hip and the knee, can now be surgically replaced when they become
damaged through injury or the wear and tear of age.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Sometimes called "the shock absorber of the body" what pearly white, slippery connective tissue at the epiphysis of a
long bone in the human body provides a smooth surface that reduces friction so that the joint can move properly
without being damaged? Hyaline cartilage. Hyaline cartilage, also called articular cartilage, covers the surface at the
ends of long bones where they articulate: that is, where they form a joint that allows one bone to move in relation to
another bone. Cartilage is a type of connective tissue, as well as acting like a lubricant to reduce friction at the joint of
bones.
Show video from LIMBS International Website:
Project 4 Awesome http://www.limbs.org/videos.php Time – 2:53
**If this is the first time ever introducing LIMBS to the students, you may want to also watch another couple of
videos –
Promotional Video from homepage http://www.limbs.org/videos.php Time: 3:04
Founder’s Story http://www.limbs.org/videos.php Time – 3:50
**If you have not yet ever talked about LIMBS, then you will want to lead a class discussion on what LIMBS is about
and have the students react and reflect.
Discussion Prompts: The LIMBS International Organization is a non-profit organization helping people respond to
natural disasters, challenges in their life, as well as thrive in their environment despite the many challenges of losing a
leg. Some key terms are associated with LIMBS International, such as: technology, polycentric knee, sustainability,
self-sustaining and empowering…
(Teacher may choose to write these on the board.)
Class Discussion - Teacher asks Students:

Who can tell me what those terms mean and how they relate to LIMBS International?

What do these phrases from the LIMBS International Website mean to you?
Sustainable Solutions for the World
Design, Create, Test and Train
Teacher: Ask the students: What is the major component of the LIMBS prosthetic that makes the amputee be able to
live a more normal life?
Answer: the knee allows for bending and doing more normal activities. Other low-cost prosthetics often are
just straight and rigid, which makes it difficult to walk normally or do other labor, recreation and sports
activities. Think of the pirate “peg leg” and how difficult that would make normal activities of walking, running,
climbing, carrying a bucket of water, riding a bicycle…
**If you have previously discussed LIMBS as a class, then move on to this activity after watching the Project 4
Awesome video.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Student activity : Day 2
Have students individually reflect and respond on the article and the LIMBS International video – by writing their
thoughts on an index card or piece of paper, or in their journal. Or, you may choose to have them do a quick Thinkpair-share activity.
Prompts for their writing/discussion: How do you feel about the work LIMBS International is doing? Is this an
organization you might want to help? What charitable organizations have you learned about in the past? How does
LIMBS compare to other charity organizations you have heard of or been involved with? Have some students share
their reflections.
Teacher Asks students:
How is the knowledge of friction important to the biomedical engineers (biomechanics) designing the prosthetic
knee?
Answers:
Use lubricants to avoid wear and tear on mechanical parts
Make sure the friction does not cause difficulty in movement of knee joint.
Student Activity:
Materials: Poster board or cardstock, Art supplies, The Physics of Figure Skating Article from back of lesson
Possible Introduction to activity: Show a short video clip of Olympic Figure Skating, the short programs for
figure skating competitions are only a few minutes.
Have the students read the hand-out titled: Friction in the Real World: The Physics of Figure Skating(See at
attached at end of lesson)
After reading this information (expository text) have the students work in pairs to create a graphic organizer or
visual aid of some kind (picture/diagram/chart…) to demonstrate the information in this passage in a more
easily identifiable way, instead of paragraphs.
Take for a grade or hang around the room. This picture/diagram should be titled Friction in the Real World, or
Physics of Figure Skating.
In addition, finding examples of friction in the real world and explaining how they relate to Newton’s Laws of
Motion would be a cool research assignment.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Student Activity/Student Reflection:
Teaching the concept of friction to grade school children.
**This could be used as a formative assessment
Materials: Poster board or Cardstock, Art supplies
Have students work in groups or individually to create a poster showing friction in the real world. If they were
teaching about friction to a group of 4th graders, how would they represent friction?
Conclusion:
Close this lesson with a discussion on friction in the real world. Positive vs. Negative friction (helpful vs.
unhelpful).
Extension:
Feel free to explore more about these concepts, including the building of a prosthetic knee, in more detail if
time allows, by doing a classroom web search on the presentation station. Or, you may want to assign
homework or a classroom research project for students to research the polycentric knee or other prosthetic
devices, and how they are made.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Friction in the Real World – The Physics of Figure Skating
To see physics in action in everyday life, look no further than figure skating.
As the men and women hit the ice to show off their spins and combinations in the 22nd Winter Olympics, scheduled to
take place February 2014, in Sochi, Russia, there's a perfect chance to watch examples of basic scientific concepts,
such as friction, momentum, and the law of equal and opposite reactions.
Friction
On one level, the difference between dancing on a floor and skating on ice is the lack of friction. Smooth ice provides
very little resistance against objects, like ice skates, being dragged across its surface. Compared to, say, a wooden floor,
ice has much less friction.
So what is friction exactly? It's a force that resists when two objects slide against each other, dissipating their energy of
motion. Friction arises because the molecules on both surfaces bond with each other, and resist when the surfaces try
to move away and break the bonds. The more rough and jagged something is, the more easily more of its molecules
will come into contact with molecules on the surface it touches, and thus the greater force of friction they will exert.
The general low level of friction on ice allows a skater to glide along the surface smoothly without friction stopping the
motion as soon as it's begun.
Newton’s First Law
Remember Isaac Newton's first law of motion — an object in motion tends to stay in motion unless acted on by a
force? This concept is also known as inertia, and it's why ice skaters, whose motion isn't being acted on by
a powerful enough force of friction, tend to stay in motion unless they use force to stop themselves. At the same time,
if there were no friction at all on ice, skating would be impossible, because it is the friction between the skate and the
ice when a skater pushes off that starts the motion to begin with. And friction is also what allows a skater to ever come
to a stop.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Momentum
Momentum, which is basically how much force it would take to stop a moving object. Essentially, the heavier
something is and the faster it's going, the more momentum it will have, and the harder it will be to slow it down.
Angular momentum applies to a body rotating around a fixed object. The amount of angular momentum, say, a
spinning skater has depends on both the speed of rotation, and the weight and distribution of mass around the center.
So, for two skaters of the same mass rotating at the same speed, the one with its mass more extended in space will
have the greater angular momentum.
A fundamental law of physics holds that momentum is always conserved, meaning that unless some outside force
enters a system, its total momentum must stay constant.
This law of physics explains why when a figure skater pulls in her arms when executing a turn, she spins more quickly.
With arms outstretched, her mass is distributed over a greater space. When she draws her arms inwards, that
distribution is reduced, so her speed must pick up to counteract this difference and keep her total momentum constant.
Newton's Third Law
One of the most well-known tenets of physics — for every action, there is an equal and opposite reaction — was first
discovered by Isaac Newton.
And it's this idea that allows skaters to move across the ice. When they push off against the ice, or "stroke" with their
skates, they are applying a force down and back against the ground. Well, the ground just pushes right back, supplying
a force forward and up that propels the skaters into a glide or jump, depending on the particulars of the force they
applied.
Since the forward push is resisted only by the slight friction of the ice, the skater can glide easily.
Applications of friction
Friction plays an important part in many everyday processes. For instance, when two objects rub together, friction
causes some of the energy of motion to be converted into heat. This is why rubbing two sticks together will eventually
produce a fire.
Friction is also responsible for the wear and tear on bike gears and other mechanical parts. That's why lubricants, or
liquids, are often used to reduce the friction — and wear and tear — between moving parts.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International
Types of Friction
There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces
that aren't moving relative to each other, while kinetic friction acts between objects in motion.
In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more
viscous fluids are thicker, so honey has more fluid friction than water.
The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed,
all the atoms inside the material move, creating internal friction.
In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact,
causing friction.
Coefficient of friction
Two solid objects moving against each other experience kinetic friction. In this case, the friction is some fraction of the
perpendicular force acting between two objects (the fraction is determined by a number called the coefficient of
friction, which is determined through experiments). In general, the force is independent of the contact area and
doesn't depend on how fast the two objects are moving.
Friction also acts in stationary objects. Static friction prevents objects from moving and is generally higher than the
frictional force experienced by the same two objects when they are moving relative to each other. Static friction is
what keeps a box on an incline from sliding to the bottom.
Learning for LIMBS Grade 8.8 — Science
© LIMBS International